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Test Taking Strategies

The best way to take a test is to take it prepared. By the time you enter the test room,

all of this information should be deeply ingrained in your head. Nevertheless, there are

some ways you can successfully maneuver around a particularly difficult question.

Strategy One: Do the questions in order from easiest to hardest. This is to boost your confidence on any single question you might not know. If you are unsure of a question, skip it and come back to it later — it is better to miss one question than to miss two just because you spent so much time on a question you had no idea how to answer.

Strategy Two: Eliminate wrong answers. If you don’t know the answer to a question, don’t immediately guess. See if you can get rid of any answer choices that you know are wrong. By doing so, you may discover the correct answer through process of elimination; even if you do not end up eliminating enough answers to find the correct one, your odds of guessing correct are way higher! This method is especially helpful for (I)-(II)-(III) type questions as well. If you know that a statement is wrong, cross out every answer choice with that statement.

For more practice with test taking strategies, visit:

https://docs.google.com/document/d/1tgvIQfB_i1zwSla_iXx6E9yuVhz0PTwF11hf74tqypg

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Tested Topics

The following topics are tested on the Weather or Not exam:

1. Air Masses

2. Fronts

3. Clouds

http://www.nws.noaa.gov/os/brochures/cloudchart.pdf

4. Precipitation

5. Weather forces

El Niño, La Niña, Jet Stream, Coriolis Effect, etc.

6. Pressure systems & significance of air pressure in weather

7. Water cycle

8. Winds

Global wind patterns

9. Hurricanes (can include historic storms)

10. Storms (can include historic storms)

Thunderstorms, derechos, nor’easters, etc.

11. Tornadoes/Waterspouts (can include historic storms)

12. Lightning

13. Atmosphere

14. Seasons

15. Blizzards/Snow/Snow Storms (can include historic storms)

16. Floods (can include historic floods)

17. Droughts (can include historic droughts)

18. Radars/Satellites (GOES and POES)

19. Weather maps/graphs/charts

20. Weather Instruments

21. Forecasting/Prediction

22. Michigan Weather

23. Weather Scientists – varies every year

http://macombso.org/index.php/esoevents/weatherornot

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Additional Resources

Good places to learn about the weather:

http://ww2010.atmos.uiuc.edu

http://www.nws.noaa.gov/os/index.shtml

http://www.rap.ucar.edu/weather/

http://www.education.noaa.gov/Special_Topics/Science_Olympiad.html

http://www.weatherwizkids.com/

How to Forecast the Weather - Dan Ramsey (1983)

http://www.amazon.com/How-Forecast-Weather-Dan-Ramsey/dp/0830602682

Field Guide to Weather - National Audubon Society

http://www.amazon.com/National-Audubon-Society-American-Weather/dp/0679408517

http://www.amazon.com/National-Audubon-Society-First-Field/dp/0590054880

Rules, Strategies, and Resources (Weather or Not)

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Competition Rules

Description of Event:

Meteorology is a weather-based event designed to test students' basic understanding of the meteorological principles and ability to interpret and analyze meteorological data.

Team Size:

2 students

Event Parameters:

The event does not allow any resources during competition, except for a single sheet of paper with notes (written/typed/ double-sided etc.) and a non-graphing calculator.

Rules, Strategies, and Resources (Meteorology)

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Competition Rules

Test Format:

A Meteorology test usually is in the form of a written test or a PowerPoint with slides on it. Occasionally, a test may come in the form of stations that each team rotates between. In the written test, it is generally a good idea to split it if possible, so each person has less work to do, and you can spend time reviewing later on. Also, if time is a tiebreaker, that can be used to your advantage. Unfortunately, in the other formats, this cannot be done, but all other teams have the same disadvantage. As long as you are able to answer all of the questions in an educated fashion, your prospects are pretty bright.

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Competition Rules

Making a Note Sheet:

Note Sheets are often allowed in Study Events. Usually the event will call for a certain number of double-sided, 11" by 8.5" paper (regular printer paper). In general, the strategies for making these note sheets are as follows:

Get as much information as you can! Research your event, and try to put everything on the note sheet. If you run out of room, take out extraneous info that you already know. With that said…
Keep information readable, neat, and easy to access. Remember that you will be taking a timed test! If you are unable to find a piece of information, it's just as useless as not having it in the first place. Experiment with tables, font size, and formatting to fit your needs.
Have diagrams! Often the test will have a section or two with labeling, so having diagrams on your note sheet will help you immensely.
Use colors. Colors aid in visually organizing your information, making it easier to find and more appealing to the eyes.
Use sheet protectors, in case of bad weather or other potential threats.

Rules, Strategies, and Resources (Meteorology)

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Event Rotation

The focus of Meteorology rotates between three topics (everyday weather, severe

storms, and climate), each of which spends one year as the focus before being replaced

by the next topic in the rotation.

All tests contain the following:

Basic knowledge, which appears every year:

Unit 01: Weather and Seasons
Unit 02: The Earth’s Atmosphere
Unit 05: Weather Instruments and Measurement
Unit 06: Temperature and Conversions
Unit 07: Atmospheric Humidity
Unit 09: Cloud Types and Formation
Unit 16: Air Pressure
Unit 17: Wind and Global Weather Patterns
Unit 18: Fronts and Air Masses

Knowledge from one of the three rotations:

(1) Severe Storms, (2) Climate, and (3) Everyday Weather

Rules, Strategies, and Resources (Meteorology)

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Event Rotation

Topic #1: Severe Storms

Years Tested: 2008, 2011, 2014, 2017, 2020… (every three years)

For this topic, you will need to know information from these units:

Unit 11: Rain and Thunderstorms
Unit 12: Floods and Droughts
Unit 13: Snow and Winter Storms
Unit 14: Hurricanes
Unit 15: Tornadoes
Unit 19: Mid-Latitude Cyclones

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Event Rotation

Topic #2: Climate

Years Tested: 2009, 2012, 2015, 2018, 2021… (every three years)

For this topic, you will need to know information from these units:

Unit 03: Energy and the Earth
Unit 04: Climate Change and the Human Impact
Unit 17: Wind and Global Weather Patterns
Unit 20: Climate Zones

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Event Rotation

Topic #3: Everyday Weather

Years Tested: 2010, 2013, 2016, 2019, 2022… (every three years)

For this topic, you will need to know information from these units:

Unit 03: Energy and the Earth
Unit 08: The Water Cycle
Unit 09: Cloud Types and Formation
Unit 10: Light and Optics
Unit 16: Air Pressure
Unit 17: Wind and Global Weather Systems
Unit 18: Fronts and Air Masses
Unit 22: Weather Forecasting

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Most of the information in this slideshow came from the textbook Meteorology Today:

An Introduction to Weather, Climate, and the Environment, 9th Edition by C. Donald Ahrens.

If you are participating in the Division B Meteorology event,

I highly recommend getting this book!

*** Many of the figures used in this slideshow come from this textbook. ***

Purchase link can be found here:

Click the link to view!

http://www.amazon.com/Meteorology-Today-Introduction-Weather-Environment/dp/0495555738

Additional Resources

Rules, Strategies, and Resources (Meteorology)

Chapter 13 Preview

Chapter 14 Preview

Chapter 15 Preview

FREE PREVIEWS!

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“Spring is the time of year when it is

summer in the sun and winter in the shade.”

― Charles Dickens, English Writer

Weather, Climate,

and Seasons

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What is weather?

Weather, Climate, and Seasons

When we talk about the weather, we are talking

about the condition of the atmosphere at any particular time and place. Weather is always changing and is comprised of the following elements:

Air Temperature - the degree of hotness or coldness of the air
Air Pressure - the force of the air above an area
Humidity - a measure of the amount of water vapor in the air
Clouds - a visible mass of tiny water droplets and/or ice crystals that are above the earth’s surface
Precipitation - any form of water, either liquid or solid (rain or snow), that falls from clouds and reaches the ground
Visibility - the greatest distance one can see
Wind - the horizontal movement of air

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What is climate?

If we were to measure these seven weather elements

over a specified interval of time (e.g. 30 years), we would obtain the average weather, or climate, of a particular region.

Climate is therefore the accumulation of daily and seasonal weather events in a particular region over a long period of time, usually over 30 years.

The concept of climate also includes the extremes of weather — for example, the heat waves of summer and the cold spells of winter — that occur in a particular region. The frequency of these extremes helps climatologists distinguish among climates that have similar averages.

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How are weather and climate different?

Suppose we photographed the earth once every 5,000 years for a billion years. If we

were to view a timelapse of all those photos, we would observe a change in climate. Over the course of the timelapse, glaciers would move up and down continents, mountains would rise up and get torn down by erosion, tectonic plates would move and change the landscape of the earth, and volcanoes would erupt and spew heat-trapping gases all over the atmosphere.

But what if we photographed the earth once every minute for an entire day? If we were to view a timelapse of those photos, we would observe a change in weather. Clouds would move across the sky, transferring water from one location to another. The wind would blow from location to location — it might be gusty at one moment but completely calm an hour later.

The earth and its atmosphere are dynamic systems that are constantly changing. Yet it takes longer for entire global weather patterns to shift dramatically. Give a few minutes and the weather could change; however, a minute would not be enough to change the climate.

Hence, weather is short-term while climate is long-term.

A good way to remember: “Climate is what you expect. Weather is what you get.”

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How are weather and climate different?

Weather, Climate, and Seasons

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Studies of Weather

There are many studies that involve different aspects

of weather and atmosphere. A few notable ones are listed below:

Meteorology - Meteorology is the study of the atmosphere and its phenomena. For example, a meteorologist might study the behavior of the atmosphere or work in forecasting.
Climatology - Climatology is the scientific study of climate. A climatologist attempts to discover and explain the impacts of climate so that society can plan its activities, design its buildings and infrastructure, and anticipate the effects of adverse conditions. Although climate is not weather, it is defined by the same terms, such as temperature, precipitation, wind, and solar radiation. For example, a climatologist (or climate scientist) may study the interaction between the atmosphere and ocean to see what influence such interchange might have on the planet many years from now. (Unit 4)

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Studies of Weather

Hydrology - Hydrology is the scientific study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic (water) cycle, water resources, and environmental watershed sustainability. (Unit 8)
Nephology - Nephology is the study of clouds. A nephologist, for example, might study the formation, behavior, and forecast of different types of clouds. (Unit 9)

Although there are more branches of meteorological science, these are the four we will look at for the sake of the competition. In this specific unit, we will look at the work of a meteorologist, which is covered more in depth on the next slide.

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What is a meteorologist?

Most people associate the term meteorologist with the weather forecaster on the television or the radio. However, not every single weather forecaster on TV is a professional meteorologist — a professional meteorologist is an individual with fundamental knowledge concerning how the atmosphere behaves, along with substantial background in mathematics, physics, and chemistry.

A meteorologist uses scientific principles to explain and forecast atmospheric phenomenon. Approximately half of the meteorologists and atmospheric scientists in the United States forecast weather for the National Weather Service, the military, or for a television or radio station. The other half work primarily in research, education, or consultancy.

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What is a meteorologist?

Scientists who conduct atmospheric research may investigate climate change, snowflake formation, temperature patterns, and the like. Much of the work of a research meteorologist involves simulating the atmosphere to see how it behaves. Researchers often work closely with scientists from other fields, such as chemists, physicists, and environmental scientists.

Meteorologists not only provide services to the general public, but they also supply valuable resources to city planners, contractors, farmers, and large corporations. In addition, meteorologists working for private weather firms create the forecasts and graphics that are found in newspapers, on television, and on the Internet. Overall, there are many different types of meteorologists, with positions ranging from weather forecaster to energy trader.

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What causes weather?

Weather is essentially driven by the heat in the atmosphere, and this heat comes from the Sun. Warm air rises in the Equatorial regions because they receive the most energy from the Sun. These warm air masses move poleward and are deflected by the Coriolis Effect, creating a dynamic system of warm and cold air masses that produce winds and act as a giant transporter of heat and energy.

Also, regions near the poles get little to no sunlight during the winter, causing colder temperatures. These differences in temperature between the equator and poles create a restless movement of air and water to distribute heat energy from the Sun across the planet. When air in one region is warmer than the surrounding air, it becomes less dense and begins to rise, drawing more air in underneath. Elsewhere, cooler air sinks, pushing air outward to flow along the surface and complete the cycle.

Weather, Climate, and Seasons

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What causes weather?

Weather, Climate, and Seasons

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What are Earth’s climate systems?

Earth is composed of five “spheres” that interact with each other and affect climate. The five spheres are as follows:

Atmosphere - The atmosphere contains all of the gases on the planet — it is a thin layer of mixed gases which make up the air we breathe. This thin layer also helps the Earth from getting too hot or too cold.
Hydrosphere - The hydrosphere contains all of the liquid water on the planet. Approximately 70% of the earth’s surface is covered by water. Of that, 97% can be found in the oceans.
Geosphere (land) - The geosphere is the portion of the earth that includes the earth's interior, rocks and minerals, landforms and the processes that shape the earth's surface. This includes land, which covers 27% of the earth’s surface. This land topography greatly influences weather patterns, as we will see in later slides. This sphere is also known as the lithosphere.
Cryosphere (ice) - The cryosphere is the frozen water part of Earth’s climate system. Ice is the world's largest supply of freshwater, providing nearly three-fourths of the total supply. It covers the remaining 3 percent of the earth's surface, including most of Antarctica and Greenland. Ice plays an important role in regulating climate because it is highly reflective.
Biosphere - The biosphere is the part of Earth that supports any living animal, plant, or organism.

Weather, Climate, and Seasons

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What are Earth’s climate systems?

Weather, Climate, and Seasons

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What are Earth’s climate systems?

Weather, Climate, and Seasons

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What are Earth’s climate systems?

Two additional “spheres” include:

Anthroposphere - The anthroposphere is the part of the environment that has been made or modified by humans for use in human activities and human habitats.
Pedosphere - The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere, and biosphere.

Weather, Climate, and Seasons

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What are Earth’s climate systems?

Weather, Climate, and Seasons

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The earth revolves around the sun in an elliptical path once every 365.25 days (one year). As the earth revolves around the sun, it spins on its own axis, completing one spin in 24 hours (one day). The average distance from the earth to the sun is approximately 150 million kilometers (93 million miles). However, because the earth’s orbit is in an ellipse rather than a circle, the actual distance from the earth to the sun varies during the year.

The earth is closest to the sun (perihelion) in January at a distance of 147 million kilometers (91 million miles) and farthest from the sun (aphelion) in July at a distance of 152 million kilometers (94.5 million miles). So shouldn’t January be warmer than July if the seasons are determined by distance from the sun? Why isn’t this the case?

Are seasons related to distance?

Weather, Climate, and Seasons

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Seasons are regulated by the amount of solar energy received at the earth’s surface. This is determined primarily by two aspects:

The angle at which sunlight strikes the surface - solar energy that strikes the earth’s surface directly (perpendicularly) is more intense than solar energy that strikes the same surface at an angle. Everything else being equal, regions experiencing more direct solar rays will receive more heat than regions experiencing angled solar rays. In addition, energy is more likely to be scattered and absorbed when shown at an angle, as it must travel through more atmosphere. As a result, when the sun is high in the sky, it can heat the ground to a much higher temperature than when it is low on the horizon.
The length of time the sun shines each day - longer daylight hours mean that more energy is available from sunlight. More surface heating takes place in a location with clear, long days than in a location with clear, short days.

Summer is warmer because summer days have more daylight hours than winter days. In addition, the noontime summer sun is higher in the sky than the noontime winter sun. So what is the factor that makes both of these events possible? Such a factor would be responsible for the seasons.

Why do we have seasons?

Weather, Climate, and Seasons

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Both events are possible because Earth is tilted on its axis as it revolves around the sun. The earth’s axis points to the same direction in space all year long; because of this, the Northern Hemisphere is tilted toward the sun in the summer (June) and away from the sun in the winter (December).

Thus, Earth’s 23.5° axial tilt is the reason why we have seasons. When the earth’s axis points toward the sun, it is summer for that hemisphere. When the earth’s axis points away, it is winter.

Why do we have seasons?

Weather, Climate, and Seasons

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The Tropics

Weather, Climate, and Seasons

The Tropic of Cancer is located in the Northern Hemisphere at 23.5°N.

The Tropic of Capricorn is located in the Southern Hemisphere at 23.5°S.

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What is the difference between a solstice and an equinox?

Weather, Climate, and Seasons

An equinox occurs when the sun is not tilted, and the center of the sun is directed at the equator. Daytime and nighttime during an equinox are approximately equal in duration. This occurs during spring and fall.

A solstice occurs when the Earth is tilted 23.5° and the center of the sun is directed at the Tropic of Cancer or the Tropic of Capricorn. This occurs during summer and winter.

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Seasons in the Northern Hemisphere

Weather, Climate, and Seasons

On June 21, the Northern Hemisphere is directed toward the sun — during noon on this day, solar rays bear down upon this hemisphere more directly than during any other time of year. The sun is at its highest position in the noonday sky directly above the Tropic of Cancer. This day, called the summer solstice, is the astronomical first day of summer for the Northern Hemisphere and the longest day of the year. (As we will see later, the seasons are flipped in the Southern Hemisphere. Hence, this day is the winter solstice for areas in the South.)

If the earth’s axis were not tilted, the noonday sun would always be directly overhead at the equator, and there would be 12 hours of daylight and 12 hours of darkness at each latitude every day of the year. However, since the earth is tilted, each latitude in the Northern Hemisphere will have more than 12 hours of daylight. The farther north you go, the longer the daylight hours become. When you reach the arctic circle, daylight lasts for 24 hours! At the North Pole, the sun actually rises above the horizon on March 20 and has six months before it sets on September 22.

The same applies to Antarctica. During the summer solstice, the arctic and antarctic regions experience 24 hours of continuous sunlight.

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24 Hour Sunlight: Land of the Midnight Sun

Weather, Climate, and Seasons

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So… shouldn’t the poles be warmer during the solstice?

Weather, Climate, and Seasons

Wait, you may say. If the polar latitudes receive 24 hours of sunlight during the solstices, why aren’t they warmer? Well, although the duration of sunlight is greater, the angle is smaller.

Once sunlight enters the atmosphere, it can be scattered, reflected, or absorbed before it reaches the surface. The greater the thickness of atmosphere that sunlight must penetrate, the higher the chances of one of these occurring.

Even if solar energy does reach the surface of the earth, it does not heat the surface effectively. A portion of the sun’s energy is reflected by ice and snow, while some of it melts frozen soil. The amount actually absorbed is spread over a large area. So, even though northern cities experience 24 hours of continuous sunlight on June 21, they are not warmer than regions farther south.

Once again, the poles are colder because polar regions receive less radiation at the surface, & what radiation they do receive does not effectively heat the surface. All of this is related to Earth’s tilt.

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Seasons in the Northern Hemisphere

Weather, Climate, and Seasons

By September 22, the earth will have moved so that the sun is directly above the equator. This is the autumnal equinox, and it marks the astronomical beginning of fall in the Northern Hemisphere.

At the poles during the autumnal equinox, the sun appears on the horizon for 24 hours, due to the bending of light by the atmosphere. The following day, the sun disappears from view, and a six month period of darkness begins.

Every day after the autumnal equinox, the northern half of the world experiences fewer hours of daylight and lower positions of the sun. This reduces sunlight duration and light angle, resulting in lower temperatures and cooler breezes.

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Weather, Climate, and Seasons

What causes the leaves to change color?

Contrary to popular belief, it is not the first frost that causes the leaves of deciduous trees to change color. The yellow and orange colors begin to show through due to the shorter days and cooler nights that cause a decrease in the production of the green pigment chlorophyll.

The molecular structure of chlorophyll a.

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Weather, Climate, and Seasons

Indian Summer

In some years around the middle of autumn, there is an unseasonably warm spell, especially in the eastern two-thirds of the United States. This warm period is referred to as an Indian Summer and may last from several days up to a week or more.

It usually occurs when a large high-pressure area stalls near the southeast coast. The clockwise flow of air around this system moves warm air from the Gulf of Mexico into the central or eastern half of the nation.

The warm weather ends abruptly when an outbreak of polar air occurs and returns the conditions of winter. Normally, a period of cool autumn weather must precede the warm weather period to be called an Indian Summer.

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Seasons in the Northern Hemisphere

Weather, Climate, and Seasons

On December 21, the shortest day of the year, the Winter Solstice occurs. On this day, the Northern Hemisphere is tilted as far away from the sun as it will be all year. The sun shines directly above the Tropic of Capricorn.

On each winter day after December 21, the sun climbs a bit higher in the sky. The periods of daylight grow until days and nights are equal in duration — thus, we have another equinox.

This equinox is the vernal equinox, and it occurs on March 20 to mark the astronomical arrival of spring. The Northern Hemisphere becomes warmer, and the cycle of seasons repeats once again.

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Weather, Climate, and Seasons

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Is December 21 really the first day of winter?

Weather, Climate, and Seasons

December 21 is often called the first day of winter; however, by that time, “winter” weather may have already begun. How is this possible? Is December 20 considered to be in the fall, even though winter conditions might have already developed?

December 21 actually marks the first astronomical first day of winter, just as June 21 marks the first astronomical first day of summer. These are the days when the sun is directly over the Tropic of Capricorn and Tropic of Cancer, respectively. Similarly, the astronomic first day of spring is March 20 (when the sun crosses the equator moving northward), and the astronomic first day of autumn is September 22 (when the sun crosses the equator moving southward). There, the astronomical (or official) beginning of any season is simply the day on which the sun passes over a particular latitude, and has nothing to do with how cold or warm the following day will be.

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Is December 21 really the first day of winter?

Weather, Climate, and Seasons

In the middle latitudes, summer is defined as the warmest season, and winter is defined as the coldest season. If the year was divided into 4 seasons with each season consisting of 3 months, then the meteorological definition of summer over much of the Northern Hemisphere would be the three warmest months of June, July, and August. Similarly, winter would be the coldest months of December, January, and February; autumn would be the transition months of September, October, and November; and spring would be the transition months of March, April, and May.

So although the official first day of winter is December 21, the meteorological definition states that winter has already been around for several weeks.

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Seasons Recap

Weather, Climate, and Seasons

So, to summarize…

An equinox occurs when the sun is not tilted and the center of the sun is directed at the equator. Daytime and nighttime during an equinox are approximately equal in duration.
A solstice occurs when the Earth is tilted 23.5° and the center of the sun is directed at the Tropic of Cancer or the Tropic of Capricorn.
The vernal equinox occurs when the sun is overhead at the equator. It marks the beginning of spring. In the Northern Hemisphere, the vernal equinox occurs during the month of March. In the Southern Hemisphere, it happens in the month of September.
The summer solstice marks the beginning of summer. In the Northern Hemisphere, the summer solstice occurs during the month of June. In the Southern Hemisphere, it occurs in the month of December. The summer solstice is the longest day of the year.
The autumnal equinox occurs when the sun is overhead at the equator. It marks the beginning of autumn. In the Northern Hemisphere, the autumnal equinox occurs in September. In the Southern Hemisphere, it occurs in the month of March.
The winter solstice marks the beginning of winter. In the Northern Hemisphere, the winter solstice occurs in the month of December. In the Southern Hemisphere, it occurs in the month of June. The winter solstice is the shortest day of the year.
In June, the sun is directed at the Tropic of Cancer. In December, the sun is directed at the Tropic of Capricorn.
The sun always rises in the east and sets in the west. This is because the earth rotates or spins toward the east.

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Seasons with Bill Nye

Weather, Climate, and Seasons

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Weather, Climate, and Seasons

Seasons in the Southern Hemisphere

When one hemisphere of the world is pointed at the sun, the other is pointed away. Hence, the Northern and Southern Hemispheres have flipped seasons. When it is winter in one hemisphere, it is summer in the other. Similarly, when it is spring in one hemisphere, it is autumn in the other.

Here are the dates for the Southern Hemisphere:

The summer solstice occurs on December 21, when the sun is directed at the Tropic of Capricorn.
The autumnal equinox occurs on March 20, when the sun moves past the equator from south to north.
The winter solstice occurs on June 21, when the sun is directed at the Tropic of Cancer.
The vernal equinox occurs on September 22, when the sun moves past the equator from north to south.

*The dates 3/20, 6/21, 9/22, and 12/21 are approximate and may vary from year to year.

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How is Earth’s tilt related to daytime length?

Weather, Climate, and Seasons

During the summer months, the hemisphere is pointed toward the sun, so it takes longer for the sun to travel across the horizon. During the winter months, the hemisphere is pointed away from the sun, so it takes a shorter time for the sun to move across the horizon.

In the diagram below, the sun takes the blue path during the summer, the season during which the sun is most directed at that region. Thus, sunrise is at 5:00 AM and sunset is at 9:00 PM.

However, during the wintertime, the sun is not as high in the sky because the region is not as pointed toward the sun. Thus, the sun takes the red path during the winter, and sunrise is at

8:30 AM (as opposed to 5:00 AM), and sunset is at 5:30 PM (as opposed to 9:00 PM).

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Paths of the Sun

Weather, Climate, and Seasons

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Weather, Climate, and Seasons

Lags in Seasonal Temperature

Although sunlight in the Northern Hemisphere is the most intense on June 21, the warmest weather tends to occur later, during the months of July and August. Similarly, sunlight is the least intense on December 21, but the colder temperatures occur in January and February. This phenomenon is known as seasonal lag.

Although incoming energy from the sun is greatest in June, it still exceeds outgoing energy from the earth for a period of at least several weeks. Only when incoming solar energy and outgoing earth energy are in balance are the highest (or lowest) average temperatures attained. When incoming solar energy exceeds outgoing earth energy, temperatures rise. When outgoing earth energy exceeds incoming solar energy, temperatures fall.

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Why is summer longer for the Northern Hemisphere?

Weather, Climate, and Seasons

Because the earth travels more slowly when it is farther from the sun (March to September) than when it is closer (September to March), it takes the earth a little more than 7 days longer to travel from March 20 to September 22 than from September 22 to March 20. Consequently, spring and summer in the Northern Hemisphere last about a week longer than fall and winter. In addition, spring and summer in the Northern Hemisphere are longer than spring and summer in the Southern Hemisphere. This is the other way around for the Southern Hemisphere (fall and winter are longer).

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Weather, Climate, and Seasons

What is the importance of the moon?

Because of its proximity to Earth, the moon exerts a strong gravitational pull on the planet. It is this gravitational pull that causes the oceans to rise and fall — this is what we call tides. The tidal effect of the moon helps to recirculate the oceans.

In addition, the moon also influences climate. The moon’s gravity has the effect of slowing down the Earth’s rotation. If the Earth’s rotation were faster, every day would be shorter. Shorter days would result in a substantial temperature drop on the planet because there would be less time for the sun to heat the Earth.

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The Importance of the Moon

Weather, Climate, and Seasons

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The Phases of the Moon

Weather, Climate, and Seasons

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Types of Tides

Weather, Climate, and Seasons

A spring tide occurs during a full or new moon, when the sun, the earth, and the moon are in alignment. This occurs twice each month, and the tides associated with it are exceptionally strong, with higher than average high tides and lower than average low tides.

A neap tide occurs during the first or last quarter, when the sun and moon are perpendicular (at right angles) to each other. This also occurs twice each month, and the tides associated with it are much weaker, with lower than average high tides and higher than average low tides.

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Perigee and Apogee

Weather, Climate, and Seasons

Apogee and perigee refer to the distance from the Earth to the moon. Apogee is the point at which the moon is farthest from the earth (407,000 km). Perigee is the point at which the moon is closest to the earth (357,000 km). The average distance between the moon and Earth is approximately 382,500 km, or 237,700 miles.

Below is a picture of the moon, taken at its apogee (left) and its perigee (right).

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How does the moon’s distance affect the tides?

Weather, Climate, and Seasons

When the moon is at apogee, the furthest distance from the Earth, it has less gravitational pull which, along with other factors that influence the tides, can contribute to lower tides or lower variation in the high and low tide levels. When the moon is at perigee, closer to the Earth, there is much more gravitational pull which contributes to the opposite effect: higher tides or greater variation between the high and low tides.

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How the Tides Work

Weather, Climate, and Seasons

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“Suddenly, from behind the rim of the moon, in long, slow-motion moments of immense majesty, there emerges a sparkling blue and white jewel, a light, delicate sky-blue sphere laced with slowly swirling veils of white, rising gradually like a small pearl in a thick sea of black mystery.”

― Edgar Mitchell, Apollo 14 Astronaut

The Earth’s Atmosphere

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The Earth’s Atmosphere

What drives the atmosphere?

The radiant energy that Earth receives from the sun drives the atmosphere into patterns of wind and weather and allows the earth to maintain an average surface temperature of about 15°C,

or 59°F. This is simply the average — temperatures on the planet may drop below -85°C (-121°F) on a frigid night at the poles or rise to above 50°C (122°F) on a hot day in a subtropical desert.

The earth’s atmosphere is a thin, gaseous envelope comprised of mostly of nitrogen and oxygen, with small amounts of other gases, such as carbon dioxide and water vapor. Although the atmosphere extends upward for many hundreds of kilometers, almost 99% of the atmosphere lies within a mere 30 km (19 mi) of the earth’s surface! In fact, if the earth were to shrink to the size of a beach ball, its inhabitable atmosphere would be thinner than a piece of paper. There is no definite limit to the atmosphere; it becomes thinner and thinner until it eventually merges with empty space.

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The Earth’s Atmosphere

The Atmosphere’s Composition

Nitrogen occupies about 78% of the earth’s atmosphere, and oxygen occupies about 21% of the earth’s atmosphere. These percentages hold fairly constant up to an elevation of about 80 km, or 50 mi. Note that the percentages stay fairly constant and not the amount — the higher you go, the thinner the air will be (there is less oxygen as you move up in elevation, but oxygen still occupies around 21% of the atmospheric air at that level). Water vapor and carbon dioxide are also important gases in the air, as they are greenhouse gases that play significant roles in the earth’s heat and energy balance.

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The Earth’s Atmosphere

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The Earth’s Atmosphere

Layers of the Atmosphere

The layers of the atmosphere, from lowest to highest, are as follows:

Troposphere
Stratosphere
Mesosphere
Thermosphere
Exosphere

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The Earth’s Atmosphere

Troposphere

The troposphere, which is located from the surface up to about

11 km, contains all the weather we are familiar with. This region is kept well stirred by rising and descending air currents — it is common for air molecules to circulate through a depth of more than 10 km in just a few days.

The higher you move in this layer, the colder it gets. In other words, the troposphere is the region of circulating air extending upward from the earth’s surface to where the air stops becoming colder with height.

The name troposphere comes from the Greek tropein, meaning to turn or change.

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The Earth’s Atmosphere

Stratosphere

The stratosphere, the second layer of the atmosphere, is located between 11 km and 50 km above the earth’s surface. Here, the air temperature begins to increase with height, forming a temperature inversion (occurs when temperature increases with height). Because of this, the stratosphere is a stratified layer, as the amount of vertical motion in the stratosphere is reduced since warm air will always rise to the top of the layer, and cold air will always sink.

The reason for this inversion in the stratosphere is due to the fact that ozone in this layer absorbs energy from ultraviolet (UV) rays, which warms up the stratosphere. If ozone were not present, the air in this layer would probably become colder with height, as it does in the troposphere.

Due to the stratosphere’s stability, most airplanes fly in this layer.

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The Earth’s Atmosphere

Mesosphere

Above the stratosphere is the mesosphere, which can be found between 50 and 85 kilometers above Earth’s surface. The air at this level is extremely thin and the atmospheric pressure is very low. 99.9% of the atmosphere’s mass is below this layer.

Although the percentages of nitrogen and oxygen in the mesosphere are about the same as at sea level, the air’s low density results in fewer oxygen molecules at this level. Thus, without proper breathing equipment, hypoxia (oxygen-starving of the brain) may result.

Many meteors and rock fragments burn up in this layer. In addition, temperature falls as altitude increases, ultimately reaching the lowest average temperature of -90°C, or -130°F. This region is known as the mesopause, and it is the coldest region of Earth’s atmosphere (temperatures can go as low as -100°C, or -148°F).

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The Earth’s Atmosphere

Thermosphere

The fourth layer of the atmosphere is the thermosphere, which can be found between 85 km and about 500 km above the earth’s surface. In this layer, oxygen molecules absorb energetic solar rays, warming the air — thus, as altitude increases in this layer, temperature rapidly increases. Because there are relatively few molecules in the thermosphere, the absorption of a small amount of energetic solar energy can cause a large increase in air temperature. The low density of the thermosphere also means that an air molecule will move an average distance of over one kilometer before colliding with another molecule.

Auroras and the space shuttle’s orbit can be found in this layer.

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The Earth’s Atmosphere

Exosphere

At the top of the thermosphere, molecules can move distances of 10 km before they collide with other molecules. Here, faster moving molecules can actually shoot off into space!

This layer is the exosphere, and is the fifth and final layer of the atmosphere. This is the upper limit of Earth’s atmosphere, as it merges into space. Satellites can be found in this layer.

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The Earth’s Atmosphere

The Karman Line (100 km) is commonly represented as the boundary between Earth’s atmosphere

and outer space.

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The Earth’s Atmosphere

Atmospheric Pauses

Atmospheric pauses are boundaries between layers. They are often defined by where temperature stays the same with height — this is known as an isothermal zone. In addition, the lapse rate, or the rate at which the air temperature decreases with height, is zero in these regions.

The boundary between the troposphere and stratosphere is the tropopause. The height of the tropopause varies; it is normally found at higher elevations over equatorial regions and decreases in elevation toward the poles. Generally, the tropopause is higher in summer and lower in winter at all latitudes on the planet.

The stratopause can be found between the stratosphere and mesosphere. The mesopause can be found between the mesosphere and thermosphere, and it is the coldest region of the earth’s atmosphere. The thermopause is the atmospheric boundary of Earth’s energy system and is located at the top of the thermosphere. Because it is the lower limit of the exosphere (thin remainder of atmospheric particles, mainly hydrogen and helium), it is also called the exobase.

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The Earth’s Atmosphere

Temperature Changes with Altitude

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The Earth’s Atmosphere

Ozone

Ozone is an important gas that encircles Earth’s atmosphere. At the surface, ozone is the primary ingredient of photochemical smog. However, the majority of atmospheric ozone (97%) is found in the stratosphere as a part of Earth’s protective ozone layer. This layer is very important, as it shields living organisms on Earth from harmful ultraviolet (UV) rays. Unfortunately, the ozone layer is currently being depleted by pollutants, but we will cover that topic more in depth in Unit 4.

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The Earth’s Atmosphere

Layers of Composition

Over the last 14 slides, we’ve categorized the atmospheric layers based on temperature. However, the atmosphere can also be divided into layers based on its composition.

Below the thermosphere, the composition of air remains fairly uniform by turbulent mixing, with 78% nitrogen and 21% oxygen. This lower, well-mixed region is known as the homosphere.

In the thermosphere and exosphere, the air is unable to keep itself stirred due to infrequent collisions between atoms and molecules. As a result, heavier gases such as oxygen and nitrogen settle on the bottom while lighter gases such as hydrogen and helium float to the top. Thus the composition of air is no longer 78% nitrogen and 21% oxygen. This region is known as the heterosphere.

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The Earth’s Atmosphere

Ionosphere

The ionosphere is a region of Earth’s upper atmosphere, from about 60 km (37 mi) to 1000 km (620 mi) in altitude, that includes the thermosphere and parts of the mesosphere and exosphere. It is not really a layer, but rather an electrified region within the upper atmosphere where fairly large concentrations of ions and free electrons exist. In other words, this is the region where the earth’s atmosphere is ionized by solar and cosmic radiation. The ionosphere plays a major role in AM radio communications (FM radio stations don’t need to worry about the ionosphere, as FM radio waves are shorter than AM waves and are able to penetrate the ionosphere without being reflected).

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The Earth’s Atmosphere

The Layers of the Atmosphere

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The Earth’s Atmosphere

Layers of the Atmosphere

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“We are all connected; To each other,

biologically. To the earth, chemically. To

the rest of the universe, atomically.”

― Neil deGrasse Tyson, Astrophysicist

Energy and the Earth

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Energy and the Earth

What is energy?

Energy is the ability or capacity to do work on some form of matter. Matter is anything that has mass and occupies space. Work is done on matter when matter is either pushed, pulled, or lifted over some distance.

Energy can take on many forms, and it can change from one form to another, but the total amount of energy in the universe remains constant. Energy cannot be created or destroyed.

The total amount of energy stored in any object (internal energy) determines how much work it is capable of doing. This is called potential energy because it represents the potential to do work. On the other hand, objects in motion have kinetic energy, or energy of motion. The faster something moves, the greater its kinetic energy.

The most important form of energy in terms of weather and climate is energy from the sun, or radiant energy.

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Energy and the Earth

Energy Formulae

The potential energy (PE) of any object is given by

PE = mgh

where m is the object’s mass in kilograms, g is the acceleration due to gravity (9.8 m/s² on Earth), and h is the object’s height above the ground in meters. PE is measured in joules, or J.

Example:

Calculate the potential energy of a 2 kg rock located 17 m above the ground.

PE = mgh = (2 kg)(9.8 m/s²)(17 m) = 333.2 J

A 2 kg rock located 17 m above the ground has 333.2 J of potential energy.

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Energy and the Earth

Energy Formulae

The kinetic energy (KE) of any object is given by

KE = ½ mv²

where m is the object’s mass in kilograms and v is the object’s velocity in meters per second.

KE is measured in joules, or J.

Example:

Calculate the kinetic energy of a 2 kg rock traveling at a speed of 29 m/s.

KE = ½ mv² = (½)(2 kg)(29 m)² = 841 J

A 2 kg rock traveling at a speed of 29 m/s has 841 J of kinetic energy.

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Energy and the Earth

Temperature and the Density of Air

Temperature is a measure of the average speed of the atoms and molecules of a substance. Higher temperatures correspond to faster average speeds.

When air is warmed, the molecules move faster and slightly farther apart. Thus, warmer air is less dense. If the air is cooled, the molecules would slow down and crowd closer together. Thus, colder air is more dense. This difference in densities is why air circulates in the troposphere, allowing us to experience weather: warm air rises and cool air sinks!

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Heat is the energy in the process of being transferred from one object to another because of the temperature difference between them.

The heat capacity of a substance is the ratio of the amount of heat energy absorbed by that substance to its corresponding temperature rise. The heat capacity of a substance per unit mass is called specific heat. In other words, specific heat is the amount of heat needed to raise the temperature of one gram of a substance one degree Celsius.

For example, if it takes 1 unit of heat (measured in calories) to raise the temperature of Substance A by 1°C and 5 units of heat to raise the temperature of Substance B by 1°C, the specific heat of Substance B is 5 times greater than that of Substance A. In other words, Substance B must absorb 5 times as much heat as the same quantity of Substance A to raise its temperature by the same amount.

Energy and the Earth

Heat

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Energy and the Earth

Water has a very high specific heat — it takes a lot of heat to raise the temperature of one gram of water by 1°C. So why is this important?

Not only does water heat slowly, it cools slowly as well. A given volume of water can store a large amount of energy while undergoing only a small temperature change. Because of this, water has a strong modifying effect on weather and climate.

Areas near bodies of water usually experience warmer winters and cooler summers than areas further inland. It is for this reason that Michigan experiences milder temperature variations throughout the year — the Great Lakes help to moderate climate. We will go more in depth on this topic in the Michigan Weather unit, Unit 21.

Water and Heat Capacity

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Energy and the Earth

Climate Moderation

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Energy and the Earth

Latent Heat and Sensible Heat

Latent and sensible heat are types of energy released or absorbed in the atmosphere. Latent heat is related to changes in phase between liquids, gases, and solids. Sensible heat is related to changes in temperature of a gas or object with no change in phase. Latent heat released during any kind of storm increases instability in the atmosphere, potentially causing severe weather. Sensible heat causes change in temperature due to contact with colder or warmer air of surfaces.

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Energy and the Earth

Latent heat is the energy absorbed by or released from a substance during a phase change from a gas to a liquid or a solid or vice versa. If a substance is changing from a solid to a liquid, for example, the substance needs to absorb energy from the surrounding environment in order to spread out the molecules into a larger, more fluid volume. If the substance is changing from something with lower density, like a gas, to a phase with higher density like a liquid, the substance gives off energy as the molecules come closer together and lose energy from motion and vibration.

For example, when water is boiled over a stove, energy is absorbed from the heating element and goes into expanding the water molecules into a gas, known as water vapor. When liquid water is put into ice cube trays and placed in the freezer, the water gives off energy as the water becomes solid ice. This energy is removed by the freezer system to keep the freezer cold.

Latent Heat

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Energy and the Earth

Because of latent heat, evaporation is a cooling process. For a substance to evaporate, heat from the environment is needed to expand the liquid molecules into a gas — thus, the environment becomes cooler. This is how sweat works: when sweat evaporates, it uses the heat from your body to do so, thus cooling your body down.

Similarly, condensation is a warming process. When gas molecules turn into a liquid, they release heat to the surroundings in order to bring the molecules closer together, making the environment warmer.

Latent Heat

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Energy and the Earth

Sensible Heat

Sensible heat is the energy required to change the temperature of a substance with no phase change. The temperature change can come from the absorption of sunlight by the soil or the air itself. It can also come from contact with the warmer air caused by release of latent heat (by direct conduction). Energy moves through the atmosphere using both latent and sensible heat acting on the atmosphere to drive the movement of air molecules that create wind and vertical motions.

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Energy and the Earth

Heat can be transferred in many ways. One way is conduction, or the transfer of heat from molecule to molecule within a substance. Heat transferred in this fashion always moves from warmer to colder regions. Generally, the greater the temperature difference, the more rapid the heat transfer.

When materials can easily pass energy from one molecule to another, they are considered to be good conductors of heat. Solids, especially metals, are better conductors of heat than air. Air is an extremely poor conductor of heat! Hot ground only warms a shallow layer of air a few centimeters thick by conduction.

Despite this, air can carry energy rapidly from one region to another. How does this happen?

Conduction

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Energy and the Earth

The transfer of heat by the mass movement of a fluid, such as water or air, is called convection. Convection happens naturally in the atmosphere. On a warm, sunny day, certain areas of the earth’s surface absorb more heat from the sun than others; as a result, the air near the earth’s surface is heated somewhat unevenly.

The heated air expands and becomes less dense than cooler air around it. As a result, this warmer air rises. In this manner, large bubbles of warm air, called thermals, rise and transfer heat energy upward. Cooler, heavier air flows toward the surface to replace the rising air. This cooler air becomes heated in turn, rises, and the cycle is repeated.

In this way, a convective circulation, or thermal cell, is produced in the atmosphere. In our atmosphere, any air that rises will expand and cool, and any air that sinks is compressed and warms.

Convection

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Energy and the Earth

The horizontally moving part of the convection circulation, called wind, carries properties of the air in that particular area with it. This transfer of these properties by horizontally moving air is called advection.

For example, wind blowing across a body of water will pick up water vapor from the evaporating surface and transport it elsewhere in the atmosphere. If the air cools, the water vapor may condense into cloud droplets and release latent heat. Thus, heat is advected by the water vapor as it is swept along with the wind. This is an important way to redistribute heat energy in the atmosphere.

Advection

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Energy and the Earth

Another form of energy transfer is radiation; this is the method with which we receive energy from the sun. Energy transferred through radiation does not require the space between two transferring objects to be heated.

Radiation travels in the form of waves that release energy when they are absorbed by an object. All things, no matter how big or small, emit radiation!

44% of radiation from the sun is in the form of visible light, and 7% is in the form of ultraviolet light. The majority of earth’s energy is radiated via infrared light, light that cannot be seen by humans.

In addition, all objects not only radiate energy, they absorb it as well. If an object radiates more energy than it absorbs, it gets colder. If it absorbs more energy than it emits, it gets warmer. An object that absorbs all radiation that strikes it is called a blackbody.

Radiation

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Energy and the Earth

Stefan-Boltzmann Law and Wien’s Law

Objects that have a very high temperature emit energy at a greater intensity than objects at lower temperatures. The Stefan-Boltzmann law states that as the temperature of an object increases, more total radiation is emitted each second. This is shown by this mathematical notation:

For example, a doubling of temperature increases radiation emission by a factor of 2⁴, or 16.

Wien’s law allows us to find the radiation of the sun and the earth. The equation can be used for the sun and the earth.

For the earth, λ_max is the wavelength in micrometers at which maximum radiation emission occurs, T is the object’s temperature in Kelvin, and the constant is 2897 μm•K.

In this case, the constant 2897 μm•K is rounded to 3000 μm•K, and the average surface temperature of the earth, 288 K, is rounded to 300 K. Thus, we conclude that the earth emits maximum radiation at wavelengths of around 10 μm. The sun, on the other hand, produces a result of approximately 0.5 μm.

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Energy and the Earth

When sunlight strikes small objects, such as air molecules and dust particles, the light itself is deflected in all directions. This distribution of light is called scattering.

Air is better scatterer of blue light — thus, when we look at the daytime sky, blue light strikes our eyes from all directions, turning the sky blue.

Sunlight can also be reflected from objects. An object’s albedo is a measurement of how well it reflects light; in other words, albedo represents the reflectivity of the surface. Snow, for example, has a high albedo, while asphalt, on the other hand, has a low albedo.

Water has a rather low albedo, with an average value of around 10%. It has the highest albedo when the sun is low on the horizon and the water is a little choppy. The earth has an albedo of 30% (see slide 106).

Scattering

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Energy and the Earth

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Energy and the Earth

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Energy and the Earth

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Energy and the Earth

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Energy and the Earth

Interactions between charged particles from the sun (called solar wind) and the magnetosphere (the earth’s magnetic field) results in the formation of auroras in the thermosphere. The aurora found in the Northern Hemisphere is called the aurora borealis (or northern lights), while the aurora found in the Southern Hemisphere is known as the aurora australis (or southern lights).

The aurora is most frequently seen in polar latitudes, but on rare occasions when the sun is very active, they can be spotted in the southern United States.

The average height of an aurora is 105 km. Rarely will auroras be found below 80 km or above 200 km. Auroras will almost always be located in the thermosphere.

Auroras

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Energy and the Earth

So, in summary, energy for the aurora comes from the solar wind, which disturbs the earth’s magnetosphere. This disturbance causes energetic particles to enter the upper atmosphere, where they collide with atoms and molecules. The atmospheric gases become excited and emit energy in the form of visible light.

Other light coming from the atmosphere, a faint glow at night much weaker than the aurora, is called airglow. It is detected at all latitudes and is not related to solar wind activity.

Auroras

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Energy and the Earth

Aurora Formation

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Climate Change and

the Human Impact

“Climate change is happening, humans are causing it, and I think this is perhaps the

most serious environmental issue facing us.”

― Bill Nye, the Science Guy

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Climate Change and the Human Impact

Types of Climate Change

The climate has been changing ever since the beginning of time; there are many different factors that go into the repeating alterations of the environment we live in. We will cover this in the second half of this unit.

However, in the past few centuries, human activity has begun to have a larger impact on the state of our climate. Anthropogenic Climate Change is the term used to describe the process by which human activities change the climate. We will introduce a bit of this in the next few slides.

The animated spiral to the left, created by climatologist Ed Hawkins, illustrates the change in global temperature since 1850.

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Climate Change and the Human Impact

Greenhouse Gases and CO₂

Greenhouse gases are gases that trap heat in the atmosphere. One of the most important greenhouse gases in the atmosphere is carbon dioxide (CO₂).

Since 1958, the atmospheric concentration of CO₂ has risen more than 20% — this means that CO₂ is entering the atmosphere at a greater rate than it is being removed. Because CO₂ is a greenhouse gas, its overabundance leads to a warming of the planet.

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Climate Change and the Human Impact

The increase in CO₂ is mainly due to the burning of fossil fuels. In fact, 80% of fossil fuel emissions from human activity is in the form of carbon dioxide, usually from the combustion of fossil fuels.

20% of the observed increase in CO₂ comes from another source: deforestation. By cutting trees, CO₂ is directly released into the atmosphere. This is can dry up the climate in the deforested area, and since trees use up CO₂ to conduct photosynthesis, it can also decrease the rate of CO₂ removal in the atmosphere as well.

Further, deforestation can speed up the process of soil erosion, or the wearing away of land by water, ice, or wind.

Through which sources are carbon dioxide released? Electricity is responsible for 37% of emissions, transportation 31%, industry 15%, residential and commercial 10%, and other 6%.

What is responsible for the extra CO₂?

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Climate Change and the Human Impact

An Explanation of Climate Change

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Climate Change and the Human Impact

Wait… if the climate is warming, why is it still cold?

Just a few years ago, the Midwest experienced a powerful polar vortex that brought temperatures below -30°F. Similarly, the northeast underwent a severe blizzard this winter season. So, do these occurrences disprove the claim that the climate is warming? Watch this video to find out:

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Climate Change and the Human Impact

Other Greenhouse Gases

Carbon dioxide (CO₂) is just one of the greenhouse gases that contribute to the Greenhouse Effect. Other notorious gases include:

Water Vapor (H₂O) - water vapor strongly absorbs a portion of the earth’s outgoing radiant energy and plays a significant role in the earth’s heat-energy balance.
Methane (CH₄) - methane also traps heat in the atmosphere, and has been increasing by about one-half of one percent per year. Most methane appears to derive from the breakdown of plant material by certain bacteria, wet oxygen-poor soil, the biological activity of termites, and biochemical reactions in the stomachs of cows.
Nitrous Oxide (N₂O) - also known as laughing gas, nitrous oxide has been rising annually at the rate of about one-quarter of a percent. It often forms in the soil through a chemical process involving bacteria and certain microbes. UV light from the sun destroys it.
Chlorofluorocarbons (CFCs) - CFCs were once used as propellants in spray cans and are now used as refrigerants, propellants, and solvents. They have an important effect on our atmosphere as they not only raise global temperatures, they also play a part in destroying the gas ozone in the stratosphere.

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Climate Change and the Human Impact

Destruction of the Ozone Layer

As we learned in Unit 2, the ozone layer is a natural protective shield in the stratosphere that protects life on Earth. When chlorofluorocarbons (CFCs) enter the atmosphere, UV rays break them apart, and the gas chlorine is released. Chlorine is a major destroyer of atmospheric ozone (another gas that destroys ozone easily is bromine). One chlorine atom can destroy over 100,000 ozone molecules before it is removed!

Ozone concentrations in the stratosphere has been decreasing over parts of the Northern and Southern Hemispheres. The reduction in stratospheric ozone levels over springtime Antarctica has plummeted at such an alarming rate that during September and October, there is an ozone hole over the region.

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Destruction of the Ozone Layer

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Climate Change and the Human Impact

Consequences of Ozone Destruction

Because ozone protects Earth from the sun’s dangerous ultraviolet rays, the depletion of the ozone layer allows more UV radiation to reach the earth’s surface. This increases the risk of dangerous levels of UV exposure, which may lead to skin cancer, eye damage (cataracts), harm to ecosystems, crop damage, extinction, and many other issues.

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Climate Change and the Human Impact

Pollutants

Man-made impurities in the atmosphere are known as pollutants. Examples of pollutants include:

Nitrogen Dioxide (NO₂) - released by automobile engines and reacts with gases in the atmosphere to produce ozone at the surface (remember, tropospheric ozone is photochemical smog, which isn’t a good thing).
Carbon Monoxide (CO) - 75% of this poisonous gas in urban areas comes from road vehicles. CO is a major pollutant of city air.
Sulfur Dioxide (SO₂) - the burning of sulfur-containing fuels, such as coal and oil, releases this colorless gas into the air. When the atmosphere is sufficiently moist, the SO₂ may transform into sulfuric acid, which may fall as acid rain and corrode metals, damage ecosystems, and turn freshwater lakes acidic. Additionally, high levels of SO₂ may cause serious respiratory problems in humans, such as bronchitis and emphysema.

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Selective Absorbers and the Greenhouse Effect

The earth’s atmosphere absorbs and emits infrared radiation. Unlike the earth, the atmosphere does not act like a blackbody, as it absorbs some wavelengths of radiation and is transparent to others. Objects that selectively absorb and emit radiation, such as gases in our atmosphere, are known as selective absorbers.

The absorption characteristics of water vapor, CO₂, and other gases such as methane and nitrous oxide inhibit to some degree the passage of outgoing infrared radiation. Thus, the role that water vapor, CO₂, and other greenhouse gases play in keeping the earth’s average surface temperature higher than it otherwise would be is known as the greenhouse effect.

The greenhouse effect is necessary for life to exist on Earth! The presence of heat-trapping gases in the atmosphere prevents the planet from getting too cold. Without CO₂ or water vapor in the atmosphere, the earth’s average temperature would be -18°C, or 0°F! It’s only when we dump too many greenhouse gases into the atmosphere that the greenhouse effect brings negative impacts to our planet.

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Climate Change and the Human Impact

The Greenhouse Effect

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Climate Change and the Human Impact

During the past century, the earth’s average surface air temperature has undergone a warming of about 0.6°C, or 1°F. Continued warming may lead to a rise in sea level and a shift in global precipitation patterns in the future.

The main cause of global warming is CO₂, whose concentration has been increasing due to the burning of fossil fuels and deforestation. However, increases in methane, nitrous oxide, and chlorofluorocarbons have also contributed to the effect.

Overall, water vapor accounts for 60% of the atmospheric greenhouse effect, CO₂ accounts for about 26%, and the remaining greenhouse gases contribute about 14%.

The continuing rise of CO₂ and other greenhouse gases may cause the earth’s average surface temperature to rise an additional 3°C by the end of the 21st century.

Global Warming

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Climate Change and the Human Impact

Earth’s Feedback System

A feedback is a process where an initial change in a process will tend to reinforce the process (positive feedback) or weaken the process (negative feedback).

An important positive feedback is the water vapor-greenhouse feedback. Rising ocean temperatures due to warming will cause an increase in evaporation rates; this increases the amount of water vapor in the air and strengthens the greenhouse effect, further warming and planet and continuing the cycle. Since the results of the greenhouse effect (more water vapor) strengthen the greenhouse effect (warmer planet) in this case, this water vapor situation is an example of positive feedback.

On the other hand, an increase in evaporation would lead to an increase in cloudiness. Clouds, however, reflect and radiate away more energy than they retain, cooling the earth’s climate. Thus, because the results of the greenhouse effect (more clouds) weaken the greenhouse effect (cooler planet) in this case, this cloud situation is an example of negative feedback.

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An Introduction to Air Pollution

On Slide 122, common types of pollutants were introduced. This topic will be expanded upon in the next few slides.

Every breath we take is filled with nitrogen and oxygen. However, we also take in other gases and particles, some of which are called pollutants. These contaminants come from car exhaust, chimneys, forest fires, factories, power plants, and other sources related to human activities. Estimates show that, worldwide, nearly one billion people in urban environments are continuously being exposed to health hazards from air pollutants.

Air pollutants are airborne substances (either solids, liquids, or gases) that occur in concentrations high enough to threaten the health of people or animals, to harm vegetation and structures, or to toxify a given environment. Air pollution can come from both natural sources and human activities. A volcano eruption that causes ash and dust to spill out into the atmosphere is an example of a natural source of air pollution. Car exhaust, on the other hand, is a man-made source of pollution.

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Pollution caused by human activity enters the atmosphere from both fixed sources and mobile sources. Fixed sources encompass industrial complexes, power plants, homes, and office buildings. Mobile sources include cars, ships, and planes.

Primary air pollutants are pollutants that enter the atmosphere directly (for example, from a smokestack). Secondary air pollutants are pollutants that form when a chemical reaction occurs between a primary pollutant and some other component of air.

The picture to the left is of the smoke that was blown over the Pacific Ocean by strong Santa Ana winds on October 28, 2003. The smoke originated from the massive wildfires that burned across Southern California at the time (Source: NASA).

Human-Induced Pollution

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As shown on the diagram below, carbon monoxide is the most abundant primary air

pollutant in the United States. The primary source of all pollutants is transportation

(motor vehicles, planes, etc.), with fuel combustion from stationary sources coming in

a distant second (factories, power plants, sewage treatment, etc.).

Primary Pollutants and Sources

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The term particulate matter represents a group of solid particles and liquid droplets that are small enough to remain suspended in the air. Collectively, they are known as aerosols; this category includes soot, dust, smoke, pollen, arsenic, tiny liquid droplets of sulfuric acid, PCBs, oil, and various pesticides.

Particulate matter dramatically reduce visibility in urban environments. Examples include iron, copper, nickel, and lead. Lead particles are especially dangerous, as they can contaminate food and water supplies. Consumption of lead can lead to brain damage, convulsions, and death.

Of the nearly 7 million metric tons of particulate matter emitted over the United States each year, about 40% comes from industrial processes, with highway vehicles accounting for about 17%.

Principal Air Pollutants

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Particles with diameters less than 10 micrometers (μm) are referred to as PM₁₀. These particles are very dangerous, as they do not settle to the ground in about a day or so after being emitted, and they are small enough to penetrate the lung’s natural defense mechanisms.

Particles with diameters less than 2.5 μm, such as those in diesel soot, are even more dangerous. They are referred to as PM_2.5.

Rain and snow remove many of these particles from the air; in fact, the predominant removal mechanism occurs when these molecules act as condensation nuclei for cloud droplets. Many of these suspended particles are hygroscopic, or readily absorb water. When water forms on these particles, they grow in size, forming haze that scatters incoming light and gives the sky a milky white appearance.

Particulate Matter

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Carbon Monoxide (CO) is a major pollutant of city air, and it is colorless, odorless, and poisonous — it can kill without warning! Because of this, it is often known as the silent killer. As mentioned on Slide 131, CO is the most plentiful of the primary pollutants.

Optional Read - CO leak kills 9-year-old boy and critically injures

10-year-old girl in their sleep: http://goo.gl/vivWl6

Sulfur Dioxide (SO₂) is a colorless gas that comes primarily from the burning of sulfur-containing fossil fuels, such as coal and oil. Its primary source includes power plants, heating devices, smelters, petroleum refineries, and paper mills. However, it can enter the atmosphere naturally during volcanic eruptions. When inhaled, sulfur dioxide can cause respiratory problems, such as asthma and bronchitis.

Optional Read - Smoggy days may lead to higher blood pressure:

http://goo.gl/eP5Ay9

Major Pollutants

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Volatile Organic Compounds (VOCs) represent a class of organic compounds that are mainly hydrocarbons, or compounds composed of hydrogen and carbon. The most abundant of these is methane.

Nitrous Oxides are gases that form when nitrogen in the air reacts with oxygen during high temperature combustions of fuel. The two primary nitrogen pollutants are nitrogen dioxide (NO₂) and nitric oxide (NO). Together, they are referred to as NO_x, or oxides of nitrogen.

The primary sources of nitrogen oxides are motor vehicles, power plants, and waste disposal systems. High concentrations may lead to heart and lung problems.

In addition, nitrogen oxides play a key role in producing photochemical smog, which we will discuss on the next slide.

Major Pollutants

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Today, the word smog refers to fog or haze that combines with other atmospheric pollutants. The main component of photochemical smog (or smog that forms when chemical reactions take place in the presence of sunlight) is the gas ozone (O₃).

While ozone that naturally forms in the stratosphere protects life on Earth from the sun’s UV rays, tropospheric ozone is a secondary pollutant that forms from a complex set of reactions involving nitrogen oxides, hydrocarbons, and sunlight.

Tropospheric Ozone

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Stratospheric ozone, on the other hand is good. Located in the stratosphere at an altitude of near 25 km, ozone prevents too much UV light from reaching the earth’s surface. This is beneficial, as UV radiation can cause skin cancer and destroy DNA.

If the concentration of stratospheric ozone decreases, the following are expected to occur:

An increase in the number of cases of skin cancer.
A sharp increase in eye cataracts and sunburning.
Suppression of the human immune system.
Damage to crops and negative impact on animal livelihoods.
A reduction of ocean phytoplankton growth.
A cooling of the stratosphere that could alter stratospheric wind patterns.

Stratospheric Ozone

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The air quality index (AQI) measures the air quality in a particular region. The index includes the pollutants carbon monoxide, sulfur dioxide, nitrogen dioxide, particulate matter, and ozone. On any given day, the pollutant measuring the highest value is the one used in the index. The measurement is then converted into a number that ranges from 0 to 500, with 100 and below being normal. Each AQI category is color coded.

Air Quality Index

AQI Value	Color	Air Quality	General Health Effects	Recommended Actions
0-50	Green	Good	None.	None.
51-100	Yellow	Moderate	There may be a moderate health concern for a very small number of individuals. People unusually sensitive to ozone may experience respiratory symptoms.	When O₃ AQI values are in this range, unusually sensitive people should consider limiting prolonged outdoor exposure.
101-150	Orange	Unhealthy for Sensitive Groups	Mild aggravation of symptoms in susceptible persons.	Active people with respiratory or heart disease should limit prolonged outdoor exertion.
151-200	Red	Unhealthy	Aggravation of symptoms in susceptible persons, with irritation symptoms in the healthy population.	Active children and adults with respiratory or heart disease should avoid extended outdoor activities. Everyone else, especially children, should limit prolonged outdoor exertion.
201-300	Purple	Very Unhealthy	Significant aggravation of symptoms and decreased exercise tolerance in persons with heart or lung disease, with widespread symptoms in the healthy population.	Active children and adults with existing heart or lung disease should avoid outdoor activities and exertion. Everyone else, especially children, should limit outdoor exertion.
301-500	Maroon	Hazardous	Significant aggravation of symptoms. Premature onset of certain diseases. Premature death may occur in ill or elderly people. Healthy people may experience a decrease in exercise tolerance.	Everyone should avoid all outdoor exertion and minimize physical outdoor activities. Elderly and persons with existing heart or lung disease should stay indoors.

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Chemical smog often occurs with clear skies, light winds, and generally warm sunny weather. However, there are other important factors that impact air pollution.

Wind Speed - wind speed plays a role in diluting pollution. When large amounts of pollutants are spewed into the air, wind speed determines how quickly the pollutants mix with the surrounding air and how fast they move away from their source. Strong winds tend to lower the concentration of pollutants by spreading them apart as they move downstream. Weak winds cause the pollutants to become more concentrated, as they do not disperse as much.

Factors That Affect Air Pollution

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Stability and Inversions - when temperature decreases rapidly with height, the atmosphere tends to be unstable (high lapse rate). Vertical air currents are favored, and pollutants tend to be mixed vertically. When temperature decreases slowly or even increases with height (known as a temperature inversion, see slide 75), the atmosphere is stable. Any air parcel that tries to rise in a stable atmosphere will at some point be cooler and heavier than the warmer air surrounding it (since temperature goes up as altitude increases in an inversion). Hence, the inversion acts like a lid on vertical air motions.

A special type of inversion called a radiation inversion (or surface inversion) occurs during the

night and early morning hours when the winds are light and the sky is clear. Here, the air at

the surface is cooler than the air at the base of the inversion (do note that this inversion is

rather shallow and does not persist for too high of an altitude). Because of this, air pollution

coming out of shorter smokestacks will sink and spread out, contaminating the area. This is

why smokestacks (as shown on Slide 115) are really tall! Unfortunately, while tall stacks may

improve the air quality in the immediate area, they may also contribute to acid rain by

allowing pollutants to be swept great distances.

Factors That Affect Air Pollution

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Topography - at night, cold air tends to drain downhill, where it settles into low-lying basins and valleys. The cold air can strengthen a pre-existing radiation inversion, and it can carry pollutants downhill from surrounding hillsides. Locations surrounded by mountain valleys are especially prone to pollution. Air pollution concentrations in mountain valleys are greatest during the winter months, when there is no heating to vent pollutants upwards.

Factors That Affect Air Pollution

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The greatest potential for an episode of dangerous air pollution occurs when everything mentioned in the previous slides comes together simultaneously. The ingredients for severe atmospheric pollution include the following:

Many sources of air pollution, preferably clustered close together
A deep high-pressure area that becomes stationary over a region
Light surface winds that are unable to disperse the pollutants
A strong subsidence inversion* produced by the sinking of air aloft
A shallow mixing layer with poor ventilation
A valley where pollutants can accumulate
Clear skies so that radiational cooling at night will produce a radiation (or surface) inversion, which can cause an even greater buildup of pollutants near the ground
Adequate sunlight to produce secondary pollutants, such as ozone

Light winds and poor vertical mixing can produce a condition known as atmospheric stagnation, which can lead to the buildup of pollutants for several days to several weeks. This can lead to air pollution disasters, like the 1948 Donora Smog that killed 20 people and sickened thousands of others.

*Subsidence inversions are long-lasting inversions that form when a widespread layer of air descends.

The Potentials of Pollution

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A Quick Review

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Cities are generally warmer than surrounding rural areas — this region of city warmth is known as the urban heat island. The urban heat island is due to industrial and urban development, as energy is used to heat up buildings and asphalt rather than evaporate water from vegetation and soil. In addition, slower release of heat in the city allows it to be warmer than rural areas at night.

The heat island is strongest (1) at night when compensating sunlight is absent, (2) during the winter when nights are longer and there is more heat generated in the city, and (3) when the region is dominated by a high-pressure area with light winds, clear skies, and less humid air.

On clear nights when the heat island is pronounced, a small thermal low-pressure area may form over the city. As a result, a light breeze called a country breeze blows from the countryside into the city*, further concentrating pollutants, an event that is worsened if an inversion is present.

*This happens because wind blows from high pressure to low pressure, which we will cover later in Unit 16.

Air Pollution and Urban Environments

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Acid Rain

Air pollution emitted from industrial areas can be carried many kilometers downwind. These particles, which include sulfur and nitrogen oxides, can leave the atmosphere by settling to the ground in dry form (dry deposition) or by falling to the ground in the form of rain or snow (wet deposition). A notable example of wet deposition is

acid rain.

Acid rain forms when nitrogen & sulfur oxides turn into dilute drops of nitric acid (HNO₃) and sulfuric acid (H₂SO₄) during a complex series of reactions. When precipitation occurs, it carries the acids to the ground.

High concentrations of acid deposition can damage plants and water resources. Acid rain is known for acidifying lakes that house large fish populations, killing trees in forests, and damaging the foundations of many structures throughout the world.

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A Broader View of Air Pollution — Putting It All Together

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An Introduction to Earth’s Changing Climate

Over the last nineteen slides, we covered the cause and effect of air pollution on the planet we live on. Now we will cover a bigger topic: Earth’s climate and how its change will impact our future.

From the ice age 2.5 million years ago to the unusually warm mid-Holocene maximum 6,000 years ago, the climate has always been changing. And it will continue to change into the future, regardless of what we do.

However, a new, unique kind of climate change is now occurring, as the planet is warming due to a complete change of the atmospheric composition of the entire planet. Consequently, Arctic sea ice does not persist in summer to the extent it once did, ice sheets over Greenland and Antarctica are melting rapidly, and sea levels are rising worldwide. This result, as mentioned in Slide 113, is the result of anthropogenic, or man-made, climate change.

Nevertheless, natural causes still power much of the change that we experience on the planet. To finish this unit, we will look at both natural and human-related causes of climate change.

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Temperature Trends in the 20th Century

Between 1900 and 1945, the average temperature rose nearly 0.5°C, and from 1970 on, it rose even more dramatically. In fact, 1983 to 2012 was likely the warmest 30-year period of the last 1400 years (and the years 2013 to 2016 were even warmer).

The greatest warming has occurred in the Arctic and over the mid-latitude continents in winter and spring, while other areas, such as the Southeast United States, have not experienced much warming at all. Also, most of the warming globally has occurred at night.

Between 1880 and 2012, global temperatures have increased by about 0.85°C, or 1.5°F. This may not seem like a lot, but it actually is when the global perspective is taken into account. Over the past 10,000 years, global temperatures have not varied by more than 2°C.

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The Fundamental Questions

With this information, we must ask the following fundamental questions regarding climate change:

Is the warming trend over the past few years a result of natural variation or human activity?
Which is the bigger contributor to global warming: natural events or human activity?
Can climate change be stopped, and if so, what can be done to do so?

As of today, here are the answers:

It is the result of both; human activity is accompanied by natural occurrences to allow for and strengthen the warming trend over the past few years.
While natural events do contribute to the warming, the warming trend we have experienced over the past few decades is more of a result of the combustion of fossil fuels that produces greenhouse gases (like CO₂) that lead to an enhanced greenhouse effect.
The climate will always change, even if we humans leave the planet. However, the warming that is caused by human activity can be slowed in many ways, including through (1) driving less and carpooling or riding the bus, (2) weatherproofing the home to prevent heat loss, (3) replacing regular lightbulbs with fluorescents to lower CO₂ production, (4) cutting hot water use, (5) adjusting the thermostat to reduce power use,

(6) turning off unnecessary energy, (7) purchasing sources of renewable energy, (8) planting trees and

vegetation, (9) recycling and reusing, and (10) shopping smart for items that are reusable and/or recyclable.

Through these ways, we can all reduce our carbon footprints and contribute to a healthier planet.

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Why does Earth’s climate change?

Earth’s climate changes for three “external” reasons:

changes in incoming solar radiation
changes in the composition of the atmosphere
changes in Earth’s surface

Natural phenomena can cause the climate to change by all three reasons, while human activity can only cause the last two. In addition, “internal” factors that redistribute energy instead of altering the amount of it (such as ocean circulation patterns) also play a role.

For the natural causes of climate change, we will look at five important factors: (1) positive and negative feedback mechanisms,

(2) plate tectonics, (3) Earth’s orbit, (4) variations in solar output, and (5) atmospheric particles.

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Positive Feedback Mechanisms

As mentioned on Slide 127, the earth has a number of checks and balances called feedback mechanisms that regulate climate change. Positive feedback mechanisms enforce a process and, if left unchecked, may lead to a runaway greenhouse effect.

Examples of positive feedback include:

Water vapor-greenhouse feedback: the greenhouse effect of water vapor raises air temperature, which in turn causes more water to evaporate into the air. Thus, the temperature increases further, and more water evaporates. The cycle continues, strengthening the process of global warming.
Snow-albedo feedback: increases in temperature melt snow and ice, which decrease the albedo (reflectivity), causing more energy to reach the surface. Thus, temperatures increase further, melting more snow and ice, and the cycle repeats.

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Negative Feedback Mechanisms

As mentioned on Slide 127, the earth has a number of checks and balances called feedback mechanisms that regulate climate change. Negative feedback mechanisms weaken a process and, if left unchecked, may lead to a runaway ice age.

Examples of negative feedback include:

Infrared radiation feedback: as the planet warms, more infrared radiation is released. This slows temperature rise and helps to stabilize the climate.
Chemical weathering-CO₂ feedback: temperature increases cause more chemical weathering, which removes CO₂ from the atmosphere, consequently lowering temperature.
Cloud feedback: increase in temperature causes more evaporation, forming clouds that radiate away more energy and decrease temperatures.

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Positive vs. Negative Feedback

To summarize:

In a positive feedback system, the result strengthens further occurrences of the result.

Example: global warming causes rise in water vapor, which strengthens global warming.

In a negative feedback system, the result weakens further occurrences of the result.

Example: global warming causes more clouds, which weaken global warming.

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Plate Tectonics

The theory of plate tectonics states that Earth’s outer shell is composed of huge plates that slowly move. This can explain past climates, as the continents were one giant landmass (called Pangaea) millions of years ago.

Plate tectonics can affect climate in many ways. Two ways include:

Ocean Currents: the location of the continents can alter the path of ocean currents that transport heat from low to high latitudes and influence global wind systems and climate in high and

mid-latitude regions.

Mountains: the movements of the plates may form mountains, which may affect wind movement, the formation of clouds, and precipitation patterns. We will cover this in depth in a later unit.

*Note that many of these natural changes occur over large periods of time.

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Earth’s Orbit

The Milankovitch theory is an important theory that describes the impact that Earth’s orbit may have on climate. It focuses on three cyclic movements that combine to form variations in the amount of solar energy that falls on Earth.

The first of these cycles is the shape (or eccentricity) of the orbit. Every 100,000 years, the Earth’s orbit changes from elliptical to nearly circular — the greater the eccentricity, the greater the solar energy variation. In addition, the more eccentric orbit changes the length of seasons in each hemisphere by changing the length of time between the vernal and autumnal equinoxes.

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Earth’s Orbit

The second of these cycles is the wobble (or precession) of Earth as it rotates on its axis. This cycle occurs over a period of about 26,000 years. Currently, the earth is closer to the sun in January and farther from the sun in July (as mentioned in Slide 40). Because of precession, the reverse will be true 13,000 years from now, and the Northern Hemisphere will experience greater seasonal variation. In about 26,000 years, we will be back to where we are today.

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Earth’s Orbit

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Earth’s Orbit

The third of these cycles is the variation in tilt (or obliquity) of Earth, and this cycle occurs over a period of about 41,000 years. During this 41,000 year period, the earth’s tilt varies from 22½° to 24½° (it is currently approximately 23½°, as mentioned in Unit 1). The smaller the tilt, the less seasonal variation there is between summer and winter in middle and high latitudes; in other words, winter tends to be milder while summer tends to be cooler.

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Earth’s Orbit

In summary, the three Milankovitch cycles that combine to form variations in solar radiation at Earth’s surface are:

Eccentricity: changes in the shape of Earth’s orbit around the sun.
Precession: wobbling of Earth’s axis of rotation.
Obliquity: changes in the tilt of Earth’s axis.

*Do note that these three cycles take tens of thousands of years to complete (100,000 years, 26,000 years, and 41,000 years, respectively), so they cannot be used to fully explain the dramatic global temperature rise over the past few decades.

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Variations in Solar Output

The sun’s energy output (or brightness) varies by a fraction of one percent with sunspot activity. Sunspots are huge magnetic storms on the sun that show up as cooler (darker) regions on the sun’s surface. They occur in cycles, with the number and size reaching a maximum approximately every 11 years. During periods of maximum sunspots, the sun emits more energy than during periods of sunspot minimums. Fluctuations in solar output may account for small climatic changes over time scales of decades and centuries, but it may take some time before we can fully understand the relationship between solar activity and climate change on Earth.

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Atmospheric Particles

Microscopic liquid and solid particles (aerosols) that enter the atmosphere from both natural and human-induced sources can have an impact on climate. This effect is complex and depends on a number of factors, such as:

size
shape
color
chemical composition
vertical distribution above the surface

In the following few slides, we will cover the particles that are released into the atmosphere naturally.

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Atmospheric Particles

Particles near the surface can enter the atmosphere in many different ways, some of which include:

wildfires that release tiny smoke particles into the air, which are swept into the atmosphere by dust storms.
smoldering volcanoes that release significant quantities of sulfur-rich aerosols into the lower atmosphere.
tiny drifting aquatic plants (called phytoplankton) that produce a form of sulfur that slowly diffuses into the atmosphere and combines with oxygen to form sulfur dioxide, which in turn forms sulfate aerosols.

The overall effect of these surface particles is to cool the surface by preventing sunlight from reaching the surface.

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Atmospheric Particles

Volcanic eruptions play a big role in affecting the climate. During eruptions, fine particles of ash and dust enter the atmosphere.

The most impactful eruptions are those rich in sulfur gases, which combine with water vapor in the atmospheric to produce tiny, reflective sulfuric acid particles. When these particles grow in size, a dense layer of haze is formed, which may reside in the stratosphere for several years and reflect incoming energy, cooling the air at the earth’s surface. This is shown by the eruption of Mount Pinatubo in June 1991 — by mid-1992, global temperatures were 0.5°C (or 0.9°F) below average. In other words, volcanic eruptions rich in sulfur may be responsible for cooler periods of the geologic past.

Optional Read - The Year Without a Summer: http://goo.gl/VPYWRc

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Anthropogenic Climate Change

Now that we’ve looked at several natural forces that drive climate change, we must consider the unnatural forces — factors that are brought about by human activity.

Over the past few decades, as technology has become more advanced, we have begun to inject vast quantities of particles and greenhouse gases into the atmosphere without fully understanding their long-term impacts. However, this same growth in technological advances allows us to register and define our role in the environment we live in.

In the next few slides, we will cover the effects of human activity on the change in climate and connect everything together to the warming that we are experiencing today.

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Sulfate Aerosols

One of the most important human activity related aerosols (tiny solid and liquid particles that enter the atmosphere) is the highly reflective sulfate aerosol. The majority of sulfate aerosols in the atmosphere are related to human activities and come primarily from the combustion of sulfur-containing fossil fuels.

Sulfate aerosols reflect incoming sunlight, which tends to lower Earth’s surface temperature during the day. It also modifies clouds by increasing their reflectivity. This may explain why global temperatures did not rise much between the 1940s and 1970s, when sulfate pollution was common. In the 1980s and 1990s, when sulfate pollution decrease, global warming intensified.

So are sulfate aerosols good since they slow down the rate of global warming? Of course not! They are still unnatural forms of pollution that have the capacity to damage health and ecosystems.

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Greenhouse Gases

As mentioned before, greenhouse gases strongly absorb infrared radiation and play a major role in atmospheric warming. Everything else being equal, higher CO₂concentrations in the atmosphere are associated with higher surface temperatures.

Currently, the annual average concentration of CO₂ is 400 ppm, and this figure is rising by about 2 ppm per year. By the end of the century, atmospheric concentrations can be anywhere between 421 and 1313 ppm, all depending on how much CO₂ is emitted by human activity in the coming decades and how natural processes interact with the increase in atmospheric CO₂.

Other greenhouse gases, such as methane, nitrous oxide, and CFCs, all readily absorb infrared radiation as well, and they also play a big role in warming the planet. Fortunately, they are not as prevalent as CO₂ in the atmosphere — but regardless, they are still very dangerous.

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Land Use Changes

Modifications of Earth’s surface also contribute to changes in climate. For example, much of the rainfall in the Amazon River Basin is returned to the atmosphere through evaporation and transpiration (evaporation of water from leaves). However, the deforestation of the region will likely cause a decrease in evaporative cooling, leading to a warming in that area by several degrees Celsius.

In addition, the reflectivity of the deforested area will change as a result of overgrazing and excessive cultivation of grasslands. This causes an increase in desert conditions, known as desertification.

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Global Warming

Global Warming is real: each decade since the 1980s has been warmer than that of the previous decade. Perhaps this can be seen where you live: the growing season, for example, may be longer, and the leaves may be changing color later than normal.

It is important not to base global warming on a specific weather event. A cold spell may not be an indicator against warming, since temperatures are averaged globally.

To bring it all together, we must use everything we have learned in the past few slides to answer this question: is the warming trend we are currently experiencing mostly a result of greenhouse gases and an enhanced greenhouse effect? As we will cover in the next few slides, uncertainties do remain about the specifics, but the general consensus is that such a statement may in fact be true.

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Radiative Forcing Agents

As we learned in previous units, an atmosphere without any greenhouse gases would make the planet uninhabitable. The Earth-atmosphere system is usually in a state of radiative equilibrium, but factors that disrupt this equilibrium are known as radiative forcing agents. Common radiative forcing agents include an increase in greenhouse gases, changes in the sun’s energy input, and volcanic eruptions rich in sulfur.

While we do know that an increase in greenhouse gases in the atmosphere caused by human activity is a major contributor to global warming, do note that the interactions between Earth and its atmosphere are so complex that it is difficult to completely prove that Earth’s warming is due entirely to this enhanced greenhouse effect.

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So… what about the Ozone Hole?

Earlier in this unit, we covered the ozone hole that has developed over much of Antarctica. Does the ozone hole play a huge role in climate change? The answer, in fact, is no:

Ozone readily absorbs ultraviolet light, but the sun only emits a small fraction of its total energy as UV. There are too few UV waves to result in warming; most of the warming on our planet is caused of infrared radiation. Ozone, unlike water vapor and CO₂, is not good at absorbing infrared energy.
The main temperature related effect of ozone depletion is in the lower stratosphere, and not the troposphere, where the warming we know is taking place. This makes sense, given the ozone layer’s location in the atmosphere.

It’s important to note that the ozone hole is not really linked to the warming of the planet. It may raise our vulnerabilities to the dangers of UV radiation, but our planet’s warming is caused by other factors.

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The Many Components of a Climate Model

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Uncertainties about the Future — Greenhouse Gases

While projections do try to employ as many resources as possible to produce to most accurate result, there are many uncertainties that remain. Some of the uncertainties are listed below:

How will water and land affect increasing concentrations of CO₂? As of now, oceans and vegetation on land absorbs about 50% of the CO₂ emitted by human sources, but this varies year to year. In addition, studies have shown that warming of the planet reduces both ocean and land intake of CO₂… so, will global warming be strengthened as temperature increases?
How much will humans change the planet’s land in the future? Deforestation and other alterations in land use can impact the rate of warming of the planet.
What is the rate at which human activities will add greenhouse gases into the atmosphere in the future? Will society continue to emit large quantities of greenhouse gases into the air by burning fossil fuels? Or will the world transform its energy system toward renewables and save the atmosphere from tons and tons of excessive carbon dioxide? The path that the future will take is unpredictable and will result in completely different results in terms of global temperatures.

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Uncertainties about the Future — Clouds

As the atmosphere warms and more water vapor is added to the air, global cloudiness might increase as well. How would these clouds affect the climate?

Clouds reflect incoming sunlight back to space, which cools the climate, but they also emit infrared radiation to Earth, which tends to warm it. Just how the climate will change will depend on the types of clouds that form, their height above the surface, and their physical properties, such as water content, depth, and droplet size distribution.

For example, high, thin clouds (composed mostly of ice) appear to warm the climate, as they allow a lot of sunlight to pass through while absorbing more infrared radiation from Earth’s surface than they emit toward space. Low, stratified clouds, on the other hand, tend to cool the climate, as they reflect much of the sun’s energy and radiate to space much of the infrared energy they receive from Earth’s surface.

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Climate Change and the Human Impact

Uncertainties about the Future — Oceans

The oceans play a major role in Earth’s climate system, but they are not fully understood.

For example, the oceans have a large capacity for storing heat energy; in fact, more than 90% of energy trapped by greenhouse gases has gone into the oceans rather than the atmosphere. Only a slight change in the rate of oceanic heat storage can change the rate of warming. Variations in ocean circulation, for example, may explain why some decades warm faster than others.

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Climate Change and the Human Impact

What are the consequences of Climate Change?

If the world continues to warm at its current rate, what are some of the consequences?

Landmasses at higher latitudes will experience extremely fast warming.
Organisms that live in climate zones defined by temperature will be badly hit.
Organic matter will decompose at a faster rate, further adding CO₂ into the air.
Trees may become more susceptible to insects and disease.
Heat waves will become more frequent, and more heat-related deaths will result.
Precipitation will increase over middle and high latitude land areas and decrease over parts of the subtropics. This will add stress to agriculture.
Severe floods and droughts will become more common.
The melting of glaciers and ice sheets will result in sea level rise. Today’s improved climate models estimate that global sea level will rise by an addition 40-60 cm (16-24 in.) or more by the end of the 21st century. This, of course, depends on how fast ice in Greenland and Antarctica melts.

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Climate Change and the Human Impact

Conclusions of the 2013 IPCC Report

Following are some conclusions about global warming and its future impact on our climate system, summarized from the 2013 Fifth Assessment Report of the Intergovernmental Panel on Climate Change, or IPCC. This summary comes from the textbook Meteorology Today by C. Donald Ahrens.

Point 1:

The primary source of the increased atmospheric concentration of carbon dioxide since the pre-industrial period results from fossil fuel use, with land-use change providing another significant but smaller contribution. The atmospheric concentration of carbon dioxide exceeds by far the natural range over the last 800,000 years (180 to 300 ppm) as determined from ice cores.

Point 2:

Almost the entire globe experienced surface warming during the period 1901 to 1912. Average Northern Hemisphere temperatures during the period from 1983 to 2012 were very likely higher than during any other 30-year period in at least the past 1400 years.

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Climate Change and the Human Impact

Conclusions of the 2013 IPCC Report

Point 3:

It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by anthropogenic factors, including the increase in greenhouse gas concentrations. Temperatures of the most extreme hot days, hot nights, cold nights, and cold days are all very likely to have increased since 1950 due to anthropogenic forcing. It is likely that anthropogenic forcing has increased the risk of heat waves.

Point 4:

The frequency and intensity of heavy precipitation events have likely increased in North America and Europe. In other continents, there are larger uncertainties due to limited observations.

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Conclusions of the 2013 IPCC Report

Point 5:

There is high confidence that ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90 percent of the energy accumulated between 1971 and 2010. It is virtually certain that the upper ocean (0-700 m) warmed from 1971 to 2010. Such warming seawater to expand, contributing to sea level rise.

Point 6:

It is very likely that the mean rate of global averaged sea level rise was 1.7 [1.5 to 1.9] millimeters/year between 1901 and 2010, 2.0 [1.7 to 2.3] millimeters/year between 1971 and 2010, and 3.2 [2.8 to 3.6] millimeters/year between 1993 and 2010. The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia.

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Climate Change and the Human Impact

Conclusions of the 2013 IPCC Report

Point 7:

Over the last two decades, there is high confidence that the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent.

Point 8:

Global mean surface temperature by 2016–2035 will likely be 0.3°C to 0.7°C warmer than in 1986–2005. There is more uncertainty in the amount of warming by 2081–2100 compared to 1986–2005; the amount will likely fall between 0.3°C and 4.8°C, depending on changes in greenhouse gas emissions and other factors.

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Climate Change and the Human Impact

Conclusions of the 2013 IPCC Report

Point 9:

Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent by the end of this century.

Point 10:

Global mean sea level will continue to rise during the 21^st century. The rate of sea level rise will very likely exceed that observed during 1971 to 2010 due to increased ocean warming and increased loss of mass from glaciers and ice sheets. Sea level rise will not be uniform: about 30 percent of coastlines will experience at least 20 percent greater or lesser change than the global average. Sea level rise due to thermal expansion of the ocean is expected to continue for many centuries.

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Climate Change and the Human Impact

Curbing the Impact of Climate Change

The most obvious way to reduce the rate of climate change is to lessen our dependency on fossil fuels. Increasing the use of alternative energy sources could also help the cause — growing technologies such as solar and wind power can produce almost no greenhouse gases while producing the energy currently obtained from oil and coal.

A new idea called geoengineering works to decrease the impact of climate change by (1) removing greenhouse gases from the atmosphere or by (2) changing the amount of sunlight that reaches Earth. This study proposes many unique ideas, from fertilizing the oceans with plants that absorb CO₂ to placing reflecting mirrors above the atmosphere of Earth. To read more about geoengineering, visit this link: http://goo.gl/wTyFRI

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Climate Change and the Human Impact

A Summary of the Unit

In Unit 3, we covered how energy is connected to the Earth. And in this unit, we added to that by explaining how this energy balance can be extremely damaging if disrupted. From air pollution to anthropogenic climate change, humans may be responsible for the drastic warming that the planet has experienced over the past few decades.

Climate change, however, is not the end all of life on this planet. As mentioned on Slides 149 and 182, there is much that can be done to curb the potential damages of our planet’s warming. Cutting down on greenhouse gas emissions will do much to benefit society, as it could slow down the enhancement of Earth’s greenhouse effect and reduce global warming. At the same time, lowering pollution can reduce acid rain, diminish haze, slow the production of photochemical smog, and produce significant health benefits.

Sure, climate change models only predict the future to the best of its ability, and the final results may not always match the expected. However, even if the greenhouse warming proves to be less than what is projected, these measures will certainly benefit society and the future of humanity.

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Weather Instruments

and Measurement

“One accurate measurement is

worth a thousand expert opinions.”

― Grace Murray Hopper, Navy Admiral

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Weather Instruments and Measurement

An Intro to Weather Instruments

Weather instruments play a vital role in the world of meteorology. Every single measurement that we associate with a weather forecast would not be possible without a weather instrument to help determine its value. In fact, without the help of weather instruments, our current level of forecasting would be insanely difficult, if not impossible, to attain.

Imagine a world without thermometers, barometers, or radar. We wouldn’t be able to give temperature a value, nor would we be able to quantify air pressure. Precipitation and rainfall intensity would be impossible to visualize. It is for this reason that weather instruments help make the very topic of meteorology possible to comprehend.

In this unit, we will explore 30 different weather instruments that contribute to the reliability and accuracy of weather forecasting.

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Weather Instruments and Measurement

Actinometer

An actinometer is an instrument used to measure the heating power of solar radiation. They are primarily used in meteorology to measure solar radiation as transmitted directly by the sun, scattered by the atmosphere, or reflected by the earth.

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Weather Instruments and Measurement

Aerometer

An aerometer is an instrument used to measure the weight and density of gases.

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Weather Instruments and Measurement

Aerovane

An aerovane, or skyvane, is an instrument used to indicate both wind speed and direction. It consists of a bladed propeller that rotates at a rate proportional to the wind speed. Its streamlined shape and a vertical fin keep the blades facing into the wind. When attached to a recorder, a continuous record of both wind speed and direction is obtained. NOTE: This is not the same as an aerometer!

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Weather Instruments and Measurement

Anemometer

An anemometer is an instrument used to measure wind speed. The cups on the anemometer catch the wind, turning a dial attached to the instrument that measures wind speed.

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Weather Instruments and Measurement

Barometer

A barometer is a weather instrument that measures air pressure. There are two types of barometers: mercury barometers and aneroid barometers. Mercury barometers use a glass tube filled with mercury to measure air pressure. Aneroid barometers use an aneroid cell.

Mercury Barometer Aneroid Barometer

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Weather Instruments and Measurement

Barometer: Mercury Barometer

The mercury barometer was invented by Evangelista Torricelli in 1643. His barometer, similar to those in use today, consisted of a long glass tube open at one end and closed at the other. Air pressure pushes down on the surface of the mercury, making some of it rise up the tube. The greater the air pressure, the higher the mercury rises.

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Weather Instruments and Measurement

Barometer: Aneroid Barometer

The aneroid barometer is the more common form of barometer, and it contains no fluid. Instead, this instrument contains a small, flexible metal box called an aneroid cell. Air is partially removed before the cell is sealed, so that small changes in air pressure causes the cell to expand and contract. Any change is amplified by levers that control an arm to point to the correct air pressure.

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Weather Instruments and Measurement

Barograph

A barograph also measures air pressure, but it also contains a pen that allows it to document a continuous record of barometric air pressure changes. The record is noted on chart paper, which is attached to a drum that is rotated by an internal mechanical clock.

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Weather Instruments and Measurement

Ceilometer

A ceilometer is an instrument that measures the height of cloud bases. The term ceiling is used to describe the height of the lowest layer of clouds above the surface. Most ceilometers used today are laser-beam ceilometers that send intense pulses of infrared radiation into the sky and use the time it takes for the beams to return after getting reflected to calculate the height of the cloud base.

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Weather Instruments and Measurement

Compass

A compass is an instrument that is used to measure direction. In meteorology, it plays a major role in measuring wind direction.

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Weather Instruments and Measurement

Dew Cell

A dew cell is an instrument that is used to determine dew point temperature. It determines the amount of water vapor in the air by measuring the air’s actual vapor pressure.

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Weather Instruments and Measurement

Doppler Radar

Doppler radar is used to measure precipitation intensity. It uses two types of images: reflectivity images to measure the intensity of the precipitation and velocity images to measure the speed of the precipitation as it moves toward or away from the radar.

How does it work? First, radar sends out short pulses of microwaves, which are reflected back when they come into contact with a foreign object (called a target). The amount of time it takes for the waves to return to the transmitter is used to calculate the distance. The image formed by the returning signal is known as the echo.

The more waves are returned to the transmitter, the more intense the precipitation. The radar image typically uses different colors to indicate the strength of precipitation, with light green or blue representing light precipitation and red or pink representing heavy precipitation.

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Weather Instruments and Measurement

Doppler Radar

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Weather Instruments and Measurement

Disdrometer

A disdrometer is used to measure the size, speed, and number of raindrops during precipitation. Some disdrometers can distinguish between rain, graupel, and hail.

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Weather Instruments and Measurement

Evaporimeter

An evaporimeter (also known as an atmometer) is an instrument that measures the rate of evaporation of water into the atmosphere.

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Weather Instruments and Measurement

Hygrometer

A hygrometer is an instrument that measures relative humidity. Several types of hygrometers exist. Hair hygrometers use human or horse hair to measure relative humidity, with strands of hair attached to different levers that transfer changes to a dial that displays the correct humidity (the length of human hair increases by 2.5% as relative humidity increase from 0 to 100 percent. Electrical hygrometers use electric currents to measure relative humidity, while infrared hygrometers do so by measuring the amount of infrared energy absorbed by water vapor in a sample of air.

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Weather Instruments and Measurement

Lidar

Lidar, a device similar to radar, sends out light and uses the time of reflection to measure the movement of atmospheric particles. In meteorology, lidar is used to perform a range of measurements that include profiling clouds, measuring winds, studying aerosols and quantifying various atmospheric components.

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Weather Instruments and Measurement

Nephelometer

A nephelometer is an instrument that measures the suspension of small particles in a liquid or a gas. They are often used for measuring air quality and determining visibility.

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Weather Instruments and Measurement

Psychrometer

A psychrometer is a weather instrument that measures relative humidity. It consists of two liquid-in-glass thermometers mounted side by side and attached to a piece of metal that has either a handle or chain at one end. Two types of psychrometers are sling psychrometers and aspirated psychrometers.

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Weather Instruments and Measurement

Pyranometer

A pyranometer is an instrument that measures solar irradiance. It is a form of an actinometer that focuses on the solar energy at the surface of the planet. They can often be found at solar farms.

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Weather Instruments and Measurement

Radiometer

A radiometer measures the intensity of radiant energy (usually infrared) to provide temperature readings at selected levels in the atmosphere.

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Weather Instruments and Measurement

Radiosonde

A radiosonde is a small, lightweight box equipped with weather instruments and a radio transmitter. It is attached to a cord that has a parachute and a gas-filled balloon tied tightly at the end. As the balloon rises, the attached radiosonde measures air temperature, pressure, and humidity. A dropsonde is a form of radiosonde that is dropped from an aircraft, usually into a storm. As the instrument descends, it measures and relays data on temperature, pressure, and humidity back to the aircraft. It also obtains data on wind speed and wind direction.

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Weather Instruments and Measurement

Rawinsonde

A rawinsonde is a radiosonde that is designed to only measure wind speed and direction. A radiosonde observation that includes wind data is a rawinsonde observation.

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Weather Instruments and Measurement

Rain Gauge

A rain gauge (or ombrometer) is an instrument designed to measure the amount of rain that falls during a given time interval. A standard rain gauge consists of a funnel-shaped collector attached to a long measuring tube. A tipping bucket rain gauge has a receiving funnel leading to two small metal collectors (buckets). A weighing-type rain gauge contains a cylinder that catches precipitation and accumulates it into a bucket.

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Weather Instruments and Measurement

Snow Gauge

A snow gauge is an instrument that measures the water equivalent of the amount of snowfall.

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Weather Instruments and Measurement

Solar Radiation Shield

Solar radiation shields are used by weather stations to protect temperature and relative humidity sensors. They can also be called Stevenson screens or instrument shelters depending on structure. Solar radiation shields look like caps, while instrument shelters are more box-like.

In addition, instrument shelters serve as a shady place for thermometers. This allows for a more accurate reading: thermometers inside shelters measure the temperature of the air, while thermometers held in direct sunlight do not.

solar radiation shield

instrument shelter

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Weather Instruments and Measurement

Thermometer

A thermometer is an instrument that measures temperature. When it is heated, the alcohol (older thermometers contain mercury, but they were discontinued because mercury was toxic) expands, moving up the tube to indicate the correct temperature. The thermometer most often used to measure surface air temperature is the liquid-in-glass thermometer; they are inexpensive and easy to read. Maximum and minimum thermometers are liquid-in-glass thermometers that are used to determine daily maximum and minimum temperatures.

A more accurate thermometer is the electrical thermometer, which uses electrical resistance to calculate air temperature. Bimetallic thermometers, consisting of two different pieces of metal welded together to form a single strip, are also used to measure temperature.

liquid-in-glass thermometer electrical thermometer bimetallic thermometer

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Weather Instruments and Measurement

Thermograph

A thermograph is an instrument that measures & records temperature. They are similar to barographs.

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Weather Instruments and Measurement

Weather Balloons

Weather balloons are used to carry radiosondes into the air to measure upper air conditions. A typical weather balloon is usually made of rubber and has a weight of about 200 grams. The balloon is filled usually with helium or hydrogen and can reach heights of about 20-30 km (stratosphere) before it bursts. The probe then returns with a parachute to the ground.

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Weather Instruments and Measurement

Weather Maps

Weather maps indicate atmospheric conditions above a large portion of Earth’s surface. Meteorologists often use these maps for weather forecasting.

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Weather Instruments and Measurement

Weather Satellites

Weather satellites are often used by meteorologists to photograph and track large scale air movements. Two very important types of satellites are the Geostationary Operational Environmental Satellite (GOES) and Polar Operational Environmental Satellite (POES).

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Weather Instruments and Measurement

Geostationary Operational Environmental Satellite (GOES)

GOES satellites, located at about 35,800 km (22,300 miles) above the earth’s surface, move at the speed of Earth’s rotation and monitor the same regions on the planet at all times. Their high altitude allows them to view a full-disk image of the earth. Because they stay fixed above a surface, they are often used to detect atmospheric triggers for severe weather conditions such as tornadoes, flash floods, hail storms, and hurricanes. When these conditions develop, the GOES satellites are able to monitor storm development and track their movements. Hurricane images are often taken by GOES satellites.

GOES satellites are also used to estimate rainfall totals during hurricanes and thunderstorms (used for administering flash flood warnings) and snowfall totals and extent of snow cover during winter storms (used for administering winter storm warnings and spring snow melt advisories.

How are satellites named? A letter is assigned to a satellite before it is launched (for example, GOES-P), and a number is assigned once it is in orbit (e.g. GOES-15). A satellite without a number designation is one that did not make it into orbit.

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Weather Instruments and Measurement

GOES-15 and GOES-13 (and soon to be GOES-16)

Two of the most important GOES satellites are GOES-15 (GOES-West) and GOES-13 (GOES-East). Each satellite views about ⅓ of the planet’s surface; GOES-15 is located over 135°W at the equator, while GOES-13 is located over 75°W at the equator. When put together, both satellites contribute to the monitoring of the entire Western Hemisphere. (As of 2016, a new and improved satellite, GOES-16, is present to measure weather conditions in the Western Hemisphere. It will take the place of GOES-13/East at the end of 2017).

A backup satellite, GOES-14, is put in place in case one of the main GOES satellites fails. This backup is kept closer to GOES-13, as a failure of the eastern GOES would be more dangerous to the United States since it monitors hurricanes approaching the US and weather conditions in the densely populated east coast.

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Weather Instruments and Measurement

Polar Operational Environmental Satellite (POES)

POES satellites, unlike GOES satellites, are lower in altitude (around 800 km above the surface) and do not monitor the same location at all times. Instead, they revolve around the earth, passing over the North and South Pole with each revolution. Since the earth is rotating, the satellite will pass over a different longitude each time it goes around the earth. Thus, it takes several hours before a POES will travel over the same area (the exception is at the poles, as POES satellites will always travel over there — because of this, satellite data for polar areas will be frequently updated).

Since POES is closer to the earth than GOES, its pictures are of a better resolution. In addition, POES satellites have many different functions, including weather analysis and forecasting, cloud imaging, climate research and prediction, global sea surface temperature measurement, and ozone levels in the atmosphere (POES satellites were able to detect the ozone hole over Antarctica).

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Weather Instruments and Measurement

Wind Sock

A wind sock is a fabric cone designed to indicate the direction and approximate speed of the wind.

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Weather Instruments and Measurement

Wind Vane

A wind vane measures the direction of the wind.

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Temperature

and Conversions

“It doesn’t matter what temperature a

room is… it’s always room temperature.”

― Steven Wright, Comedian

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Temperature and Conversions

What is Temperature?

Temperature is a degree of hotness or coldness that can be measured with a thermometer. It is measured through how fast atoms and molecules of a substance are moving.

Two forms of temperature are wind chill and the heat index. Wind chill is the temperature your body feels when air temperature is combined with wind speed. The heat index is the temperature your body feels when air temperature is combined with humidity.

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Wind Chill and Excessive Heat Alerts

Wind chill advisories are issued when the wind chill is expected to be between -15°F and -24°F.
Wind chill warnings are issued when the wind chill is expected to be below -25°F.
Heat advisories are issued when the heat index is expected to be between 105°F and 115°F for less than three hours.
Excessive heat watches are issued when the heat index is expected to be above 115°F.
Excessive heat warnings are issued when the heat index is expected to be above 115°F for more than three hours each day for at least two consecutive days.

Temperature and Conversions

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Units of Temperature

The three units of temperature are Fahrenheit, Celsius, and Kelvin. The Kelvin scale is the one most used in science, as it begins at absolute zero, the coldest temperature possible before atoms stop moving. It is named after Lord Kelvin, the British scientist who first introduced it.

The Fahrenheit and Celsius scales are more commonly used in real life. The Fahrenheit scale is named after G. Daniel Fahrenheit, who assigned 32 to water’s freezing point and 212 to water’s boiling point. The Celsius scale, introduced later, assigns 0 to water’s freezing and 100 to boiling.

Temperature and Conversions

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Conversion: Fahrenheit to Celsius

To convert Fahrenheit to Celsius, use this formula (plug in temperature for T_°F).

(T_°F - 32) x (5/9) = T_°C

or in other words, subtract 32, divide by 9, and multiply by 5.

Example: convert 86°F to Celsius -

subtract 32 ⇨ 86 - 32 = 54
divide by 9 ⇨ 54 ÷ 9 = 6
multiply by 5 ⇨ 6 x 5 = 30
therefore, 86°F = 30°C

Temperature and Conversions

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Conversion: Celsius to Fahrenheit

To convert Celsius to Fahrenheit, use this formula (plug in temperature for T_°C).

(9/5) x (T_°C) + 32 = T_°F

or in other words, divide by 5, multiply by 9, and add 32.

Example: convert 30°C to Fahrenheit -

divide by 5 ⇨ 30 ÷ 5 = 6
multiply by 9 ⇨ 6 x 9 = 54
add 32 ⇨ 54 + 32 = 86
therefore, 30°C = 86°F

Temperature and Conversions

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Conversion: Celsius to Kelvin

To convert Celsius to Kelvin, simply add 273 (or 273.15 for more accuracy).

Example: convert 30°C to Kelvin -

add 273 ⇨ 30 + 273 = 303
therefore, 30°C = 303 K

Temperature and Conversions

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Conversion: Kelvin to Celsius

To convert Kelvin to Celsius, simply subtract 273 (or 273.15 for more accuracy).

Example: convert 303 K to Celsius -

subtract 273 ⇨ 303 - 273 = 30
therefore, 303 K = 30°C

Temperature and Conversions

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Water’s Freezing and Boiling Points

Temperature and Conversions

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The warmest part of the day usually occurs in the afternoon. Around noon, the sun’s rays are most intense. However, even though incoming solar radiation decreases in intensity after noon, it still exceeds the outgoing energy for a time. Thus, there is an energy surplus two to four hours after noon, contributing to a higher temperature.

The exact time of the highest temperature varies. When it is clear, the maximum temperature usually occurs between 3:00 and 5:00 PM. However, when there are clouds or haze, the maximum temperature usually occurs a few hours earlier. Near large bodies of water, cool air moving inland may change the temperature to cause the warmest temperatures to occur at noon or even before.

Where the air is humid, haze and cloudiness lower maximum temperatures by preventing some of the sun’s rays from reaching the ground.

Daytime Heating

Temperature and Conversions

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As the sun sets, its energy is spread over a larger area, which reduces the heat available to heat the surface. As the earth’s surface and air begins to lose more energy than it gains, they start to cool.

Both the ground and air cool by releasing infrared energy, a process called radiational cooling.

Nighttime Cooling

Temperature Records

The highest temperature ever recorded in the United States was a 134°F (57°C) measurement in Death Valley, California on July 10, 1913.

The lowest temperature ever recorded in the United States was a -80°F (-62°C) reading at Prospect Creek, Alaska on January 23, 1971. In the world, this record is a -129°F (-89°F) reading in Antarctica on July 21, 1983. The biggest swing in temperature occurred on January 15, 1972 in Loma, Montana, when the temperature rose from -54°F to 49°F.

Temperature and Conversions

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The main factors that cause variations in temperature from one place to another are called the controls of temperature. The main controls of temperature are

latitude
land and water distribution
ocean currents
elevation

On average, temperatures decrease poleward from the tropics and subtropics in both January and July. However, a bigger change is experienced in January (tighter gradient).

In the winter, temperatures are lower in the middle of continents than in regions near large bodies of water. The opposite is true during the summer. The cooling properties of land and water cause regions inland to experience warmer temperatures during the summer & cooler temperatures during the winter compared to regions near water.

The Controls of Temperature

Temperature and Conversions

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The greatest difference between the daily maximum temperature and the daily minimum temperature, called the daily range of temperature, is greatest next to the ground and becomes progressively smaller as we move farther from the surface. This daily range is more apparent on clear days than on cloudy ones. The largest daily range occurs in high deserts with low cloud cover and low CO₂ amounts in the air.

Clouds help regulate air temperature by keeping daytime temperatures lower during the day and nighttime temperatures higher during the night. In addition, humidity may also have an effect on temperature, as humid regions have lower daily ranges of temperature due to the fact that moist air prevents some of the sun’s energy from reaching the surface and by absorbing the earth’s infrared radiation and radiating a part of it to the ground.

Cities near water have smaller daily ranges of temperature (also called diurnal ranges) because water warms and cools slower than land.

Air Temperature Data

Temperature and Conversions

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The average of the highest and lowest temperature for a 24-hour period is known as the mean (average) daily temperature. The average of the mean daily temperatures for a particular date averaged over a 30-year period gives the average temperatures for that date. The average temperature for each month is the average of the daily mean temperatures for that month.

The difference between the average temperature of the warmest and coldest months is called the annual range of temperature. Usually the largest annual ranges occur over land, the smallest over water. In addition, inland cities have larger annual ranges than coastal cities. Near the equator, annual temperature ranges are small, but the farther you go, the more the temperature varies.

The average temperature of any station for the entire year is the mean (average) annual temperature, which represents the average of the twelve monthly average temperatures. However, two locations with the same mean annual temperatures may have different temperature patterns throughout the year.

Air Temperature Data

Temperature and Conversions

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“Books have the same enemies as people: fire, humidity, animals, weather, and their own content.”

― Paul Valéry, French Poet

Atmospheric Humidity

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Atmospheric Humidity

Humidity may refer to any one of the number of ways of specifying the amount of water vapor in the air. Because there are several ways to express atmospheric water vapor content, there are several meanings for the concept of humidity. The first type is absolute humidity. Absolute humidity is the mass of water vapor in a given volume of air, and can be expressed as

Absolute humidity = mass of water vapor ÷ volume of air

Absolute humidity is normally expressed as grams of water vapor in a cubic meter of air. For example, if the water vapor in 1 cubic meter of air weighs 25 grams, the absolute humidity of the air is 25 grams per cubic meter (25 g/m³). This measure of humidity is not often used in atmospheric studies because it changes when volume changes, even when the vapor content remains the same. Note that absolute humidity does not take temperature into consideration.

Absolute Humidity

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Atmospheric Humidity

Humidity can, however, be expressed in ways that do not change with volume. When the mass of the water vapor in the air in a parcel is compared with the mass of all the air in the parcel (including vapor), the result is called the specific humidity:

Specific Humidity = mass of water vapor ÷ total mass of air

You can also express humidity by comparing the mass of water vapor in a parcel to the mass of remaining dry air. Humidity expressed in this manner is called the mixing ratio:

Mixing Ratio = mass of water vapor ÷ mass of dry air

Both specific humidity and mixing ratio are expressed as grams of water vapor per kilogram of air (g/kg). They remain constant as long as water vapor is not added to or removed from the parcel.

Specific Humidity and the Mixing Ratio

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Atmospheric Humidity

If we want to report the moisture content of the air around us, we have several options:

Absolute humidity tells us the mass of water vapor in a fixed volume of air, or the water vapor density.
Specific humidity measures the mass of water vapor in a fixed total mass of air, and the mixing ratio describes the mass of water vapor in a fixed mass of the remaining dry air.
The actual vapor pressure of air expresses the amount of water vapor in terms of the amount of pressure that the water vapor molecules exert.
The saturation vapor pressure is the pressure that the water vapor molecules would exert if the air were saturated with vapor at a given temperature.

Reporting Moisture Content

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Atmospheric Humidity

The most common way to describe atmospheric moisture is relative humidity. Instead of measuring the amount of water vapor in the air, relative humidity tells us how close the air is to being saturated. In other words, relative humidity is the ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at that particular temperature (and pressure). It is the ratio of the air’s water vapor content to its capacity.

Relative Humidity = water vapor content ÷ water vapor capacity

Relative Humidity

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Atmospheric Humidity

Relative humidity is given as a percent. Air with a 50% relative humidity contains one-half the amount required for saturation. Air with a 100% relative humidity is said to be saturated because it is filled to capacity with water vapor. Air with a relative humidity greater than 100% is said to be supersaturated — this rarely occurs, however, and even when it does, the relative humidity of the atmosphere seldom exceeds 101%.

A change in relative humidity can happen in two primary ways:

by changing the air’s water vapor content - as more and more water vapor molecules are added to the air, the air approaches saturation, and relative humidity increases.
by changing the air temperature - as temperature increases, saturation becomes less likely due to high molecular speeds, and relative humidity decreases.

Relative Humidity

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Atmospheric Humidity

The dew point is the temperature to which air would have to be cooled, with no change in air pressure or moisture content, for saturation (100% relative humidity) to occur.

For example, if the air is filled to 100% capacity (saturation) at 10°C, the dew point would be 10°C as long as the amount of water vapor in the air does not change.

The dew point is an important measurement used in predicting the formation of dew, frost, fog, and even the minimum temperature. In addition, it is a good indicator of the air’s actual water vapor content. High dew points indicate high water vapor content, while low dew points indicate low water vapor content (the more water vapor there is in the air, the less you have to decrease temperature for the air to be completely saturated to capacity). Addition of water vapor to the air increases the dew point; removing water vapor lowers it. (put graph 4.12 on quiz)

Dew Point

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Atmospheric Humidity

In the wintertime, dew points are highest over the Gulf and lowest over the interior plains. This makes sense, as moist air from the Gulf keeps the water vapor levels in the Gulf states high, while cold, dry winds from Canada keep the water vapor levels of the plains low.

In the summertime, the Gulf states still have the highest dew points, but states in the central and eastern portions of the country have higher dew points than in winter. This is because, in the summertime, these states constantly receive humid air flowing up from the Gulf of Mexico. The lowest dew point (the driest air) can be found in the West, where mountains effectively shield the region from surrounding moisture.

The difference between air temperature and dew point can indicate whether the relative humidity is low or high. When the air temperature and dew point are far apart, the relative humidity is low. When the air temperature and dew point are equal, the air is saturated and the relative humidity is 100%. Because relative humidity does not measure the amount of water vapor in the air, but rather how saturated the air is, an area with 100% relative humidity may still contain less water vapor than an area with a lower relative humidity.

Dew Point

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Atmospheric Humidity

Relative humidity varies from the equator to the poles. The highest relative humidities can be found over the tropics and the poles, while relative humidities near latitude 30° are low (that is where many of the earth’s deserts are found).

While locations near water tend to be more humid than locations inland, there are still many differences in humidity among coastal states. For example, New Orleans, Louisiana (near the Gulf) contains 50% more water vapor in the air than along Southern California. Why is this the case?

The Pacific water is much cooler than the Gulf water. Because the winds blowing across the water usually match the temperature of the water they are over, winds blowing across the Pacific are cooler than winds blowing across the Gulf. As air over both regions is saturated with water vapor, the dew point temperature of the air over the cooler Pacific Ocean is much lower than the dew point temperature of the air over the warmer Gulf. Consequently, air from the Gulf contains much more water vapor. This makes sense, as high temperatures and high dew points over the Gulf Coast produce higher relative humidities, while high temperatures and low dew points over the Western states produce lower relative humidities.

Why might two coastal states have differing humidities?

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Atmospheric Humidity

In warm weather, the main source of body cooling is through the evaporation of sweat. This process, however, is slower on more humid days. When humidity is low, sweat evaporates quicker, giving us the sensation that the temperature is cooler than it actually is. When humidity is high, on the other hand, body moisture does not readily evaporate and instead collects on the skin. This makes us feel muggy, and makes the temperature seem warmer than it actually is.

The wet-bulb temperature is the lowest temperature that can be reached by evaporating water into the air (this is different from the dew point in that it evaporates water into the air instead of cooling it). The larger the difference between the wet-bulb temperature and the actual temperature, the easier it is for perspiration to evaporate at the skin’s surface. When the wet-bulb temperature exceeds the skin’s temperature, no net evaporation occurs, and body temperature rises rapidly. Fortunately, this rarely happens.

Humidity and Comfort

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Atmospheric Humidity

As mentioned in Unit 6, temperature is combined with humidity to give us an apparent temperature, or what the air temperature feels like. This is known as the heat index.

On hot and muggy days, many problems may arise. They include heat cramps, which is caused by an imbalance of salts and water in the body, heat exhaustion, which is caused by excessive water loss and may lead to fatigue, headache, nausea, and fainting, and heatstroke, which results in a total failure of the circulatory system when temperatures rise above 41°C (106°F).

Humidity-Related Problems

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Atmospheric Humidity

No. At the same temperature and at the same level, humid air weighs less than dry air.

If you were to weigh a given volume of completely dry air, it would weigh slightly more than the same volume of water vapor. If we replace dry air molecules with water vapor molecules, the total weight decreases. Since density is mass per unit volume, hot humid air at the surface is less dense (lighter) than hot dry air.

This has an important influence on our weather. The lighter the air is, the more likely it is to rise. Thus, humid air will rise more readily than dry air, causing water vapor to condense and fall back to earth as precipitation.

Is Humid Air Heavier Than Dry Air?

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“We forget that the water cycle

and the life cycle are one.”

― Jacques Yves Cousteau, Undersea Explorer

The Water Cycle

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The Water Cycle

The circulation of water in the atmosphere plays a big role in maintaining life on Earth. There are many different ways in which water can transform from one phase to another, and these transformations form the basis of the earth’s water cycle.

Because the oceans occupy over 70% of the earth’s surface, much of the water in the atmosphere comes from this common source. Thus, scientists often label the oceans as the beginning of the water cycle’s circulation.

Here, the sun’s energy turns enormous quantities of liquid water into water vapor in a process called evaporation. Winds then transport the moist air to other regions, where the water vapor changes back into liquid, a process called condensation. These condensed particles of liquid water fall back to the ground when they get heavy, a process known as precipitation. This cycle of moving and transforming water molecules from liquid to vapor and back again is known as the hydrologic cycle, or the water cycle. If the precipitation falls back into the ocean, the water cycle gets to repeat all over again.

An Introduction to the Water Cycle

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The Water Cycle

The water cycle, however, is not a perfect entity that follows the same path every single time. There are many complexities to this cycle, many of which involve mechanisms that interfere with water’s movement. For example, falling raindrops may evaporate before hitting the ground, and the droplets that make it to the ground may be intercepted by vegetation before making it into the ocean.

What if the water does not hit the ocean when it hits the earth as precipitation? There are many possibilities where the ocean is not the receiving source; if that is the case, the water will continue on its journey through many other processes that occur on land. When moisture is carried through plants from roots to small pores on the underside of leaves, a process called transpiration (or evapotranspiration) occurs. In this situation, the water changes into vapor when it enters the atmosphere after leaving the plant.

Although evaporation from plants (transpiration) and continental areas plays a big role in the amount of water vapor in the atmosphere, it makes up only 15% of the water vapor that is evaporated into the air every year. The other 85% comes from the oceans.

Other Modes of Water Movement

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The Water Cycle

What if the precipitation hits the ground instead of a body of water? At this moment, many things may occur. The first of which is infiltration, or the process by which rainwater soaks into the ground through soil and underlying rock layers. Rain that does not infiltrate or evaporate is known as runoff and undergoes collection, the process by which water that falls from the clouds as rain, snow, hail, or sleet collects in the oceans, rivers, lakes, and streams. Eventually, the runoff drains into streams and rivers, which find their way back to the ocean.

Water on Land

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The Water Cycle

Although we’ve mainly focused on the processes of evaporation (when water turns from liquid to vapor) and condensation (when water turns from vapor to a liquid), water can change phases by completely skipping the liquid state. When this occurs, sublimation or deposition occurs.

Sublimation occurs when solid ice turns into water vapor without entering the liquid phase. This happens when an occasional water molecule gains enough energy to break away from the surrounding molecules and enter the air above.

Deposition is just the opposite. This transformation occurs when water vapor turns into solid ice without entering the liquid phase.

The Many Phases of Water

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The Water Cycle

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Cloud Types

and Formation

“The clouds — the only

birds that never sleep.”

― Victor Hugo, French Poet

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Cloud Types and Formation

In Unit 7, we talked about the dew point, the temperature at which saturation occurs. When surfaces such as twigs, leaves, and blades of grass cool below this temperature, water vapor begins to condense upon them, forming tiny visible specks of water called dew. If the air temperature drops below freezing, the dew will freeze and become tiny beads of ice called frozen dew.

Dew is more likely to form on nights that are clear and calm than on nights that are cloudy and windy. These atmospheric conditions are usually associated with large fair-weather, high-pressure systems.

The cloudy, windy weather that prevents rapid cooling near the ground and the forming of dew often signals the approach of a rain-producing storm system.

Dew Formation

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Cloud Types and Formation

On cold, clear, calm mornings, when the dew-point is at or below freezing, water vapor can change directly into ice without becoming liquid first. Recall from Unit 8 that this process is called deposition. The white crystals that result are called frost.

Remember from Unit 7 that if air cools without any change in water vapor content, the relative humidity increases. If air cools to the dew point, the relative humidity becomes 100%, and the air is saturated. If the air continues to cool beyond the dew point, some of the vapor will condense into tiny cloud droplets. We will cover the development of clouds in further depth later on in this unit.

Frost Formation

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Cloud Types and Formation

Both dew and frost require a surface to form on, as water vapor needs something to attach to before it condenses. The same goes for the formation of clouds.

Although the air looks clean, it is actually not. Every day, a volume of air about the size of your index finger contains between 1,000 and 150,000 particles. These particles play an important role in the formation of clouds, as they provide the surface for water vapor to condense on. Because of this, these particles are called condensation nuclei. Without condensation nuclei, relative humidities of several hundred percent would be needed before condensation would occur.

Condensation nuclei are relatively light, so they can remain suspended over the air for many days. They are most abundant over industrial cities and decrease over cleaner country and over the oceans.

Condensation Nuclei

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Cloud Types and Formation

There are two main types of condensation nuclei: hygroscopic and hygrophobic.

Hygroscopic (water-seeking) particles easily condense water vapor, typically when the relative humidity is lower than 100%. A notable example is table salt; that is why it is harder to pour salt on a humid day, as water vapor easily condenses onto the salt crystals and sticks them together.

Hydrophobic (water-repelling) particles resist condensation even when relative humidity is over 100%. Major examples of hydrophobic nuclei include oils, gasoline, and wax.

Because there are usually many nuclei present in the atmosphere at any point, haze, fog, and clouds will form at relative humidities near or below 100%.

Types of Condensation Nuclei

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Cloud Types and Formation

As relative humidity gradually approaches 100%, visibility decreases, and the landscape becomes masked with a swath of gray. When the visibility lowers to less than 1 km (0.62 mi), and the air is wet with countless millions of tiny floating water droplets, a cloud on the ground called fog is formed.

With the same water content, fog that forms in dirty city air is often thicker than fog that forms over the ocean. Over the ocean, the smaller number of condensation nuclei results in fewer, but larger, fog droplets. In the city, abundant nuclei particles allow for the fog to become very thick.

Fog that forms in polluted air can turn acidic as tiny water droplets combine with chemicals such as sulfur and nitrogen oxides. Acid fog is very dangerous, especially for people with respiratory problems.

Fog

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Cloud Types and Formation

Fog, like any cloud, forms in one of two ways:

by cooling - air is cooled below its saturation point (dew point).
by evaporation and mixing - water vapor is added to the air by evaporation, and the moist air mixes with relatively dry air.

With these in mind, we will now look at the many different types of fog that can form.

Fog Formation

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Cloud Types and Formation

Radiation and conduction are the primary means for cooling nighttime air near the ground. Fog produced by the earth’s radiational cooling is called radiation fog, or ground fog. It forms best on clear nights when a shallow layer of moist air near the ground is covered by drier air. The moist air is cooled rapidly by colder ground and quickly becomes saturated, allowing fog to form. Because longer nights lead to longer cooling, and thus a larger chance of fog formation, radiation fogs are most common in late fall and winter.

A light breeze of less than 5 knots also helps the formation of radiation fog, as it brings more of the moist air in contact with the cold ground (although stronger winds are not helpful because they mix the moist air on the ground with drier air above). Since clear skies and light winds are associated with high-pressure systems, radiation fog is associated with this type of pressure.

Since cold, heavy air drains downhill and collects in low-lying areas, radiation fog is frequently called valley fog. Radiation fogs form upward from the ground as the night progresses and are usually deepest around sunrise. As the day goes on, much of the fog will dissipate as the sun’s energy evaporates it from the surface.

Radiation Fog

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Cloud Types and Formation

Advection fog is fog that forms when warm moist air is cooled to saturation over a cold surface. The surface must be sufficiently cooler than the air above so that the transfer of heat from air to surface will cool the air to its dew point and produce fog.

A good example of advection fog can be observed along the Pacific Coast during the summer. Warm moist air from the Pacific Ocean is carried by westerly winds over the colder coastal waters, cools to the dew point, and forms advection fog.

Unlike radiation fog, advection fog always involves a movement of air.

Advection Fog

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Cloud Types and Formation

Fog that forms as moist air flows up along an elevated plain, hill, or mountain is called upslope fog. Typically, upslope fog forms during the winter and spring on the eastern side of the Rockies, where the eastward-sloping plains are nearly a kilometer higher than the land further east.

Occasionally, cold air moves from the lower eastern plains westward. The air gradually rises, expands, becomes cooler, and — if sufficiently moist — a fog forms. Upslope fogs that form over an extensive area may last for many days.

Upslope Fog

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Cloud Types and Formation

Fog that forms by the mixing of two unsaturated masses of air is known as evaporation (mixing) fog because evaporation initially enriches the air with water vapor.

A common form of evaporation-mixing fog is steam fog, which forms when cold air moves over warm water. It is common to see steam fog forming over lakes on autumn mornings, as cold air settles over water still warm from the long summer. On occasion, over the Great Lakes, columns of condensed vapor rise from the fog layer, forming whirling steam devils, which appear similar to the dust devils on land. Over the ocean in polar regions, steam fog is referred to as arctic sea smoke.

A warm rain falling through a layer of cold moist air can produce fog. Fog of this type usually develops in the shallow layer of cold air just ahead of an approaching warm front or behind a cold front, which is why this type of evaporation fog is also known as precipitation fog, or frontal fog. Snow covering the ground is an especially favorable condition for the formation of frontal fog.

Evaporation (Mixing) Fog

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Cloud Types and Formation

A cloud is a visible aggregate of tiny water droplets or ice crystals suspended in the air. Clouds were first identified and classified in the early 19th century.

Luke Howard, an English naturalist, developed a cloud classification system that forms the basis of the system used today. He named a sheet like cloud stratus, a puffy cloud cumulus, and a wispy cloud cirrus.

Today, there are four major cloud groups, each containing their own group of clouds. The high clouds include cirrus clouds, cirrostratus clouds, and cirrocumulus clouds. The middle clouds include altostratus clouds and altocumulus clouds. The low clouds include stratus, stratocumulus, and nimbostratus clouds. The clouds with vertical development include the cumulus and cumulonimbus clouds.

Thus, in other words, each cloud was first categorized by its altitude, and then specified into a further grouping depending on its appearance.

Clouds

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Cloud Types and Formation

High clouds in middle and low latitudes generally form above 6000 m (20,000 ft). Because the air at these high altitudes is cold and dry, high clouds are composed exclusively of ice crystals. In addition, high clouds tend to be rather thin.

There are three main types of high clouds: cirrus, cirrocumulus, and cirrostratus.

The most common high clouds are cirrus clouds, thin, wispy clouds blown by high winds. Their long streamers are also called mares’ tails. Cirrus clouds usually move across the sky from west to east, and generally point to fair, pleasant weather.

Cirrocumulus clouds are small, rounded, white puffs that may occur individually or in long rows. Because the ripples that it forms in the sky resemble the scales of a fish, the term mackerel sky often describes a sky full of cirrocumulus clouds. These clouds indicate fair but cold weather.

Cirrostratus clouds are thin, sheetlike, high clouds that often cover the entire sky; they are so thin that the sun and moon can be seen through them. The ice crystals in these clouds bend light passing through them and will often produce a halo around the sun or moon. These clouds frequently form ahead of an advancing storm, and can be used to predict rain or snow within 12 to 24 hours, especially if followed by middle level clouds.

High Clouds

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Cloud Types and Formation

High Clouds

cirrus cirrocumulus cirrostratus

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Cloud Types and Formation

Middle clouds have bases between 2,000 and 7,000 m (6500 to 23,000 ft) in the middle latitudes. These clouds are composed of water droplets and some ice crystals. This group of clouds includes altocumulus and altostratus clouds.

Altocumulus clouds are middle clouds that are composed mostly of water droplets and are rarely more than 1 km thick. They appear as gray, puffy masses, sometimes rolled out in parallel waves or bands. The individual puffs of altocumulus appear larger than those of cirrocumulus clouds. The appearance of these clouds on a warm, humid summer morning often precede thunderstorms by late afternoon.

Altostratus clouds are gray or blue-gray clouds composed of ice crystals and water droplets. Their gray color, height, and dimness of the sun are good clues to identifying these clouds. Altostratus clouds often form ahead of storms that have widespread and relatively continuous precipitation.

Middle Clouds

Altocumulus (left) and altostratus (right) clouds

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Cloud Types and Formation

Low clouds, with their bases lying below 2000 m (6500 ft), are almost always composed of water droplets.

Nimbostratus clouds are dark gray clouds associated with continuously falling rain or snow. The intensity of this precipitation is usually light or moderate.

Stratocumulus clouds are low, lumpy clouds that appear in rows, patches, or rounded masses with the blue sky visible between the individual cloud elements. Often they appear near sunset as the spreading remains of a much larger cumulus cloud. Occasionally, the sun will shine through the cloud breaks producing bands of light called crepuscular rays that appear to reach down to the ground.

Stratus clouds are uniform grayish clouds that covers the entire sky. It resembles a fog that does not reach the ground. Normally, no precipitation falls from the stratus, but sometimes it is accompanied by a light mist or drizzle. This cloud commonly occurs over Pacific and Atlantic coastal waters in summer.

Low Clouds

Nimbostratus (left)

Stratocumulus (middle)

Stratus (right)

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Cloud Types and Formation

The puffy cumulus cloud looks like a piece of floating cotton with sharp outlines and a flat base. The base appears white to light gray, and on a humid day, may be only 1000 m (3300 ft) above the ground and a kilometer or so wide. Unlike stratocumulus clouds, cumulus clouds are detached instead of in groups or patches. Cumulus clouds that show only slight vertical growth are called cumulus humilis and are associated with fair weather. On warm summer mornings, however, cumulus clouds often develop into larger and more vertically developed towering cumulus clouds, or cumulus congestus. These clouds may result in showery precipitation.

If the cumulus congestus continues to grow vertically, it develops into a cumulonimbus cloud, or thunderstorm cloud. While this cloud’s base may be no more than 600 m above the earth’s surface, its top may extend upward to the tropopause, over 12,000 m (39000 ft) higher. The lower (warmer) part of the cloud is usually composed of only water droplets, while ice crystals dominate higher regions of the cloud. These anvil-like clouds may contain all forms of precipitation, including large raindrops, snowflakes, snow pellets, and even hail.

Vertically-Developing Clouds

Cumulus humilis (left)

Cumulus congestus (center)

Cumulonimbus (right)

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Cloud Types and Formation

In addition to these main clouds, there are also several other clouds that deserve mention. These are as follows.

Lenticular clouds are formed when moist air crosses a mountain barrier, and are shaped like flying saucers. They frequently form on the downward side of mountains. They often appear motionless to the viewer.
Banner clouds are formed when a cloud forms over and extends downwind of an isolated mountain peak.
Mammatus clouds frequently form under cumulonimbus clouds when air sinks. They are baglike sacs that hang beneath the cloud and resemble a cow’s udder. For mammatus clouds to form, the sinking air must be cooler than the air around it & have a high liquid water or ice content. They usually indicate severe weather.
Asperatus clouds are dark, wavy clouds that often dissipate without a storm forming. They occur most frequently in the Plains during the morning or midday hours following convective thunderstorm activity.
Condensation trails, or contrails, are cirrus-like trails produced by jet aircraft flying at high altitudes.

From left to right: lenticular, banner, mammatus, asperatus, contrails.

Some Unusual Clouds

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Cloud Types and Formation

In addition to these main clouds, there are also several other clouds that deserve mention. These are as follows.

Fractus clouds are small, ragged cloud fragments that are usually found under an ambient cloud base. They form or break off from a larger cloud, and are generally sheared by strong winds, giving them a jagged, shredded appearance. Fractus clouds have irregular patterns, appearing much like torn pieces of cotton candy. They change constantly, often forming and dissipating rapidly. They do not have clearly defined bases. Sometimes they are persistent and form very near the surface.
Nacreous clouds, or mother-of-pearl clouds, are soft, pearly clouds that form in the stratosphere at altitudes above 30 km. They are best viewed in polar latitudes during the winter months when the sun is able to illuminate them because of their high altitude.
Noctilucent clouds are wavy bluish-white clouds that may be seen in the upper mesosphere at altitudes above 75 km. To an observer, they appear bright against a dark background.

From left to right: fractus, nacreous, noctilucent.

Some Unusual Clouds

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Cloud Types and Formation

In meteorology, descriptions of sky conditions are defined by the fraction of sky covered by clouds.

A clear sky is one where no clouds are present.
When there are between one-eighth and two-eighths clouds covering the sky, there are few clouds present.
When cloudiness increases to between three-eighths and four-eighths, the sky is described as scattered. This sky can also be called partly cloudy.
Clouds covering between five-eighths and seven-eighths of the sky denote a sky with broken clouds. This sky can also be called mostly cloudy.
When a sky is completely covered by clouds, it is considered overcast.

Because open spaces between clouds are less visible at a distance, cloudiness is usually overestimated when clouds are near the horizon.

Cloud Observations

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Cloud Types and Formation

The following prefixes are used to identify clouds:

“Cirro” = high clouds (e.g. cirrostratus, cirrocumulus)
“Alto” = middle clouds (e.g. altostratus, altocumulus)
“Strato” = low clouds (e.g. stratus, stratocumulus)
“Cumulo” = vertically growing clouds (e.g. cumulus, cumulonimbus)
“Nimbo” = rain clouds (e.g. nimbostratus, cumulonimbus)

Prefixes

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Cloud Types and Formation

Sometimes, you may see streaks of precipitation under clouds that do not seem to touch the ground. These observable streaks or shafts of precipitation that fall from a cloud but evaporate or sublime before reaching the ground are called virga.

Virga

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Cloud Types and Formation

To Summarize...

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Cloud Types and Formation

Atmospheric Stability

We determine the stability of air by comparing the temperature of a rising parcel (a small volume of air) to that of its surroundings. If the rising air is colder than its environment, it will be more dense (heavier) and will tend to sink back to its original level. In this case, the air is stable because it resists upward movement.

If the rising air is warmer, it is less dense (lighter) than the surrounding air, and it will continue to rise until it reaches the same temperature as its environment. This is an example of unstable air. To figure out the air’s stability, we need to measure the temperature both of the rising air and of its environment at various levels above the earth.

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Cloud Types and Formation

Cloud Development

How do clouds form? All air contains water, but near the ground it is usually in the form of an invisible gas called water vapor. When warm air rises, it expands and cools. Cool air can’t hold as much water vapor as warm air, so some of the vapor condenses onto tiny pieces of dust that are floating in the air and forms a tiny droplet around each dust particle. When billions of these droplets come together, they become a visible cloud.

Why are clouds white? Clouds are white because their water droplets or ice crystals are large enough to scatter the light of the seven wavelengths (red, orange, yellow, green, blue, indigo, and violet), which combine to produce white light.

Why do clouds turn gray? If the clouds get thick enough or high enough that all the light above does not make it through, they attain a gray or dark colored look. Also, if there are lots of other clouds around, their shadows can add to the gray or multicolored gray appearance.

Why do clouds float? A clouds forms when air is heated by the sun. As it rises, it slowly cools. When it reaches the saturation point, water condenses, forming a cloud. As long as the cloud and the air that it’s made of is warmer than the outside air around it, the cloud floats.

How do clouds move? Clouds move with the wind. High cirrus clouds are pushed by the jet stream, which travels at speeds over 100 miles per hour. When clouds are part of a thunderstorm, they usually move at speeds of 30 to 40 mph.

Which factors affect cloud height? The factors that affect cloud height are: (1) amount of water vapor, (2) temperature, (3) wind, and (4) air masses.

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Cloud Types and Formation

Topography and Clouds

One of the most important concepts in meteorology is the effect of mountains on cloud formation. Because horizontally moving air cannot move through mountains, it must move over it. The forced lifting that results is known as orographic lift.

When large masses of air approach long mountain chains like the Sierra Nevada or the Rockies, they often rise. As a result, they cool and condense, forming clouds. Clouds formed in this manner are called orographic clouds. Because of this phenomenon, the side of the mountain facing the wind (windward side) often experiences precipitation.

The opposite is true of the other side of the mountain, the leeward side. As the air moves downhill, it warms. Consequently, this sinking air is much drier, since much of its moisture was removed as precipitation on the windward side. Due to the absence of precipitation on the leeward side, this region is known as the rain shadow.

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Cloud Types and Formation

Changing Cloud Forms

Under certain conditions, a layer of altostratus may change into altocumulus. This happens if the top of the cloud deck cools while the bottom warms, as differing rates of absorption and emission of infrared radiation may lead to small convection cells within the cloud itself. The same process may happen with cirrocumulus and stratocumulus clouds.

When the wind is fairly uniform throughout a cloud layer, new cloud elements appear evenly distributed across the sky. However, if wind speed or direction changes with height, the new clouds become arranged in rows and are known as cloud streets. When the changes in wind speed and direction reach a critical value, and an inversion caps the cloud-forming layers, wavelike clouds called billows may form along the top of the cloud layer.

Occasionally, altocumulus show vertical development and produce tower-like extensions. The clouds often resemble floating castles and, for this reason, they are called altocumulus castellanus. When altocumulus castellanus clouds appear, they indicate that the mid-level of the troposphere is becoming more unstable, and that showers and thunderstorms may be likely in the near future.

When surface heating increases on a warm, humid summer day, a stratocumulus layer may change into a sky dotted with growing cumulus clouds.

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Cloud Types and Formation

As we learned in this unit, clouds — especially low, thick ones — are great emitters of infrared radiation. Their tops radiate infrared energy upward and their bases radiate energy back to the earth’s surface where it is absorbed and, in a sense, radiated back to the clouds. This process keeps calm, cloudy nights warmer than calm, clear ones. If the clouds remain into the next day, they prevent much of the sunlight from reaching the ground by reflecting it back to space. Since the ground does not heat up as much as it would in full sunshine, cloudy, calm days are normally cooler than clear, calm days.

In other words, clouds moderate temperature throughout the days and nights, preventing temperature from going to either the high or low extremes. During the day, clouds help keep the daytime temperatures cool, while during the nights, clouds help keep the nighttime temperatures warm.

Clouds and Temperature Regulation

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Light and Optics

“And when it rains on your parade,

look up rather than look down. Without

the rain, there would be no rainbow.”

― Gilbert K. Chesterton, English Novelist

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Light and Optics

Every time we look up toward the sky, we see the effects of light and optics. In clear weather, the sky can appear blue while the horizon appears as a milky white; during a glowing sunset, the sky can be streaked with reds and pinks. Yet at night time, the colors of the sky recede and get replaced with a black that shines no light other than the stars that are scattered about.

Before a rainstorm, we often see the clouds change color, from a harmless white to a menacing dark gray. However, as the rain pulls back, the sky returns to its normal appearance, this time accompanied with a vibrant display of color — a rainbow.

In this unit, we will analyze the effect of light and optics on the very world we live on and how these phenomena connect to the everyday weather that we experience.

Introduction to Optics

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Light and Optics

Ever wonder why the clouds in the sky have a unique ability to change color at the blink of an eye? To answer this question, we must look into the concepts of scattering and reflection.

When sunlight bounces off a surface at the same angle at which it strikes the surface, we say that the light is reflected. However, when it is deflected in all directions, it is considered to be scattered.

In the atmosphere, scattering is usually caused by small objects, such as air molecules, fine particles of dust, water molecules, and some pollutants.

Cloud droplets are large enough to scatter vast amounts of sunlight, resulting in a situation where most light is scattered — hence, when we look at a cloud, it appears white because countless cloud droplets scatter all wavelengths of visible sunlight in all directions. However, as the cloud grows larger and taller, less light can penetrate all the way through, resulting in lower levels of scattering. This gives taller clouds a darker appearance. (In addition, larger droplets near the cloud base become better absorbers, absorbing visible light and adding to their dark color. This is why dark clouds often signal an approaching storm.)

Scattering and Reflection

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Radiation and Cloud Thickness

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Green clouds are often used to symbolize the arrival of dangerous thunderstorms and, quite frequently, tornadoes, from weather safety tips to fictional survival stories. Is there a truth behind these clouds, and if so, why are they green?

The existence of green clouds cannot be denied, as they are fairly frequent over the Great Plains during the wrath of a severe thunderstorm. But why do they appear green? Science says that the green color may be due to reddish sunlight (especially at sunset) penetrating the storm and then being scattered by cloud particles composed of water and ice. With much of the red light removed, the scattered light casts the underside of the cloud as a faint greenish hue.

So next time when you see a green cloud, know that scattering is the culprit behind its color. And don’t forget to dive for cover, as the arrival of a green cloud may signal the approach of a dangerous storm system!

Green Clouds

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The age old question: why is the sky blue? The answer to this question is a process known as Rayleigh scattering, named after the British Physicist Lord Rayleigh. Rayleigh scattering occurs when light scatters off of molecules in the air such as oxygen and nitrogen. These air molecules are selective scatterers because each scatters shorter waves of visible light more effectively than longer waves. Because the colors of violet, blue, and green have shorter visible wavelengths, they are scattered more by air molecules. Our eyes are more responsive to blue light; thus, the waves that are scattered produce the blue color that we associate with the sky.

The following table shows the various types of scattering resulting from visible light.

Rayleigh Scattering

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Rayleigh scattering only occurs when the particles that scatter light are so small that they selectively scatter visible light with shorter wavelengths. But what if small particles, such as dust or salt, become suspended in the atmosphere — what happens then? These particles are small, but they are not as small as nitrogen and oxygen molecules!

In this case, these particles are able to scatter all colors in the visible spectrum, not just blue! Because of this, the sky appears milky white and hazy. Thus, the more particles in the air, the more scattering, and the whiter the sky becomes!

A phenomenon called crepuscular rays, as shown in the photo to the left, may occur when haze or dust particles scatter sunlight. This usually happens during a rising or setting sun, or when the sun shines through a break in the clouds.

Scattering by Larger Particles

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At noon, the sun appears white. However, at sunset, it may appear to have a shade of yellow, orange, or red. Why is this the case?

This is because, at noon, light from the sun is most intense, and all visible wavelengths of light are able to reach the eye with equal intensity, resulting in the white color. However, during sunrise or sunset, the rays coming from the sun strike the atmosphere at a low angle. Thus, they have to travel a longer distance in the atmosphere than at any other time during the day. By the time the light finishes its journey through the atmosphere, many of the shorter wavelengths have been scattered away. This leaves behind the longer wavelengths of red, orange, and yellow that ultimately produce the effects of a warm sunrise and sunset.

Red Sunsets

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But what factor may play a role in the distinct hue of the sun? The amount of particles in the atmosphere!

When the atmosphere is fairly clean, the sun tends to be yellow at sunset. But as the number of particles increases, so does the scattering of wavelengths. If the air contains many fine particles, yellow light would be scattered as well, giving the sun an orange-reddish hue. Raise the number of atmospheric particles, and longer wavelengths would incrementally be scattered away. If the atmosphere becomes loaded with particles, only the longest red wavelengths would be able to make it through the atmosphere unscattered, giving the sun a reddish look.

Red Sunsets

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Light and Optics

Light that passes through a substance is said to be transmitted. Upon entering a denser substance, transmitted light slows in speed. If it enters at an angle, the light’s path also bends. This bending is called refraction. As shown in the diagram below, light that travels from a less-dense to a more-dense medium loses speed and bends toward the normal, while light that enters a less-dense medium increases in speed and bends away from the normal. The normal is a line perpendicular to the boundary between two different mediums.

Refraction

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The refraction of light may produce many visual effects. One such effect is the bending of starlight as it enters the earth’s atmosphere. Because the starlight bends as it enters the more-dense atmosphere, the star appears higher than it actually is! We cannot perceive the bending of the path that the starlight travels along. Thus, if you see a star in the horizon, the position that you see it in the night sky is actually higher than its actual position!

In addition, the path that starlight travels through is not uniform; each region along the path may have slightly different densities. As a result, the apparent position of the star constantly changes as each region deflects the light’s path by a differing amount. The resulting twinkling or flickering of the star is known as scintillation.

Bending of Starlight

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Bending of Light by Refraction

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The bending phenomenon of refraction can also be applied to the sun and moon. Since light is bent most at the horizon, the sun and moon both appear to be higher than they really are! Because of this, they both rise about two minutes earlier and set about two minutes later than they would otherwise if there were no atmosphere.

In addition, you may have noticed that on clear days the sky is often bright for a period of time even after the sun has set. This period of time is known as twilight, when the sky is still illuminated after the sunset. The length of twilight depends on season and latitude. In general, summer twilights in the middle latitudes add 30 minutes of light to each morning and evening. The duration of twilight increases with increasing latitude especially in summer. At high latitudes during the summer, morning and evening twilight may converge into one giant nightlong twilight: this is known as a white night. Without an atmosphere, the sun would rise later and set earlier, and twilight would not exist! Darkness would arrive immediately once the sun set below the horizon!

Twilight

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Take a look at the photo below. Is the ground wet?

This image was adapted from C. Donald Ahrens’s textbook Meteorology Today.

Please see the next slide for the answer!

Mirages

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Light and Optics

The road in the previous slide may seem wet, but in actuality, the wetness is merely a mirage! The road only appears wet because blue skylight is bending up into the camera as the light passes through air of different densities.

In the atmosphere, when an object appears to be displaced from its true position, we call this phenomenon a mirage. This occurs when the atmosphere plays tricks on us! Atmospheric mirages are created by light passing through air of different densities. Usually these differences in density are caused by sharp changes in air temperature. The greater the rate of temperature change, the greater light rays are bent. For example, you may see the image on the previous slide on a hot summer day, as the road surfaces are often hotter than the air above it.

When the air near the ground is much warmer than the air above, objects appear lower than they really are and are often inverted. These mirages are called inferior mirages. On the other hand, when air near the ground is much colder than the air above, objects may appear higher than they really are. This phenomenon is known as a superior mirage.

Mirages

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Mirages

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A ring of light encircling and extending outward from the sun or moon is called a halo. This is produced when sunlight or moonlight is refracted as it passes through ice crystals. The presence of a halo indicates that cirriform clouds (cirrostratus or cirrocumulus) are present.

If the sun is near the horizon in such a configuration that it, ice crystals, and the observer are all in the same horizontal plane, the observer will see a pair of brightly colored spots on either side of the sun. These spots are called sundogs (or parhelia), and they are shown in the cover image on Slide 282.

Halos and sundogs are caused by the refraction of sunlight through ice crystals. When sunlight is instead reflected off ice crystals, a phenomenon called sun pillars may appear. These appear most often at sunrise or sunset as a vertical shaft of light extending vertically from the sun.

Halos, Sundogs, and Sun Pillars

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Rainbows occur when rain is falling in one part of the sky and the sun is shining in another. To see the rainbow, we must face the falling rain with the sun at our backs.

When we look at a rainbow we are looking at sunlight that has entered the falling drops and has been redirected back toward our eyes. As sunlight enters a raindrop, it slows and bends, with violet light refracting the most and red light the least. Most of this light passes right through the raindrop, but a portion hits the drop at a critical angle (48° for water) that allows it to be reflected toward our eyes. The angle at which these light rays emerge from the drop determines its color. When this process occurs with multiple raindrops at the same time, we are able to see the colors of a primary rainbow.

Diagrams of this process is shown on the next slide.

Rainbows

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Rainbows

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Sometimes, a larger secondary rainbow with its colors reversed can be seen above the primary rainbow. This second rainbow tends to a bit fainter than the primary one.

Double rainbows occur when sunlight enters the raindrops at an angle that allows light to make two internal reflections in each drop. The color reversal is due to the way the light emerges from each drop after going through two internal reflections.

Keep in mind that only one ray of light is able to enter your eye from each raindrop. Every time you move, light from different raindrops enters your eye. Thus, the rainbow you see is not the same as the one the person next to you is seeing! We all have our own personal “rainbows” to enjoy after a rainstorm!

Double Rainbows

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Light and Optics

When the moon is seen through a thin veil of clouds composed of tiny water droplets, a bright ring of light called a corona may appear to rest on the moon. This effect is made possible by the process of diffraction, the bending of light as it passes around objects.

Another diffraction phenomenon is known as the glory. A glory is a set of rings that appear around the shadow of an aircraft that flies above a cloud layer composed of water droplets less than 50 μm in diameter. For the glory to be seen, the sun must be behind your back.

Lastly, if you were to face a field of grass filled with dew on a clear morning, you may see a bright area around the shadow of your head. This is known as the Heiligenschein, which forms when sunlight is reflected back toward the sun along nearly the same path it took originally upon hitting a spherical dew drop. The light, however, does not travel along the same path. It actually spreads out just enough to be seen as bright white light around the shadow of your head on the lawn.

These three phenomena are shown to the right.

Coronas, Glories, and Heiligenschein

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“I get my best ideas in a thunderstorm. I have the power and majesty of nature on my side.”

― Ralph Steadman, British Artist

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Atmospheric Stability

When we talk about atmospheric stability, we are referring to a condition of equilibrium, or stability. Let’s use the rock and the hill below to explain this concept.

Rock A below is resting at the bottom of two hills. If this rock were to be pushed up along either side of the surrounding hills and then let go, it would quickly return to its original position. This situation, where an object tends to return to its original position after being disturbed, is known as stable equilibrium.

On the other hand, Rock B is resting at the top of the hill. If this rock were to be pushed in either direction, it would quickly move away from its original position. This situation, where an object departs farther from its original position after being disturbed, is known as unstable equilibrium.

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Atmospheric Stability

We can thus apply these principles to the atmosphere:

Air is in stable equilibrium when, after being lifted or lowered, it tends to return to its original position, resisting upward and downward air motions.
Air is in unstable equilibrium when a small push will cause it to move farther away from its original position. This type of air favors vertical air currents.

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Adiabatic Process

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At the earth’s surface, a parcel of air (a small volume of air) has the same temperature and pressure as the air surrounding it. If this parcel is lifted up into the atmosphere, the air pressure around it would decrease. Because of this, air molecules inside the parcel push the parcel walls outward, and the parcel expands. This expansion comes at the cost of the molecules’ own energy, so this results in lower average molecular speeds and thus a lower temperature.

If the parcel is instead lowered in the atmosphere, pressure increases. This increased pressure compresses the parcel into a smaller volume, which increases the average speed of the air and thus its temperature.

To summarize: a rising parcel of air expands and cools, while a sinking parcel is compressed and warms.

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Adiabatic Process

Rain and Thunderstorms

An adiabatic process occurs when a parcel of air expands and cools (or compresses and warms) with not interchange of heat with its surroundings. As long as the parcel is unsaturated (relative humidity less than 100%), the rate of adiabatic cooling and warming remains constant. This rate of heating/cooling is about 10°C for every 1000 m (1 km) change in elevation (5.5°F per 1000 ft), and is known as the dry adiabatic rate. Remember: the dry adiabatic rate is only valid when the air in a parcel is unsaturated!

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Adiabatic Process

As the rising parcel of air cools, its relativity humidity increases. When the rising air cools to the dew point temperature, its relative humidity reaches 100% — further lifting in this scenario would result in condensation, which releases latent heat inside the parcel.

Because the heat added during condensation offsets some of the cooling due to expansion, the air no longer cools at the dry adiabatic rate but rather at a lesser rate called the moist adiabatic rate.

Because the evaporation of liquid droplets offsets the rate of compressional warming, the moist adiabatic rate (the rate at which rising or sinking saturated air changes temperature) is less than the dry adiabatic rate! This difference is greatest when the rising air is warm, but closer together when the rising air is very cold.

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Adiabatic Process

Unlike the dry adiabatic rate, the moist adiabatic rate varies with temperature and moisture content, as warm saturated air produces more liquid water than cold saturated air. A table of moist adiabatic rates for different temperatures and pressures is shown below.

To make numbers easy to deal with, an average moist adiabatic rate of 6°C per 1000 m (3.3°F per 1000 ft) can be used as an approximation in most examples and calculations.

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Reading an Adiabatic Chart (Problem 9.96)

Rain and Thunderstorms

Unit 9 Problem Sheet Question 96, Revisited:

Suppose that you wanted to calculate the dry adiabatic rate of the air given a mountain 3 km tall with an environmental temperature of 10°C at its base. How would you do this? You could use a meteorological tool called an adiabatic chart, as shown to the right. This chart is also available in the appendix on this slide.

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Reading an Adiabatic Chart (Problem 9.96)

Unit 9 Problem Sheet Question 96, Revisited:

First, you are given that the temperature of the environment at a height of 0 km (the mountain’s base) is 10°C. Locate 10°C on the adiabatic chart, as shown in the figure.

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Reading an Adiabatic Chart (Problem 9.96)

Unit 9 Problem Sheet Question 96, Revisited:

You may have noticed that there are two lines coming out of the 10°C position on the chart: a solid line and a dotted line. Look on the chart to see what each of these lines indicate. As shown by the orange circle, the solid line represents the dry adiabatic rate and the dotted line represents the moist adiabatic rate.

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Reading an Adiabatic Chart (Problem 9.96)

Unit 9 Problem Sheet Question 96, Revisited:

Now let’s identify the other measurement we were given: the mountain has a height of 3 km. Locate this point on the adiabatic chart. This is done with a green circle, as shown.

Notice that there is a dotted line coming out from the right side of the graph, at 3 km.

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Reading an Adiabatic Chart (Problem 9.96)

Unit 9 Problem Sheet Question 96, Revisited:

Since you are looking for the dry adiabatic rate, move up the solid line starting at 10°C. In addition, since you want to find the temperature using the dry rate at a height of 3 km, move left from the dotted line starting at 3 km. This is shown with the arrows on the diagram.

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Reading an Adiabatic Chart (Problem 9.96)

Unit 9 Problem Sheet Question 96, Revisited:

Find the point where these two lines intersect, as marked with a star. Drop down to the temperature axis from this point to identify the temperature of the air parcel at 3 km using the dry adiabatic rate. (This does not mean that this calculated temperature is the same as the environmental temperature at this altitude.)

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Reading an Adiabatic Chart (Problem 9.96)

Unit 9 Problem Sheet Question 96, Revisited:

Continuing from Slide 315, we find that the temperature at 3 km using the dry adiabatic rate is -20°C. We can then calculate the rate as

dry adiabatic rate (DAR) = change in temp.

change in altitude

This is equal to (-20 - 10)°C / (3 km - 0 km), or a rate of 10°C per km. (This agrees with what we expected!)

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Reading an Adiabatic Chart (Problem 9.96)

Unit 9 Problem Sheet Question 96, Revisited:

What if we wanted to calculate the moist adiabatic rate? This is simple: just follow the same procedure detailed in the previous six, but use the dotted line instead of solid one!

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Reading an Adiabatic Chart (Problem 9.96)

Unit 9 Problem Sheet Question 96, Revisited:

As shown by the red arrow, the dotted line is used instead of the solid one, and the intersection point is found. Dropping this down to the temperature axis gives us an approximate temperature of -8°C.

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Reading an Adiabatic Chart (Problem 9.96)

Unit 9 Problem Sheet Question 96, Revisited:

Using the formula below,

moist adiabatic rate (MAR) = change in temp.

change in altitude

we calculate a moist adiabatic rate of

(-8 - 10)°C / (3 km - 0 km), or 6°C per km.

(Double check: the moist adiabatic rate should be smaller than the dry rate, and it is!)

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Atmospheric Stability

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To figure out the air’s stability, we need to measure the temperature both of the rising air and of its environment at various levels above the earth. If the rising air is colder than its environment, it will be heavier and tend to sink back to its original level: this indicates that the air is stable, as it resists upward movement. If the rising air is warmer and less dense than the surrounding air, it will continue to rise until it reaches the same temperature as its environment: this indicates that the air is unstable.

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Characteristics of Stable Atmospheres

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The environmental lapse rate is the rate at which the air temperature surrounding us would change if we were to climb upward into the atmosphere. An absolutely stable atmosphere occurs when the environmental lapse rate is less than the moist adiabatic rate.

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Characteristics of Stable Atmospheres

Since air in an absolutely stable atmosphere strongly resists upward motion, it will tend to spread out horizontally if forced to rise. If clouds form in this rising air, they will usually spread horizontally in relatively thin layers and usually have flat tops and bases. Clouds in the stratus category — such as cirrostratus, altostratus, nimbostratus, and stratus — are often associated with formation in stable air.

The atmosphere is stable when the environmental lapse rate is small, and the difference in temperature between the surface air and the air aloft is relatively small. Thus, the atmosphere tends to stabilize when air aloft warms or surface air cools. The cooling of surface air may be due to:

nighttime radiational cooling of the surface
an influx of cold surface air brought in by the wind (cold advection)
air moving over a cold surface

As a result, the atmosphere is most stable in the early morning around sunrise, when the lowest surface air temperature is recorded.

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Characteristics of Stable Atmospheres

Another way the atmosphere becomes more stable is when an entire layer of air sinks. After subsiding, the top of the layer is warmer than the bottom, and an inversion is formed. Inversions that form when air slowly sinks over a large area are called subsidence inversions, and they are frequently observed aloft and are often associated with large high-pressure areas because of the sinking air motions associated with these systems.

An inversion represents an atmosphere that is absolutely stable! This is because warm air sits on top of cooler air in an inversion, preventing vertical air motions from taking place (i.e. air that tries to move up gets colder, but since cold air is lower anyway, the air resists upward movement!). When an inversion exists near the ground, stratus clouds, fog, haze, and pollutants are all kept close to the surface.

If the lapse rate is equal to the dry adiabatic rate (for unsaturated air), a condition called neutral stability exists. This is because the unsaturated air will cool or warm at the same rate as the air around it, causing it to neither continue rising nor begin sinking. For saturated air, neutral stability exists when the environmental lapse rate is equal to the moist adiabatic rate.

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Characteristics of Unstable Atmospheres

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Absolute instability results when the environmental lapse rate is greater than the dry adiabatic rate. This is because, if the dry adiabatic rate is less than the environmental lapse rate, a parcel of unsaturated air will be warmer than the surrounding environmental air; this allows the parcel to continue to rise farther away from its original position.

*It should be noted, however, that deep layers in the atmosphere are seldom,

if ever, absolutely unstable. Absolute instability is usually limited to a very

shallow layer near the ground on hot, sunny days.

When the atmosphere becomes unstable, thunderstorms may result!

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Characteristics of Unstable Atmospheres

To summarize what we’ve covered in the last few slides: if the environmental lapse rate is less than the moist adiabatic rate, the atmosphere is absolutely stable. On the other hand, if the environmental lapse rate is greater than the dry adiabatic rate, the atmosphere is absolutely unstable. But what if the environmental lapse rate falls right in between?

In this case, the atmosphere is said to be conditionally unstable. This type of stability depends on whether the parcel of rising air is saturated or not. If the rising parcel of air is unsaturated, the atmosphere is stable; if the rising parcel of air is saturated, the atmosphere is unstable. Conditional instability means that, if unsaturated air could be lifted to a level where it becomes saturated, instability would result.

The average lapse rate in the troposphere is about 6.5°C per 1000 m (3.6°F per 1000 ft). Since this value lies between the dry adiabatic rate and the average moist rate, the atmosphere is ordinarily in a state of conditional instability!

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A Conditionally Unstable Atmosphere

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Consider the figure below of a conditionally unstable atmosphere. When a parcel of unsaturated air rises, it cools dry adiabatically and is colder at each level than the air around it (as shown in Fig. a). It will, therefore, tend to sink back to its original level because it is in a stable atmosphere. Now, suppose the rising parcel is saturated, as shown in Fig. b. In this case, the rising air is warmer than its environment at each level. Once the parcel is given a push upward, it will tend to move in that direction; the atmosphere is unstable for the saturated parcel.

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A Summary of Atmospheric Stability

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An atmosphere is absolutely unstable if the environmental lapse rate is greater (or has a smaller slope on the chart) than the dry adiabatic rate.

An atmosphere is absolutely stable if the environmental lapse rate is less (or has a steeper slope on the chart) than the moist adiabatic rate.

An atmosphere is conditionally unstable if the environmental lapse rate lies between the dry adiabatic rate and the moist adiabatic rate (or has a slope steeper than the dry adiabatic rate but flatter than the moist adiabatic rate, as shown in the green area to the right).

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Causes of Instability

The atmosphere becomes more unstable as the environmental lapse rate steepens; that is, as the air temperature drops rapidly with increasing height. This circumstance may be brought on by either air aloft becoming colder or surface air becoming warmer, as shown below.

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Causes of Instability

The cooling of air aloft may be due to

winds bringing in colder air (cold advection)
clouds or air emitting infrared radiation to space (radiational cooling)

The warming of the surface air may be due to

daytime solar heating of the surface
an influx of warm air brought in by the wind (warm advection)
air moving over a warm surface

The combination of cold air aloft and warm surface air can produce a steep lapse rate and atmospheric instability.

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Atmospheric Instability

The stability of the atmosphere changes during the course of a day. In clear, calm weather around sunrise, surface air is normally colder than the air above it, a radiation inversion exists, and the atmosphere is quite stable. However, as the day progresses, sunlight warms the surface, which subsequently warms the air above. As air temperatures near the ground increase, the lower atmosphere gradually becomes more unstable, or destabilizes. Maximum instability usually occurs during the hottest part of the day.

Mixing and lifting can also make the atmosphere more unstable,

as shown below. Just as lowering an entire layer of air makes

it more stable, the lifting of a layer makes it more unstable.

A potential instability brought about by lifting a stable layer

whose surface is humid and whose top is dry is known as

convective instability. Convective instability is associated with

the development of severe storms, which we will cover later on.

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Precipitation

Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the earth.

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What is Rain?

To a meteorology, rain is any falling drop of liquid water that has a diameter greater than or equal to 0.5 mm. Raindrops fall because of gravity. Drops of water with diameters smaller than 0.5 mm are called drizzle. Most drizzle falls from stratus clouds.

Sometimes, raindrops may encounter rapidly rising air when falling. Because large raindrops have a terminal velocity of about 9 m/s, if they encounter rising air with a speed greater than 9 m/s, they will not make it to the surface. Instead, they will be suspended in the air.

If the updraft weakens or changes direction to become a downdraft, the suspended raindrops fall to the ground as a rain shower. If the shower is excessively heavy, it may be termed a cloudburst.

Showers falling from cumuliform clouds tend to be brief and sporadic. Continuous rain often falls from layered clouds with larger areas and smaller vertical air currents, like nimbostratus clouds.

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Raindrops

Raindrops that reach the surface are often smaller than 6 mm. Why?

This is due to the fact that raindrops tend to break up into smaller raindrops when they collide with each other. Even if a drop manages to fall without colliding with any of the surrounding raindrops, a large size may still lead to instability that breaks apart the drop.

What is the shape of a falling raindrop?

You may think that raindrops look like

tears, but that is wrong! First and foremost, the shape of a raindrop depends on the drop’s size. If the raindrop is less than 2 mm in diameter, the are spherical like shown in raindrop #2. Larger raindrops with diameters greater than 2 mm look more like raindrop #3, like a hamburger bun. This is because pressure from the air flattens the drop as it falls. Just remember: raindrops are NOT tear-shaped!

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Precipitation tends to remove suspended particles from the air. Because of this, visibility tends to improve after a rainstorm.

The intensity of rain is measured by the amount that falls in a given period. Intensity of rain is always based on the accumulation during a certain interval of time. A table of rainfall intensity is shown below.

Rainfall Intensity

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A thunderstorm is a storm containing lightning and thunder. They are convective storms that form with rising air. Thus, the birth of a thunderstorm usually involves warm, moist air rising in a conditionally unstable environment.

The basic ingredients of a thunderstorm include moisture, unstable air, and lift. Moisture is needed to form clouds and rain. Warm, unstable air is needed to rise rapidly. Lift is needed to trigger thunderstorm initiation; this can form from fronts, sea breezes, or mountains.

Although they can occur year-round at all hours, thunderstorms are most likely to happen in the spring and summer months during the afternoon and evening hours.

A typical thunderstorm is 15 miles in diameter and lasts around 30 minutes.

Thunderstorms

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Thunderstorms often form where the surface air is quite warm and humid, but they may also form when the surface air temperature is below 10°C (50°F), as cold air aloft may destabilize the atmosphere.

Most thunderstorms that form over North America are short-lived, producing rain showers, gusty surface winds, thunder and lightning, and sometimes small hail. Many share an appearance similar to the one pictured below, but the majority do not reach severe status.

Thunderstorms

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Severe Thunderstorms

Rain and Thunderstorms

Severe thunderstorms are defined by the National Weather Service as thunderstorms that produce at least one of the following: large hail with a diameter of at least one inch, surface wind gusts of at least 50 knots (58 mph), or a tornado.

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Severe Thunderstorm Watches and Warnings

A severe thunderstorm watch indicates that a severe thunderstorm (damaging winds of over 58 miles per hour or hail 1’’ in diameter or greater) is likely to develop in that area.

A severe thunderstorm warning indicates that a severe thunderstorm is taking place in the area.

In the doppler radar animation to the right, the yellow boxes that appear indicate the presence of a severe thunderstorm warning.

(The green boxes represent flash flood warnings while the red boxes represent tornado warnings. The reason why there are so many tornadoes is because this radar loop was taken from the tornado outbreak that occurred on April 27, 2011!)

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Scattered thunderstorms typically form on warm, humid days and are often referred to as ordinary cell thunderstorms or air-mass thunderstorms because they tend to form in warm, humid air masses away from significant weather fronts. These thunderstorms can be considered “simple storms” because they rarely become severe, are typically less than a kilometer wide, and they go through a rather predictable life cycle from birth to maturity to decay that usually takes less than an hour to complete.

However, under the right atmospheric conditions, more intense “complex thunderstorms” may form, such as the multicell thunderstorm and the supercell thunderstorm — an intense, rotating storm that can last for hours and produce strong surface winds, large damaging hail, flash floods, and violent tornadoes.

Types of Thunderstorms

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Ordinary cell (air mass) thunderstorms tend to form in regions where there is limited vertical wind shear (where the wind speed and direction do not abruptly change with increasing height above the surface). Many ordinary thunderstorms form as parcels of air are lifted from the surface by turbulent overturning in the presence of wind.

Ordinary thunderstorms go through a fairly predictable cycle of development from birth to maturity to decay. The stages that are a part of this cycle are as follows:

cumulus (growth) stage
mature stage
dissipating stage

These stages are covered more in depth in the next few slides.

Ordinary Cell Thunderstorms

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An Overview of the Thunderstorm Life Cycle

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The first stage of the thunderstorm life cycle is the cumulus stage or growth stage. During this stage, the sun heats the earth’s surface during the day, warming the air around it and allowing parcels of warm, humid air to rise (also known as an updraft). If the air is moist, the warm air can condense into a cumulus cloud, which will continue to grow vertically as long as warm air below it continues to rise.

During the cumulus stage, there is not normally time for precipitation to form, and the updrafts help keep water droplets and ice crystals suspended within the cloud.

The cumulus clouds that form during this stage can build well above the freezing level. However, as this happens, the cloud particles begin to grow larger and heavier as they collide and join with each other.

Cumulus Stage

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As the cloud continues to build above the freezing level, the cloud particles grow larger and heavier as they collide and join with each other. These particles will continue to grow until their size exceeds the ability of the updraft to keep them suspended.

When this happens, drier air from around the cloud begins to be drawn into the cloud in a process called entrainment. The entrainment of drier air causes some of the raindrops to evaporate, which chills the air. Thus, this colder and heavier air begins to descend as a downdraft.

As the air descends, the cold ice particles in the cloud begin to melt, which further cools the air around the cloud. This cooling enhances the downdraft.

The appearance of the downdraft marks the beginning of the mature stage of a thunderstorm.

Mature Stage

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The thunderstorm is most intense during the mature stage. As the top of the cloud reaches a stable region in the atmosphere, it begins to spread horizontally, giving the cumulonimbus cloud its defining anvil shape. The cloud itself may extend up to an altitude of 12 km and spread several kilometers in diameter.

Updrafts and downdrafts are strongest near the middle of the cloud. Lightning and thunder are also present in the mature stage. Heavy rain, and sometimes hail, often fall from the cloud. At the surface, a downrush of cooler air accompanies the onset of precipitation.

When the cold downdraft hits the surface, the air spreads out horizontally in all directions. The boundary that separates the advancing cooler air from the surrounding warmer air is called the gust front. The gust front forces warm, humid air up into the storm, which enhances the cloud’s updraft.

It is not always true that, in the region of the downdraft, rainfall will reach the surface. This depends on the relative humidity beneath the storm. If the air is dry, raindrops may evaporate before the reaching the ground. However, the downdrafts from the storm may still reach the surface, producing gusty winds and a gust front!

Mature Stage

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As the storm enters the mature stage, it begins to dissipate after about 15 to 30 minutes. As the gust front moves away from the storm, it no longer enhances the updrafts. Thus, the downdrafts begin to take over the updrafts, cutting off the storm’s fuel supply. Cloud droplets no longer form, and the cloud reaches its dissipating stage.

During this stage, light precipitation falls from the cloud, accompanied by weak downdrafts. As the storm dies, the lower-level cloud particles evaporate, leaving only the cirrus anvil behind. A single ordinary cell thunderstorm may go through its three stages in one hour or less.

While these single cell thunderstorms may bring momentary cooling on a hot day when their downdrafts reach the surface, this cooling is short-lived. As the storm ends, humidity levels tend to increase as rainfall moisture evaporates, which may increase the temperature to a level even more oppressive than before!

Dissipating Stage

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Thunderstorm Life Cycle

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A Brief Summary of the Ordinary Cell Life Cycle

To summarize:

Ordinary cell thunderstorms are short-lived, rarely become severe, and form in a region with weak vertical wind shear.
As these storms develop, the updraft will eventually give way to the downdraft, and these storms essentially collapse on themselves.

However, these storms only consist of one cell, so the collapse of the cell will lead to the dissipation of the whole storm. Yet if the storm were to develop in a region with strong vertical wind shear, it would be able to develop a more complex structure with more than one cell. In this case, we can say that the storm is no longer an ordinary cell storm, but a multicell one.

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Multicell thunderstorms are thunderstorms that contain a number of cells, each in a different stage of development. These storms tend to form in regions of moderate to strong vertical wind shear. This type of shearing causes the cell inside the storm to tilt in such a way that the updraft actually rides up and over the downdraft. This updraft can thus generate new cells that go on to become mature thunderstorms.

In addition, the downdraft and precipitation inside the storm does not interfere with the updraft, as they would in an ordinary cell storm. Because of this, the storm’s fuel supply is not cut off and the storm can last for a long time! These long-lasting multicell storms can become intense and produce severe weather for brief periods of time.

The next slide contains a diagram detailing the process of multicell thunderstorm formation.

Multicell Thunderstorms

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Multicell Thunderstorm Formation

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Overshooting Top

When convection is strong and the updraft is intense, the rising air may intrude into the stable stratosphere! This phenomenon results in a phenomenon known as an overshooting top. In addition, as the air spreads out into the anvil, sinking air in this region made help to produce mammatus clouds.

At the surface, below the thunderstorm’s downdraft, cold, dense air may cause the surface air pressure to rise, often by several millibars. The relatively small, shallow area of high pressure is called a mesohigh, which increases the pressure gradient that may lead to high winds.

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Gust Front

When the cold downdraft reaches the earth’s surface, it pushes outward in all directions, producing a strong gust front that represents the leading edge of the cold outflowing air. When it passes, temperature drop sharply and the wind becomes strong and gusty, with speeds occasionally exceeding 55 knots, or about 63 mph. Unlike the rotating winds of a tornado, high winds from a strong gust front are known as straight line winds.

As warm, moist air rises along the forward edge of a gust front, a shelf cloud may form. These clouds are prevalent in areas where the atmosphere is stable near the base of the thunderstorm.

Occasionally, an elongated cloud that slowly spins about a horizontal axis forms just behind the gust front. These clouds are roll clouds.

When the atmosphere is conditionally unstable, the leading edge of the gust front may force warm, moist air upwards, producing many multicell storms with new gust fronts. These gust fronts may merge into a huge gust front called an outflow boundary. Along this boundary, air is forced upwards, often generating new thunderstorms.

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Gust Fronts, Shelf Clouds, Roll Clouds, & Outflow Boundaries

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If the downdraft of an intense thunderstorm is so localized and concentrated when it hits the ground, the radial hurst of wind that results is known as a downburst. A downburst with winds spanning less than 4 km is known as a microburst. While small, microbursts can deliver straight-line winds well over 100 knots, or 115 mph.

Microbursts can be extremely dangerous for aircraft. If a plane were to fly into a microburst at a low altitude, as shown below, it would first encounter a headwind at (a) that generates extra lift. This may lead the pilot to nose the aircraft downward to counter this extra lift, but doing so will lead to devastating consequences! In a matter of seconds, the plane will reach a powerful downdraft at (b) and a tailwind at (c) that will accelerate the plane toward the ground.

Rain and Thunderstorms

Microbursts

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While microbursts are associated with severe thunderstorms, they can also occur with ordinary cell thunderstorms and with clouds that produce isolated showers. In the Western US, microbursts may even come with virga!

In addition, thunderstorm-related downdrafts are not always cool! While the majority are cool, they can at times be extremely hot. Warm downbursts are known as heat bursts. At this time, the exact cause of this phenomenon is not yet known.

Rain and Thunderstorms

Microbursts

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A line of multicell thunderstorms is called a squall line. The line of storms may form directly along a cold front and extend for hundreds of kilometers, or the storms may form in the warm air 100 to 300 km out ahead of the cold front.

These pre-frontal squall-line thunderstorms of the middle latitudes represent the largest and most severe type of squall line, with huge thunderstorms causing severe weather over much of its length.

Rising air along the frontal boundary and the gust front, accompanied with the tilted updraft, helps to promote the development of new cells as the storm moves along. Even as old cells decay and die out, new ones constantly form, and the squall line can maintain itself for hours on end.

Squall Lines

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Squall Lines

Squall lines that exhibit weaker updrafts and downdrafts tend to be shallower than pre-frontal squall lines and have shorter lifespans. These squall lines are ordinary squall lines. While severe weather may be present in ordinary squall lines, the thunderstorms within them typically exhibit characteristics of ordinary cell thunderstorms.

Strong downdrafts often form to the rear of the squall line, as some of the falling precipitation evaporates and chills the air. The heavy cooler air then descends, dragging some of the surrounding air with it. If the cool air rapidly descends, a rather narrow band of fast-flowing air called a rear-inflow jet may form, delivering straight-line winds that may exceed 90 knots (104 mph).

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As the strong winds rush forward along the ground, it may push the squall line outward. This creates a bow shape effect on Doppler Radar, as shown to the right. This bow-shaped squall is known as a bow echo.

The rush of strong winds may produce relatively small bows of between 8 to 15 km long. These bows are called mini-bows. However, if wind shear ahead of the squall line is strong, much larger bows may form. These bows can reach over 150 km long.

The strongest straight-line winds tend to form near the center of the bow, where the sharpest bending occurs. Tornadoes can form within the bow, but they are usually small and short-lived.

Bow Echo

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Bow Echo

If the thunderstorms in a bow echo organize into a region of cyclonic rotation, a spinning low called a mesoscale convective vortex is formed. These vortices may continue for several days even after the thunderstorms associated with them dissipate, serving as a focus for additional storms.

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Derecho

If straight-line winds gusting to more than 50 knots (58 mph) persist along a path of at least 400 km (250 mi) long, the windstorm is called a derecho. In an average year, about 20 derechos occur in the United States. They occur most often in the months of June, July, and August.

Typically, derechos form in the early evening and last throughout the night. The damage from derechos can be mistaken with the damage from a tornado. However, with a derecho, debris is blown in one direction, while debris with a tornado is blown in many directions.

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Rain and Thunderstorms

When conditions are favorable for convection, a number of individual multicell thunderstorms may organize into a large circular convective weather system called a mesoscale convective complex (MCC). A MCC can be as much as 1,000x larger than an individual ordinary cell thunderstorm, and it may be able to cover up an entire state!

A MCC generates a long-lasting (6 to 12+ hours) weather system that produces widespread precipitation. They tend to form during the summer in regions where the upper-level winds are weak (often beneath a ridge of high pressure). Most MCCs reach their maximum strength in the early morning hours. Below is satellite imagery from GOES-16 showing a MCC forming over Cedar Rapids, Iowa on July 21, 2017.

Mesoscale Convective Complex

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Rain and Thunderstorms

Hail is created when small water droplets are caught in the updraft of a thunderstorm. These water droplets are lifted until they freeze into ice. Once they are large enough, they may fall out the bottom of the cloud with the downdraft; however, they may remain airborne as long as the updraft is strong enough to sustain them there.

Hail

According to the National Weather Service, in order for a thunderstorm to produce dime-sized hail, its updraft speed would need to be at least 37 mph. For golfball-sized hail, updraft speeds would need to be around 56 mph. Baseball-sized hail requires strong winds that are blowing upwards at 100 mph!

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Hail Formation

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Supercell Thunderstorms

In a region where there is strong vertical wind shear, a thunderstorm may form such that the cold downdraft never intercepts the updraft. In this type of storm, the wind shear may be so strong that horizontal spin may begin to occur in the storm, causing the strong, uninterrupted updraft to rotate. This results in an intense, long-lasting thunderstorm with a single violently rotating updraft called a supercell. The rotation within a supercell often leads to the formation of tornadoes!

The largest hail observed on Earth comes from supercells. The powerful updrafts inside supercells can keep hailstones airborne for long periods, allow frozen water droplets to accumulate on them — this leads to hailstones of ever increasing size! The stronger the updraft, the larger the hailstone would have to be in order it to be able to fall to the ground.

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Supercells are divided into three types:

Classic supercells produce heavy rain, large hail, high surface winds, and the majority of tornadoes. These supercells are often used in diagrams.
HP (high precipitation) supercells are supercells dominated by heavy precipitation, strong downdrafts (downbursts), and large hail. Tornadoes that form in HP supercells are wrapped in heavy rain and are often difficult to see.
LP (low precipitation) supercells are supercells that tend to produce little rain (but are still able to produce large hail and tornadoes). In these supercells, tornadoes and cloud features are quite visible. Sometimes, the rotation of an LP supercell can be easily seen inside a bell-shapped central tower with a corkscrew like pattern along its sides.

Types of Supercells

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A Diagram of a Supercell

Below is a diagram of a supercell, viewed from the southeast. The storm is moving from the southwest to the northeast (left to right on the diagram). The rotating air column on the south side, the mesocyclone, contains an updraft so strong that precipitation cannot fall through it. This produces a rain-free base beneath the updraft. Large hail often falls just north of the updraft, and the heaviest rain falls just north of the hail. If humid low-level air is drawn into the updraft, a rotating cloud called a wall cloud may descend from the base of the storm.

Below is a picture of a wall cloud. An enlarged version

of the diagram is recreated on the next page.

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The diagram to the left shows conditions often observed during the springtime in the Central Plains. At the surface, we find an open-wave middle-latitude cyclone with cold, dry air moving in behind a cold front, and warm humid air pushing northward from the Gulf behind a warm front (for fronts, please see Unit 18).

At approximately 5,000 ft, we find a narrow band of strong winds called the low-level jet. Directly above the moist layer where the jet can be found is a wedge of cooler, drier air moving in from the southwest.

The yellow region on the bottom represents where supercells are most likely to form. They tend to form in this region because (1) the position of cold air above warm air produces a conditionally unstable atmosphere and because (2) strong vertical wind shear induces rotation (this strong shear is caused by rapidly increasing wind speed from the surface up to the low-level jet).

Supercell Formation

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Thunderstorms and the Dryline

Drylines are narrow zones where there is a sharp horizontal change in moisture. In the United States, drylines are most frequently observed in the western half of Texas, Oklahoma, and Kansas. In this region, drylines occur most frequently during spring and early summer, where they are observed 40% of the time.

Thunderstorms may form along or just east of a dryline. The figure to the right shows why this is true. The dryline separates the hot, dry air from the warm, humid air. However, since the Central Plains of North America are elevated to the west, some of the hot, dry air from the southwest is able to ride over the slightly cooler, moist air from the Gulf. This creates a potentially unstable atmosphere east of the dryline which may promote the generation of new thunderstorms.

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Distribution of Thunderstorms

It is estimated that more than 50,000 thunderstorms occur each day throughout the world. This equates to 18 million thunderstorms annually. Thunderstorm formation is most common in equatorial regions, where storms may occur on about one out of every three days. Thunderstorms are also prevalent over water along the intertropical convergence zone (ITCZ), a belt of low pressure that circles the Earth at the equator.

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Distribution of Thunderstorms

In the United States, thunderstorms form most frequently in the Southeastern United States along the Gulf Coast, with a maximum in Florida. Thunderstorms are least common along the Pacific coast and its interior valleys. A map of thunderstorm frequency is shown below.

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Distribution of Thunderstorms

In many areas, thunderstorms typically form during a warm, summer afternoon when the surface is most unstable. There are some exceptions, though:

During the summer in the valleys of central and southern California, dry, sinking air produces an inversion that inhibits cumulus (vertical) cloud growth. In these regions, thunderstorms are most frequent in winter and spring.
On many summer days, thunderstorms develop just east of the Rocky Mountains in the afternoon. These storms later intensify into mesoscale convective systems in the evening as they move eastward over the Great Plains. Thus, for parts of the plains such as Iowa and Missouri, thunderstorms can be most frequently found between midnight and dawn.

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Distribution of Thunderstorms

There is, however, one additional aspect to note: even though the frequency of thunderstorms is greatest in the Gulf Coast, the frequency of hailstorms is greatest over the western Great Plains, as shown. Why is this the case?

The conditions over the Great Plains are more favorable for the development of severe thunderstorms, especially supercells that have strong updrafts capable of keeping hailstones suspended within the cloud for a long time. This allows hailstones to grow massively in size before falling back to Earth.
In addition, in summer along the Gulf Coast, a thick layer of warm, moist air extends upward from the surface. Thus, most hail will melt before reaching the ground. On the contrary, a thunderstorm forming in the high plains east of the Rockies will typically be located above the freezing level, so hail has a better chance of occurring.

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Pineapple Express

A Pineapple Express is a common weather pattern that brings moisture from the Hawaiian islands to the West Coast of the United States. It sometimes produces days of heavy rainfall, which can cause extensive flooding and mudslides.

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Lightning is a discharge of electricity that typically occurs in mature thunderstorms. Light can take place within a cloud, between clouds, from a cloud to the surrounding air, or from a cloud to the ground. The width of a thunderbolt is around ½ to 1 inch (about the width of a quarter).

The majority of lightning strikes occur within the cloud, with only about 20% reaching the ground.

A lightning stroke can heat the air around it to 30,000°C, or 54,000°F. This is 5 times hotter than the surface of the sun! Such extreme heating causes the air to expand explosively, initiating a shock wave that becomes a booming sound wave that travels outward in all directions from the flash. This sound is known as thunder.

Lightning and Thunder

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Distance from Lightning

A flash of lightning is always seen before its corresponding sound of thunder is heard. This is because light travels faster than sound. Light is in fact so fast that you can see it instantly after a lightning flash, but sound travels at only 330 m/s (1,100 ft/s), making it take longer for thunder to reach your ear. However, we can use this fact to calculate how far a flash of lightning is.

It takes sound about 5 seconds to travel one mile. Thus, to calculate the distance you are from the lightning, simply count the number of seconds that pass between a flash of lightning and its rumble of thunder and divide it by 5. This will give you your distance from the thunder in miles.

Example: 15 seconds pass between a flash of lightning and a

rumble of thunder. How far away is the storm?

Distance = (15 seconds) / (5 seconds/mile) = 3 miles

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The sound of thunder may differ depending on how far you are from the storm. When the lightning stroke is close (i.e. 100 m or less), thunder sounds like a clap or crack followed by a loud bang. When it is farther away, a rumbling sound may be heard as the sound waves are bounced off of obstructions.

But what if no thunder is heard after lightning is seen? Does that mean that thunder was not produced by that stroke of lightning? No; the sound waves were probably bent away or scattered by the atmosphere. This attenuates the thunder to the point that it may not be audible.

In addition, sound travels faster in warm air than in cold air. Because thunderstorms form in a conditionally unstable atmosphere where temperature drops rapidly with height, sound waves moving away from the lightning are bent upward away from the observer. Thus, observers farther than 8 km from the storm will seldom hear thunder.

The Sound of Thunder

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Lightning Formation

What causes lightning? For lightning to occur, separate regions containing opposite electrical charges must exist within a cumulonimbus cloud. In the sky, these electrical charges are created from collisions between small bits of ice and frozen raindrops. During these collisions, there is a net transfer of positive ions from larger, warmer particles to smaller, cooler ones. Because the larger, heavier particles become negatively charged while the smaller, lighter particles become positively charged, these two characteristics become stratified by height. The positive charges move to the top of the cloud, while the negative charges move to the bottom.

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Lightning Formation

Because unlike charges attract one another, the negative charge at the bottom of the cloud causes a region of the ground beneath it to become positively charged. This positive charge beneath the thunderstorm cloud follows the cloud no matter where it goes, with the densest positive charge on protruding objects, such as trees, poles, and buildings. When enough charge builds up between the cloud and the ground, a current flows between them and lightning occurs. This specific type of lightning is called cloud-to-ground lightning.

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Lightning Formation: The Stepped Leader and Return Stroke

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Negative and Positive Lightning

Negative cloud-to-ground lightning occurs when the origin point of the cloud is negatively charged and the ground is positively charged. In this case, the stroke carries negative charges from the cloud to the ground. The majority of cloud-to-ground lightning (90%) is negative.

Positive cloud-to-ground lightning occurs when the origin point of a cloud is positively charged and the ground is negatively charged. Usually associated with supercell thunderstorms, positive lightning is more dangerous as it generates a much higher current and its flash lasts longer in duration than that of negative lightning. Why? Because positive lightning originates at the top of the cloud, the bolt has to travel over such a large distance it can be up to 10 times stronger and last 10 times longer than a negative strike. The photo below shows an example of positive lightning.

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Other Types of Lightning

Ball lightning looks like a sphere of light about the size of a football that appears to float in the air or slowly dart about for several seconds. The actual cause of ball lightning is still a mystery!

Intracloud lightning is the most common form of lightning. It happens completely inside the cloud, jumping between different charged regions of the cloud. Intracloud lightning is sometimes called sheet lightning because it lights up the sky like a sheet.

Heat lightning is lightning from a thunderstorm that is too far to be heard. It is called heat lightning because it occurs most often on hot summer days when the sky is clear overhead.

Dry lightning occurs when cloud-to-ground lightning forms with storms that do not produce rain.

A bolt from the blue occurs when a positive lightning bolt that originates from the updraft of the storm travels horizontally for many miles before striking the ground.

As the electrical potential near the ground increases during a thunderstorm, a current of positive charge moves up pointed objects. This results in a continuous supply of sparks that are sent from the pointed object into the air; an effect known as St. Elmo’s Fire. When St. Elmo’s Fire is seen during a thunderstorm, a lightning flash may occur in the near future!

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Other Types of Lightning

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The Science of Lightning

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Lightning Safety Tips

About 10% of all people struck by lightning are killed. Thus, it is important to know some general lightning safety tips that will allow you to stay safe during a storm.

If you see someone struck by lightning, immediately give CPR. Lightning normally leaves its victims unconscious without heartbeat and respiration. Don’t worry; the injured person will not carry an electrical charge, so it is okay to touch them. In addition, call 9-1-1 or send for help immediately.

If you are outside during a thunderstorm, keep an eye at the sky. Look for darkening skies, flashes of lightning, or increasing winds. Lightning often precedes rain, so don’t wait for the rain to begin. If you hear the sound of thunder, go to a safe place immediately. The best protection is inside a building, away from electrical appliances and corded phones. Avoid taking a shower. Automobiles with metal frame and trucks may also provide protection (but not golf carts!).

But what if you cannot find a shelter? If this case, avoid elevated places and isolated trees. Never take shelter under a tree during a thunderstorm, as upward pointing objects tend to have a concentration of positive charge. Keep your head as low as possible, but do not lie down! Crouch down as low as possible and minimize the contact area you have with the ground.

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Lightning Safety Tips

If you’re with a group of people stay about 15 feet from each other. Stay out of water, because it’s a great conductor of electricity. Swimming, wading, snorkeling and scuba diving are not safe activities.

Do not stand in puddles and avoid metal. Stay away from clotheslines, fences, and drop your backpacks because they often have metal on them. If you’re playing an outdoor activity, wait at least 30 minutes after the last observed lightning strike or thunder.

There are some warning signs to alert you to a strike. If your hair begins to stand on end or your skin begins to tingle and you hear clicking sounds, watch out! Lightning may be about to strike, and if you are standing upright, you may be acting as a lightning rod!

What if lightning strikes your vehicle? As long as the windows are up, the occupants will most likely be unharmed, as lightning typically travels across the car’s outer surface or through wiring. However, complex electrical systems in the car may be damaged. Avoid touching any metallic interior object that may be connected to the exterior, such as the door handle or a gear shifter. If safe, pull over and wait until the storm has passed.

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Lightning Safety Tips

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Floods and Droughts

“I think we are bound to, and by, nature. We may want to

deny this connection and try to believe we control the

external world, but every time there's a snowstorm or

drought, we know our fate is tied to the world around us.”

― Alice Hoffman, American Novelist

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Floods and Droughts

In the previous unit, we learned about thunderstorms. In this unit, we will cover one of the consequences that may arise from prolonged thunderstorm activity: the flood.

Intense thunderstorms can be associated with floods and flash floods. A flash flood is a flood that develops rapidly with little or no advance warning. Such flooding often occurs when thunderstorms stall or move very slowly, causing heavy rainfall over a relatively small area. In narrow canyons and valleys, floodwaters flow faster than on flat ground and can be quite destructive.

Flooding may also occur when thunderstorms move quickly but keep passing over the same area. This phenomenon is known as training.

In the United States, flooding has killed an average of more than 100 people a year and has accounted for untold property and crop damage.

Floods and Flash Floods

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Flooding is not limited to any season of the year. For some places, flooding occurs primarily in the spring when heavy rain and melting snow cause rivers to overflow their banks. For other places, flooding may be prominent in the summer and autumn as a result of tropical storms that dump torrential rains in the area.

If thunderstorms bring repeated heavy rain to a region for days or week, a result may be a river flood. A river flood occurs when a major river system rises slowly but ends up flooding a large area, whereas a flash flood may devastate a smaller area in minutes to hours.

Sometimes a region may experience both flash flooding and river flooding. Such was the case in Colorado in early September 2013. In just one day, Boulder, Colorado reported 23 cm (9 in) of rainfall, double its previous record! The waters also caused river flooding along the South Platte River, resulting in an event that produced $2 billion in damage, caused at least 10 deaths, and destroyed thousands of homes.

River Floods

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Flood Watches and Warnings

A flood watch means that an overflow of water from a river is possible for the area.
A flash flood watch means that flash flooding is possible in or close to the watch area. Flash flood watches can be put into effect for as long as 12 hours, while heavy rains move into and across the area.
A flood warning means flooding conditions are actually occurring in the warning area.
A flash flood warning means that flash flooding is actually occurring in the warning area. A warning can also be issued as a result of torrential rains, a dam failure, or snow thaw.

The green box on the radar map below usually illustrates where a flash flood warning is active.

Floods and Droughts

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Flood Safety Tips

Before a flood, have a disaster plan and prepare a disaster supplies kit for your home and car. Include a first aid kit, canned food, can opener, bottled water, battery-operated radio, flashlight, protective clothing and written instructions on how to turn off electricity, gas, and water.

During a flood, move to a safe area quickly. Move to higher ground, such as the highest floor of your house. Avoid areas subject to sudden flooding like low spots and canyons. Avoid already flooded areas. If a flowing stream of water is above your ankles, stop, turn around, and go the other way. If you are driving, do not attempt to drive through a flooded road. The depth of the water is not obvious and the road may be washed away. If your car stalls, leave it and seek higher ground. Rapidly rising water may engulf the car, pick it up and sweep it away. According to FEMA, six inches of water will reach the bottom of most passenger cars, causing loss of control and potential stalling. Two feet of rushing water will carry away most vehicles, including SUVs and pickups. Almost two of every three U.S. flash flood deaths from 1995-2010, excluding fatalities from Hurricane Katrina, occurred in vehicles! Kids should never play around high water, storm drains or viaducts. Be cautious at night, because it’s harder to see flood dangers. If told to evacuate, do so immediately.

After a flood, always boil drinking water. Electrical equipment should be checked & dried before use.

Floods and Droughts

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Flood Safety Tips

Almost two of every three U.S. flash flood deaths from 1995-2010, excluding fatalities from Hurricane Katrina, occurred in vehicles! A chart of flood related-deaths is shown below.

Floods and Droughts

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Historical Floods

Below are some articles on historical floods. It may be beneficial for you to study these prior to the competition. These articles will be updated as new events occur.

this slide will be updated upon the completion of the rest of the slideshow

Floods and Droughts

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Floods and Droughts

When a region’s average precipitation drops dramatically for an extended period of time, drought may result. The word drought refers to a period of abnormally dry weather than produces a number of negative consequences, such as crop damage or limitations to water supply. Note that drought does not necessarily equate to a dry spell!

If a dry climate, for example, experiences no rain during the summer, a dry spell occurs. However, this dry spell is normal for the region and would not be considered a drought! But if a moist climate were to experience the same paucity of rainfall, it would be disastrous for the region, and a drought would ensue.

Can a dry climate experience a drought? Yes. If the dry Central Valley of California were to experience a dry period lasting several years, the lake and reservoirs that provide water for agriculture would diminish. Because water resources for agriculture are thus limited, this seemingly dry region would, by definition, be in a drought.

Droughts

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Palmer Drought Severity Index (PDSI)

To measure drought severity, scientist Wayne Palmer developed the Palmer Drought Severity Index, or PDSI. This index takes into account average temperature and precipitation values to define drought severity, and is most effective in assessing long-term drought that lasts several months or more. Drought conditions are indicated by a set of numbers that range from 0 (normal) to -4 (extreme drought). A table detailing the PDSI is shown below.

Floods and Droughts

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Historical Droughts

Below are some articles on historical droughts. It may be beneficial for you to study these prior to the competition. These articles will be updated as new events occur.

this slide will be updated upon the completion of the rest of the slideshow

Floods and Droughts

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Floods, Droughts, and Climate Change

In Unit 4, we talked about the science behind climate change. Now that we’ve looked at floods and droughts, it is important to ask the question: how are all these concepts related?

While the relationship between climate change and the frequencies of hurricanes and tornadoes are still being researched, the relationship between climate change, floods, and droughts cannot be any more clear: climate change will make drought and flooding events more frequent around the world.

Why is this the case? As average temperatures in regions across the world have gone up, more rain has fallen during the heaviest downpours. Very heavy precipitation events are more frequent now than they were 50 years ago. This happens because warmer air holds more moisture. If the emissions that cause the warming of our planet continue without restraint, scientists expect that the amount of rainfall during the heaviest precipitation events will increase more than 40 percent by the end of the century, worsening the threat of floods.

Floods and Droughts

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Floods, Droughts, and Climate Change

What is the link between our warming climate and the frequency of droughts? As more greenhouse gas emissions are released into the air, air temperatures will tend to increase. This leads to greater levels of moisture evaporation from land and from lakes, rivers, and other bodies of water. In addition, warmer temperatures also increase evaporation in plant soil, which can seriously harm plant life and reduce rainfall. To make things worse, if heavy rainfall were to come to drought-stricken areas, the drier soils on the ground will be less able to absorb water, resulting in a greater risk for flooding (this is why deserts are able to flood so quickly after heavy rain).

Floods and Droughts

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Floods and Droughts

Floods, Droughts, and Climate Change

Why is this so consequential? While droughts may be detrimental to the environment and ecosystems, it has disastrous consequences for human livelihood around the world. Nearly 40 percent of the world population –

1.3 billion people – relies on agriculture as its main source of income. If severe droughts lead to water shortages in an area dependent on agriculture, it puts the health and wellbeing not only of animals and crops at risk, but of the farmers and communities that depend on them too. This is especially dangerous in developing nations around the world, who may not have the technologies and wealth to deal with these natural disasters when things go wrong.

Because of the importance of normal rainfall levels for successful agriculture, the possibility of drought and water shortages play a decisive and unifying role in society’s fight against climate change.

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Snow and Winter Storms

“Thank goodness for the first snow, it was a

reminder — no matter how old you became and

how much you'd seen, things could still be new

if you were willing to believe they still mattered.”

― Candace Bushnell, American Novelist

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Snow and Winter Storms

Much of the precipitation that reaches the ground begins as snow. Snowflakes form by the process of deposition. In the summer, the freezing level is usually above 3,600 m (12,000 ft), and snowflakes falling from the cloud melt before hitting the ground. However, during the wintertime, this freezing level tends to be lower, giving snowflakes a better chance for survival.

Snowflakes can usually fall 300 m (1,000 ft) below the freezing level before completely melting. The figure below illustrates how you may be able to see the melting level in the sky.

Snow

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Fallstreaks

Snow and Winter Storms

Fallstreaks form when ice crystals and snowflakes fall from high cirrus clouds, and they behave very similarly to virga. As the ice particles fall into drier air, they usually sublimate. The appearance of fallstreaks can be attributed to the fact that winds at higher levels move cloud and ice horizontally more quickly than the slower winds at lower levels.

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Snowflake Appearance

Snow and Winter Storms

Below are four different types of snowflakes. The appearance of a snowflake and its growth rate depends on air temperature and relative humidity.

The six-sided star-shaped dendrite is the most common

snowflake form. This is because at the temperatures where

dendrites are common, between -12°C and -16°C, the

maximum growth rate of ice crystals is greatest (maximum

difference between the saturation vapor pressures between

water and ice). As a result, the dendrite crystal grows more

rapidly than any of the other crystal forms.

Notice that these four shapes are not the only shapes possible for a

snowflake. Ice crystals are constantly exposed to varying temperature

and moisture conditions, and they can also join together to form a

much larger, complex snowflake that assumes an appearance beyond

the four designs provided.

Snow’s white color can be attributed to its high albedo: it easily reflects light.

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Accretion and Aggregation

Snow and Winter Storms

Water droplets existing at temperatures below freezing are referred to as supercooled. In some clouds, especially those with warm tops, ice crystals might collide with supercooled droplets. In this situation, the liquid droplets freeze into ice and stick together through a process called accretion. The product of accretion is a icy snow pellet called graupel. Graupel can fracture into tiny pieces when it collides with cloud droplets, and these tiny pellets can recombine to become new graupel.

In colder clouds, ice crystals may collide with other crystals and fracture into smaller ice particles. These smaller particles can freeze hundreds of supercooled droplets on contact, producing even more ice crystals. These ice crystals collide and stick together through a process called aggregation. The end product of aggregation is a snowflake. In most cases, rain that hits the ground begins as snow. (In addition, the term coalescence is used to describe the process by which large raindrops overtake and collide with smaller drops in their path.)

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Types of Snowstorms

Snow and Winter Storms

Snow that falls from developing cumulus clouds often comes in the form of flurries. Flurries are typically light and fall intermittently for short durations. A more intense snow shower is called a snow squall. These brief but heavy bouts of snowfall, like flurries, often fall from cumuliform clouds. A more continuous, steady snowfall may be attributed to nimbostratus and altostratus clouds. The intensity of snowfall is based on its reduction of horizontal visibility at the time of observation, as shown by the table below.

Blowing snow is snow that is lifted from the surface by the wind

and blown about in such quantities that horizontal visibility is greatly

restricted. The combination of drifting and blowing snow, after falling

snow has ended, is called a ground blizzard.

A true blizzard is a weather condition characterized by low temperatures and strong winds greater than 30 knots. Blizzards come with long-lasting and intense snowfall, which can reduce visibility to only a few meters. Three things are needed to form a blizzard: cold air at the surface, lots of moisture, and lift.

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Can it snow when air temperature is above freezing?

Snow and Winter Storms

You may notice that snow may still fall when the air temperature is considerably above freezing. Why is this the case?

In order for falling snowflakes to survive in air with temperatures above freezing, the air must be unsaturated and the wet-bulb temperature must be at freezing or below. Recall from Unit 7 that the wet-bulb temperature is the lowest temperature that can be obtained by evaporating water into the air. When rain falls into a layer of dry air with a low wet-bulb temperature, rapid evaporation and cooling occurs.

Thus, as the snow falls in an atmosphere slightly above freezing, it begins to partially melt. However, because evaporation occurs rapidly, the water from the melt quickly evaporates, cooling the air. As snow continues to fall, the temperatures continue to drop. Eventually, the entire layer of air cools to the wet-bulb temperature and becomes saturated at 0°C.

In conclusion, snow can indeed fall in warmer than freezing temperatures. But, in order for this to happen, the air must be extremely dry in order to have a wet-bulb temperature at freezing or below.

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Annual Snowfall in the United States

Snow and Winter Storms

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Snow and Winter Storms

Thundersnow occurs when thunder and lightning occur during a snowstorm. This happens most often in late winter or early spring.

For thundersnow to form, you need a mass of cold air on top of warm, moist air closer to the ground. The sun heats the ground and pushes masses of warm, moist air upward, creating unstable air columns. As it rises, the moisture condenses to form clouds, which are jolted by internal turbulence.

In summary, you need atmospheric instability in the wintertime to form thundersnow: the air layer closer to the ground has to be warmer than the layers above, but it must also be still cold enough to create snow.

Thundersnow

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Thundersnow and Jim Cantore

Snow and Winter Storms

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Snow and Winter Storms

When falling snow encounters warmer air, it begins to melt. However, the partially melted snowflake or cold raindrop may still turn back into ice as it falls through a lower, subfreezing surface layer of air. The result is sleet, a transparent ice pellet with diameter 5 mm or less. They usually bounce when they hit a surface and do not typically stick to objects. Sleet, however, can accumulate like snow.

However, if the subfreezing surface layer is too shallow to freeze raindrops as they fall, the end product may be supercooled liquid drops known as freezing rain or glaze. Small droplets of freezing rain with diameters less than 0.5 mm are called freezing drizzle. This type of precipitation freezes to surfaces, such as trees, cars, and roads, forming a coating or glaze of ice. When small supercooled cloud or fog droplets strike an object whose temperature is below freezing, the tiny droplets freeze into an accumulation of white or milky granular ice called rime. This fog tends to freeze to the windward side of objects.

Sleet and Freezing Rain

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Snow and Winter Storms

Freezing rain can turn roads and highways into skating rinks for cars, and its destructive weight can break tree branches, power lines, and telephone cables.

A phenomenon called black ice is especially dangerous. This occurs when a sheet of ice covering a road surface appears relatively dark, disguising the fact that a rather innocuous road may in fact be a hazardous skating rink. Black ice commonly forms when light rain, drizzle, or supercooled fog droplets come in contact with surfaces that have cooled to a temperature below freezing.

Sleet and Freezing Rain

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Sleet, Freezing Rain, and Rime

Snow and Winter Storms

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Snow and Winter Storms

Ice storms are a type of winter storm caused by freezing rain. The National Weather Service defines an ice storm as a storm which results in the accumulation of at least 0.25 inches of ice on exposed surfaces. They often form when a layer of warm air is between two layers of cold air. Frozen precipitation melts while falling into the warm air layer, and then proceeds to refreeze in the cold layer above the ground, producing freezing rain or a glaze of ice.

Of the many types of precipitation that we have discussed, freezing rain is often considered the most hazardous. While heavy snowfall may make a commute unnecessarily painful, freezing rain can make it dangerous. The presence of freezing rain (or much worse, black ice), can spin cars off roads and/or into other vehicles, acting as a silent killer. Its weight can also lead to tree damage and power outages and, unlike other forms of precipitation, it cannot be plowed or runoff. The only way to rid this ice is to wait for it to melt using salt or time!

Ice Storms

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A Brief Summary

Snow and Winter Storms

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A Brief Summary

Snow and Winter Storms

The temperature profile of winter air can give us an indication of the type of precipitation that falls:

If the air temperature is below freezing at all levels, snowflakes reach the ground.
If a zone of above freezing air that causes snowflakes to partially melt is followed by a deep, sub-freezing air level at the surface, sleet reaches the ground.
If a zone of above freezing air that causes snowflakes to partially melt is followed by a shallow, sub-freezing air level at the surface, freezing rain reaches the ground.
If the air temperature is above freezing in a sufficiently deep layer of air above the surface, rain reaches the ground.

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Snow and Winter Storms

Snow grains are small, opaque grains of ice that fall in small quantities from stratus clouds. The are fairly flat or elongated with diameters less than 1 mm, and can be considered the solid equivalent of a drizzle.

Snow pellets are white, opaque grains of ice that often fall as showers from cumulus congestus clouds. They have diameters less than 5 mm, and are brittle, crunchy, and bounce or break apart upon hitting a hard surface. Graupel is an example of a snow pellet.

On the surface, the accumulation of snow pellets sometimes gives the appearance of tapioca pudding. Thus, this phenomenon is sometimes referred to as tapioca snow.

While snow pellets such as graupel may be small, they can develop into large hailstones in the presence of a vigorously convective cloud.

Snow Grains and Snow Pellets

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Hailstones

Snow and Winter Storms

Hailstones are pieces of ice that are either transparent or partially opaque. They are produced in a cumulonimbus cloud when graupel, or large frozen raindrops, act as embryos that grow by accumulating supercooled liquid droplets, a process known as accretion. It takes a million cloud droplets to form a single raindrop, but it takes about 10 billion cloud droplets to form a golf ball-sized hailstone. The longer a hailstone gets trapped in the updraft, the more exposure it has to liquid water content, and the larger the coating of ice that forms around them.

While smaller hail may completely melt in the warmer air below the cloud, larger hail often does not. As a thunderstorm moves along, it may deposit its hail in a long narrow band called a hailstreak. If the storm remains stationary for a long period of time, significant accumulation is possible.

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Snow and Winter Storms

As mentioned in Unit 9, frost consists of white ice crystals that form on a surface, like the ground or leaves of a plant. Frost is created when the air temperature drops below freezing and the water vapor in the air freezes into ice crystals.

Frost quakes are non-tectonic seismic events, or events that are not caused by the shifting of the Earth’s tectonic plates. Also known as cryoseisms, frost quakes are caused by a sudden rapid freezing of ground and bedrock, usually when temperatures go from above freezing to below zero. As moisture absorbed in the rock and soil freezes, it expands. This places a large magnitude of stress on the areas around it. Eventually, the stress is too great and the soil and rock will crack in an explosive manner, creating a loud sound and shaking the earth’s surface. Since temperatures are coldest in the overnight hours, most frost quakes occur in the middle of the night.

Frost and Frost Quakes

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Lake-Effect Snow

Snow and Winter Storms

When cold, dry air moves over a relatively warm body of water, such as the Great Lakes, heavy snow showers, called lake-effect snow, can result. Lake-effect snow affects the eastern shore of the body of water (for example, Western Michigan often receives lake-effect snow, as it is to the east of Lake Michigan), and they are often highly localized, extending from just a few kilometers to more than 100 km inland.

Lake-effect snow is most prominent during the months from November to January. During these months, cold air moves over the lakes when they are warm and not quite frozen. This leads to a large temperature difference between water and air, which helps to enhance the potential for snow.

As cold air moves over warmer water, the air is quickly warmed from below, making it less stable. The air then rapidly sweeps up moisture and soon becomes saturated. As the air warms, cumuliform clouds grow, eventually becoming clouds that produce heavy snow showers.

Generally, the longer the stretch of water over which an air mass travels, the greater the amount of warmth and moisture derived from the lake, and the greater the potential for heavy snow showers. Studies show that air must move across 80 km, or 50 mi, of open water for significant snowfall to occur. Forecasting lake-effect snow depends on the trajectory of the air is it flows over the lake.

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Lake-Effect Snow

Snow and Winter Storms

Figure 1 shows the process by which lake-effect snow forms. Figure 2 shows the areas that experience the heaviest lake-effect snow. It’s important to note that lake-effect snow is not limited to the Great Lakes region; any large unfrozen lake can enhance snowfall when cold, dry air sweeps over it.

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Nor’easters

Nor’easters can occur in the eastern United States any time between October and April, when moisture and cold air are plentiful. A nor’easter is named for the winds that blow in from the northeast and drive the storm up the east coast along the Gulf Stream, a band of warm water that lies off the Atlantic coast. Nor’easters are most intense off the coast of New England, and they are known for dumping heavy amounts of rain and snow, producing hurricane-force winds, and creating high surfs that can cause severe beach erosion and coastal flooding.

Snow and Winter Storms

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An Alberta clipper is an area of low pressure that generally forms over Alberta, Canada, east of the Rocky Mountains. They develop east of the Rockies as air flows eastward over the mountains. Once an Alberta Clipper forms, it usually moves very rapidly to the southeast across the USA's northern Plains and then to the east off the mid-Atlantic Coast. Clippers usually cause only light precipitation with very few producing major snowstorms. However, if conditions are favorable, some Alberta clippers can rapidly intensify off the East Coast once the storm taps into the relatively warm moist air over the Atlantic Ocean. The storms that rapidly intensify sometimes spread heavy snow over New England and southeastern Canada. Generally, the main weather features associated with Alberta clippers are light snow and a reinforcement of cold air over the USA.

Snow and Winter Storms

Alberta Clippers

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Snow and Winter Storms

Polar Vortices

A polar vortex is an area of low pressure — a wide expanse of swirling cold air — that is parked in polar regions. Sometimes, parts of the low pressure system break off and migrate toward the equator, bringing the cold air along with it.

Why is it called a vortex? The term isn’t meant to be scary; it’s just that the area of low pressure at the poles is cyclonic — that is, it rotates. Because the polar vortex is very sensitive to hemisphere-wide temperature variations, it is essentially “trapped” at the poles during warm periods. However, during the wintertime, the jet stream may buckle and stretch downwards (or towards the equator), allowing the cold air to reach further from the equator. The process is represented below… the presence of the polar vortex was the winter of 2013-14 was so cold!

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Snow and Winter Storms

The wind chill is the temperature your body feels when the air temperature is combined with the wind speed. The higher the wind speed, the faster exposed areas of your body lose heat and the colder you feel.
Frostbite is damage to the skin due to prolonged exposure to cold temperatures (< 32°F).
An avalanche is a moving mass of snow that may contain ice, soil, rocks, and uprooted trees. The height of a mountain, the steepness of its slope, and the type of snow lying on mountain surface all help to determine the likelihood of an avalanche. Avalanches begin when an unstable mass of snow breaks away from a mountainside and moves downhill. They can reach speeds of up to 245 mph!

Other Winter Terms

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Winter Weather Advisories

Winter weather advisories are issued for accumulations of snow, freezing rain, freezing drizzle, and/or sleet that may cause significant inconveniences or lead to life-threatening situations.
Winter storm watches are issued when there is the possibility of a blizzard, heavy snow, heavy freezing rain, or heavy sleet. They are usually issued 12 to 48 hours before the beginning of a winter storm.
Winter storm warnings are issued when hazardous winter weather in the form of heavy snow, heavy freezing rain, or heavy sleet is occurring. They are usually issued 12 to 24 hours before the event is expected.
Blizzard warnings are issued when blowing snow, gusty winds (> 35 mph), and low visibilities (< ¼ miles) are expected. They usually persist for about 3 hours.
Frost/Freeze warnings are issued when below freezing temperatures are expected.
Lake effect snow advisories are issued when accumulation of lake effect snow is expected to cause significant inconvenience.
Lake effect snow warnings are issued when heavy lake effect snow is occurring.
Wind chill advisories are issued when wind chill temperatures are forecast to be between -15°F and -24°F.
Wind chill warnings are issued when wind chill temperatures are forecast to be -25°F or lower.

Snow and Winter Storms

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Winter Weather Safety Tips

Before a winter storm, have a disaster plan and prepare a disaster supplies kit for your home and car. Include a first aid kit, emergency food supply, bottled water, battery-operated radio, flashlight, protective clothing, and blankets. During a winter storm, stay indoors and dress warmly. Eat regularly, as food provides the body with energy for producing its own heat. Also, drink lots of water. If you must go outside, wear layered clothing, mittens, and a hat. Watch for signs of hypothermia and frostbite. Remember to keep dry. Always change wet clothing to prevent the loss of body heat. If you must drive, carry a cell phone. Always keep the gas tank full, and let someone know where you’re going just in case your car gets stuck. If your car gets stuck, stay with it and wait for help unless help is visible within 100 yards. Use maps and car mats to stay warm. After a winter storm, avoid driving until conditions have improved. Avoid overexertion! Heart attacks from shoveling snow are the leading cause of deaths during the winter. From 1990 to 2006, 1,647 fatalities were attributed to cardiac-related injuries that resulted from shoveling snow.

Snow and Winter Storms

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Historical Winter Storms

Below are some articles on historical winter storms. It may be beneficial for you to study these prior to the competition. These articles will be updated as new events occur.

this slide will be updated upon the completion of the rest of the slideshow

Snow and Winter Storms

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Hurricanes

“My favorite day was Monday, September the 25th, 2006.

New Orleans, LA, site of the Superdome. I watched our people who had suffered so grievously through Katrina fill a stadium hours before a game and stay hours after the game.”

― Michael Irvin, Former Dallas Cowboys Wide Receiver

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The Hurricane

Hurricanes

A hurricane is an intense storm of tropical origins with sustained wind speeds above 74 mph (or 64 knots). Hurricanes form in the warm northern Atlantic and eastern North Pacific oceans.

This same type of storm is given different names in different regions of the world. In the western North Pacific, it is called a typhoon; in India, it is called a cyclone; and in Australia, it is called a tropical cyclone. For the sake of simplicity, all three types will be referred to using the term hurricane in this unit (however, by international agreement, a tropical cyclone is the general term for all hurricane-type storms that originate over tropical waters).

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GOES Satellite Imagery of Major Hurricane Fernanda (2017)

Hurricanes

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Anatomy of a Hurricane

Hurricanes

Hurricanes rotate in a counterclockwise direction around an eye in the Northern Hemisphere (this rotation is clockwise in the Southern Hemisphere). This eye, or center of the storm, is the calmest region of the hurricane, with light winds and fair weather. The lowest pressures are also found in the eye of a hurricane.

Adjacent to the eye is the eyewall, a ring of intense thunderstorms that spin around the storm’s center. Within the eyewall is the heaviest precipitation and the strongest winds. Much of the eyewall consists of spiral rainbands that swirl in toward the storm’s center, where they wrap themselves around the eye. The large central area of thunderstorms surrounding a storm’s circulation center, caused by the formation of its eyewall, is known as the central dense overcast (CDO).

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Anatomy of a Hurricane

Hurricanes

To understand why the surface pressure is lowest at the center of the hurricane and why the weather associated with that area is often calm and clear, one must look at the vertical profile of the hurricane along a slice that runs through its center. Near the surface, moist tropical air flows in toward the hurricane’s center, which causes huge cumulonimbus clouds to form in the eyewall. These vigorous convective clouds release a large quantity of latent heat, which causes the air to warm and creates slightly higher pressures aloft. This initiates downward motion in the eye, which explains the absence of convective clouds at the storm’s center.

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Anatomy of a Hurricane

Hurricanes

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Hurricane Formation

Hurricanes

Where and how do hurricanes form? There are certain ingredients that are necessary for a weak tropical disturbance to turn into a powerful hurricane:

The Right Environment: hurricanes form over tropical waters where the winds are light, the humidity is high throughout a deep layer extending up through the troposphere, and the surface water temperature is warm over a vast area, typically 80°F/26.5°C or higher. These conditions are usually prevalent during the summer and early fall; as a result, the hurricane season tends to run from June through November.

The Atlantic Hurricane Season begins on June 1 and ends on November 30.
The Eastern Pacific Hurricane Season begins on May 15 and ends on November 30.
The Pacific Typhoon Season lasts all year.
The North Indian Cyclone Season begins on April 1 and ends on December 31.
The Southwest Indian Cyclone Season begins on October 15 and ends on May 31.
The Australian/Southeast Indian Cyclone Season begins on October 15 and ends on May 31.
The Australian/Southwest Pacific Cyclone Season begins on November 1 and ends on April 30.

May tends to be the least active tropical month; September tends to be the most active. This does

not mean that storm formation is limited to these seasons; Hurricane Alex of the 2016 Atlantic

Hurricane Season formed in January!

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Hurricane Formation

Hurricanes

For a mass of unorganized thunderstorms to develop into a hurricane, its surface winds must converge. In the Northern Hemisphere, converging air spins counterclockwise about an area of surface low pressure, often along a region called the intertropical convergence zone (ITCZ). This region is often known for erratic weather and wind patterns, but the wind speeds in this region tend to be low. As a result, sailors named this region of our planet the doldrums.

The necessity for converging air to spin counterclockwise about an area of low pressure prevents hurricanes from forming at the equator. This is because the Coriolis force, a deflection due to the earth’s rotation, is zero at the equator. Instead, tropical cyclones tend to form in tropical regions between 5° and 20° latitude, where they are able to utilize the Coriolis force to create their spin.

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Hurricane Formation

Hurricanes

While tropical cyclones can form anywhere in the Atlantic, many (if not most) of these storms can be traced back to tropical waves that form over Africa (a tropical wave is a low pressure trough moving generally westward with the trade winds). However, only a small fraction of all these tropical disturbances end up becoming hurricanes. Nonetheless, because of the frequency of storm development from waves that enter the Atlantic from Africa, the region between the Lesser Antilles and Cape Verde is often known as the MDR, or main development region.

When the western part of Africa is wet, tropical waves that emerge off the cost are higher, and the likelihood of a major Atlantic hurricane forming is higher.

tropical wave

SAL

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Hurricane Formation

Hurricanes

Supposed that the waters are warm, the air is humid, and the winds are converging; in other words, the conditions at the surface are perfect for cyclogenesis. Why is it that, in some cases, storms are unable to form even with this favorable environment. This is because hurricane formation does not solely rely on surface conditions; conditions aloft need to be favorable as well.

One of the reasons why storms rarely form in the Atlantic MDR during June and July is because of the presence of SAL, or the Saharan Air Layer, as labeled in the previous slide. The SAL is a extremely hot and dry layer of dust that blows off the Sahara Desert and into the Atlantic. Tropical disturbances that ingest this dry air are not often able to survive; this is for two reasons: First, dry air causes evaporation of liquid water, which, since evaporation is a cooling process, reduces the warm core structure of the hurricane and limits vertical development of convection. Second, dry air in the mid levels can create what is known as a trade wind inversion. The trade wind inversion produces a layer of warm temperatures and dryness in the mid levels of the atmosphere due to the sinking and adiabatic warming of the mid level air. This inhibits deep convection and the formation of strong thunderstorms and hurricanes.

In addition, disturbances that move at a fast forward speed are often less likely to develop than storms that have little movement or are stationary.

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Hurricane Formation

Hurricanes

Another condition that inhibits storm formation is the presence of high vertical wind shear. Wind shear is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Strong wind shear tends to disrupt the convection of the storm and disperses heat and moisture, which are necessary for the storm to grow. Below is a picture of Tropical Storm Sergio (2006) heavily affected by wind shear.

As you can see, the circulation of Sergio is naked, and the convection is displaced.

Stronger winds aloft (and wind shear) often occur during El Niño events, when the waters of the Pacific are warmer than normal. As a result, during El Niño there are fewer Atlantic hurricanes than normal. However, the presence of cooler Pacific waters during a La Niña event often result in lower wind shear, which leads to more active hurricane seasons during La Niña years. We will cover El Niño and La Niña in a later unit, but you can preview that topic by clicking here. (This is also a reason why active Pacific hurricane seasons often occur alongside quiet Atlantic hurricane seasons, and vice versa.)

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The Power of Wind Shear

Hurricanes

Ever wonder why tropical systems do very poorly in regions of high wind shear? Look no further than satellite imagery of Tropical Storm Chris in 2006. The high shear near the Caribbean completely separated Chris’s center of circulation from its convection. As a result, the tropical storm did not make it very far.

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Hurricanes

For a hurricane to form, a cluster of thunderstorms must become organized around a central area of surface low pressure. It is important to note that a closed low-level circulation needs to form in order for a disturbance to be labeled as a tropical depression or storm.

The energy for a hurricane comes from the direct transfer of sensible heat and latent heat from the warm ocean surface. The warmer the water that the storm is over and the greater the wind speed, the greater the transfer of sensible and latent heat into the air above. The heat absorbed at the water’s surface by the hurricane is converted into kinetic energy and lost at its top through radiational cooling.

The maximum strength of a hurricane can achieve is proportional to the difference in air temperature between the tropopause and the surface, and to the potential for evaporation from the sea surface. As a result, warmer ocean surfaces tend to lead to hurricanes with lower pressures and, consequently, higher winds. Because there is a limit to how intense storms can become, peak wind gusts rarely exceed 230 mph — or 200 knots. Even one of the strongest recorded hurricanes in the Western Hemisphere, Hurricane Patricia of 2015 (image on left), was not able to break the 200 knot threshold (however, its wind speeds still reached a formidable 215 mph, a rarity in the tropics).

Hurricane Formation

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Hurricane Formation

Hurricanes

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Hurricanes

Most hurricanes last for less than one week, as they inevitably enter colder and more unfavorable environments. Like previously mentioned, dry air and wind shear can weaken a storm, even if it is above warm waters. Cold water is a major killer of hurricanes! As storms travel over colder water, they quickly lose their heat source. Even a small drop in water temperatures beneath the eyewall will noticeably weaken many storms.

A hurricane can also weaken if the layer of warm water beneath the storm is shallow. This is because turbulence caused by the storm’s winds can easily bring cooler water up to the surface, which causes the storm to lose intensity, especially if it is moving slowly.

Lastly, interaction with land can disrupt the circulation and greatly weaken a hurricane. Not only do they lose their energy source when they encounter land, friction causes surface winds to decrease and blow directly into the storm, which raises internal pressure. This is especially good for the United States, as the mountainous islands of Cuba and Hispaniola act as a barrier that often weakens hurricanes before they hit the Gulf Coast!

Hurricane Dissipation

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Hurricanes

Reconnaissance aircraft, called “hurricane hunters” or “recon,” are equipped with various weather instruments that allow them to fly into tropical cyclones and monitor their development. Why are data obtained from recon missions important? While satellites have greatly improved forecasters’ abilities to detect signs of cyclones before they form, they are still not sophisticated enough to determine interior barometric pressures and wind speed information from hurricanes. Thus, information from these aircraft are vital, as they allow meteorologists to accurately predict hurricane development and movement. To the left is an image taken of Hurricane Katrina’s eyewall during a hurricane hunter mission.

Is hurricane hunting a dangerous job? While flying into powerful hurricanes is indeed risky, no reconnaissance aircraft has been lost over the tropical Atlantic since 1955! However, there was a close call when an aircraft lost three of its four engines while flying into Hurricane Hugo in 1989. Nonetheless, skillful piloting brought the battered plane back home safely.

Reconnaissance Aircraft

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Hurricane Stages of Development

Hurricanes

A tropical cyclone often goes through several stages in its life. Initially, systems start off as a tropical disturbance or a tropical wave, a mass of thunderstorms with only slight wind circulation. The tropical disturbance becomes a tropical depression when the winds increase to between 20 and 34 knots, convection is steady over the center, and the storm attains a low-level circulation. When the pressure falls further and the winds reach 34 knots (39 mph), the depression becomes a tropical storm and gets a name. If the tropical storm’s winds were to exceed 64 knots (74 mph), it would become classified as a hurricane.

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Hurricane Tracks

Hurricanes

Hurricanes that form over the warm, tropical North Pacific and North Atlantic are steered by easterly winds and move west or northwest. Gradually, they swing poleward around a subtropical high (also known as the subtropical ridge or Bermuda high, a significant belt of atmospheric high pressure situated around the latitude of 30°N in the Northern Hemisphere) and get caught in the westerly flow, which curves them to the north or northeast. The hurricane’s speed tends to increase as it gains latitude.

Not all hurricanes follow this exact path; in fact, the actual movement of a hurricane depends greatly on the structure of the storm and its environment. Nevertheless, the pattern of storms stays relatively consistent during hurricane season — strange tracks are more often an oddity than a norm.

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Hurricane Tracks

Hurricanes

Below shows the tracks taken by all cyclones worldwide between 1985 and 2005. As it can be seen, storms in the Atlantic often originate from the African coast and curve around the Bermuda high. Storms in the Pacific also tend to move westward; as a result, hurricanes in the Eastern Pacific often stay offshore.

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But wait… what about South America?

Hurricanes

You may have noticed from the previous diagram that tropical cyclones do not form around South America (i.e. the South Atlantic and the eastern South Pacific). Why is this the case?

Cooler water, vertical wind shear, and unfavorable positioning of the ITCZ discourages hurricanes from developing in this region. In fact, the first Southern Atlantic hurricane of the satellite era formed in March 2004, when a rare hurricane developed off the coast of Brazil. Because such an occurrence was so uncommon, no government agency had an effective warning system, and the tropical cyclone was not named (it was later given the name Catarina during post-analysis). An image of Catarina is shown below.

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The Eastern Pacific

Hurricanes

Storms in the Eastern Pacific (or EPAC) tend to move westward away from the coast. As a result, little is often heard about these storms. The few storms that manage to curve northwestward often weaken rapidly over the cooler waters of the North Pacific (however, a few do curve northward or northeastward and slam into Mexico).

Cold sea surface temperatures are a major reason why California and the west coast do not experience hurricanes. Even off the shore of Southern California during the summer, sea surface temperatures rarely rise above 24°C, or 75°F. In fact, there is only one hurricane on record to have reached the west coast of the United States with sustained hurricane winds, ramming into California’s southern tip in October 1858.

Temperatures off the California coast are very cold!

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The Eastern Pacific

Hurricanes

One might wonder about the impact of hurricanes in the Eastern Pacific on the Hawaiian islands, which are located between 20°N and 23°N. It seems reasonable to assume that these islands are right in the path of many tropical cyclones. While this is true, storms that do reach the islands have frequently weakened considerably and often pass harmlessly to the south or northeast. Two notable exceptions are Hurricanes Iwa of 1982 and Iniki of 1992. Iniki, shown below, was the worst hurricane to hit Hawaii in the twentieth century, with sustained winds of 114 knots (about 130 mph) and gusts up to 140 knots (about 160 mph).

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The Atlantic

Hurricanes

The deadliest hurricanes in human history often form in the Atlantic basin. While hurricanes in the Eastern Pacific rarely garner any attention (such as the powerful Hurricane Fernanda in 2017, as shown 20 slides earlier), storms that form in the Atlantic are more frequently reported on as areas of concern. Why is this the case? Because steering patterns tend to drive hurricanes westward, storms in the Eastern Pacific move away from land. But storms that head westward in the Atlantic move toward land — on a collision course with North or Central America!

It must be said, however, that many storms in the Atlantic curve around the subtropical ridge well before they even come close to the eastern seaboard of the United States. But on occasions where the ridge is very strong or farther west, a few storms manage to move inland, bringing strong winds and waves and, unfortunately with some systems, immense devastation.

To the right is the track of Hurricane Earl of 2010. Earl’s track is a good example of steering in the Atlantic: a storm forms off the coast of Africa and curves around the subtropical ridge (or Bermuda high).

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The Western Pacific

Hurricanes

While we will not focus much on the Western Pacific in this unit, it is noteworthy to mention that this basin also produces its fair share of devastating storms. Like with the Atlantic, storms that form in the Western Pacific (called typhoons) move westward, a steering pattern that also brings them on a collision course with Asia. One very notable storm was Typhoon Haiyan in 2013, the strongest landfalling hurricane on record. Haiyan killed at least 6,300 people in the Philippines alone and left nearly 11 million homeless.

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The Western Pacific

Hurricanes

The Western Pacific is also known for generating more storms than the Atlantic basin. On average, the Western Pacific produces 26 storms, while the Atlantic only produces 12. Why is this difference so large?

There are two notable reasons for this occurrence. The first, as shown below, is that water temperatures in the Western Pacific are extremely warm (often upwards of 30°C; comparable temperatures in the Atlantic are often only found in the Gulf and Caribbean). The second is the presence of SAL (dry air from the Sahara Desert) in the Atlantic, which often inhibits cyclone formation.

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Hurricane Naming

Hurricanes

Have you ever wondered how storms are named? Many have the misconception that names are randomly chosen by the National Hurricane Center once a storm forms. In 2017, for instance, the naming of tropical storms Don and Hilary prompted many to believe that these names were chosen politically. However, nothing can be further from the truth! Instead, hurricane names are chosen off predetermined lists that are recycled every six years. The names used during the 2017 were first used during the 1981 season!

This, however, was not always the case. Before hurricanes were named, they were instead identified using their latitude and longitude. This method was confusing, so during World War II, storms began to be identified using letters of the alphabet. This method was improved upon in 1953, when the National Weather Service began using female names to identify hurricanes, with names given in alphabetical order. Beginning in 1979, storms also attained male names, which were alternately assigned (if the A named storm was female, for example, the B named storm would be male, and so forth). Names from different languages were incorporated as well in 1979, giving us the naming process we have today. There are six different name lists that alternate every six years for both the Atlantic and Eastern Pacific (so 12 in total). The Western Pacific uses a different process to identify storms, which will be covered in brief later.

Lastly, if all names in the list are exhausted during a hurricane season, tropical storms then become identified with letters in the Greek Alphabet. This has only happened once so far, during the record breaking 2005 season. The Atlantic and Pacific names are included on the next two slides.

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Atlantic Hurricane Names

Hurricanes

List I	List II	List III	List IV	List V	List VI
Ana	Alex	Arlene	Alberto	Andrea	Arthur
Bill	Bonnie	Bret	Beryl	Barry	Bertha
Claudette	Colin	Cindy	Chris	Chantal	Cristobal
Danny	Danielle	Don	Debby	Dorian	Dolly
Elsa	Earl	Emily	Ernesto	Erin	Edouard
Fred	Fiona	Franklin	Florence	Fernand	Fay
Grace	Gaston	Gert	Gordon	Gabrielle	Gonzalo
Henri	Hermine	Harvey	Helene	Humberto	Hanna
Ida	Ian	Irma	Isaac	Imelda	Isaias
Julian	Julia	Jose	Joyce	Jerry	Josephine
Kate	Karl	Katia	Kirk	Karen	Kyle
Larry	Lisa	Lee	Leslie	Lorenzo	Laura
Mindy	Martin	Maria	Michael	Melissa	Marco
Nicholas	Nicole	Nate	Nadine	Nestor	Nana
Odette	Owen	Ophelia	Oscar	Olga	Omar
Peter	Paula	Philippe	Patty	Pablo	Paulette
Rose	Richard	Rina	Rafael	Rebekah	Rene
Sam	Shary	Sean	Sara	Sebastien	Sally
Teresa	Tobias	Tammy	Tony	Tanya	Teddy
Victor	Virginie	Vince	Valerie	Van	Vicky
Wanda	Walter	Whitney	William	Wendy	Wilfred

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Pacific Hurricane Names

Hurricanes

List I	List II	List III	List IV	List V	List VI
Andres	Agatha	Adrian	Aletta	Alvin	Amanda
Blanca	Blas	Beatriz	Bud	Barbara	Boris
Carlos	Celia	Calvin	Carlotta	Cosme	Cristina
Dolores	Darby	Dora	Daniel	Dalila	Douglas
Enrique	Estelle	Eugene	Emilia	Erick	Elida
Felicia	Frank	Fernanda	Fabio	Flossie	Fausto
Guillermo	Georgette	Greg	Gilma	Gil	Genevieve
Hilda	Howard	Hilary	Hector	Henriette	Hernan
Ignacio	Ivette	Irwin	Ileana	Ivo	Iselle
Jimena	Javier	Jova	John	Juliette	Julio
Kevin	Kay	Kenneth	Kristy	Kiko	Karina
Linda	Lester	Lidia	Lane	Lorena	Lowell
Marty	Madeline	Max	Miriam	Mario	Marie
Nora	Newton	Norma	Norman	Narda	Norbert
Olaf	Orlene	Otis	Olivia	Octave	Odalys
Pamela	Paine	Pilar	Paul	Priscilla	Polo
Rick	Roslyn	Ramon	Rosa	Raymond	Rachel
Sandra	Seymour	Selma	Sergio	Sonia	Simon
Terry	Tina	Todd	Tara	Tico	Trudy
Vivian	Virgil	Veronica	Vicente	Velma	Vance
Waldo	Winifred	Wiley	Willa	Wallis	Winnie
Xina	Xavier	Xina	Xavier	Xina	Xavier
York	Yolanda	York	Yolanda	York	Yolanda
Zelda	Zeke	Zelda	Zeke	Zelda	Zeke

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Central North Pacific Hurricane Names

Hurricanes

List I	List II	List III	List IV
Akoni	Aka	Alika	Ana
Ema	Ekeka	Ele	Ela
Hone	Hene	Huko	Halola
Iona	Iolana	Iopa	Iune
Keli	Keoni	Kika	Kilo
Lala	Lino	Lana	Loke
Moke	Mele	Maka	Malia
Nolo	Nona	Neki	Niala
Olana	Oliwa	Omeka	Oho
Pena	Pama	Pewa	Pali
Ulana	Upana	Unala	Ulika
Wale	Wene	Wali	Walaka

These names are used one after another. When the bottom of the list is reached, the next name comes from the first name in the next list.

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Western Pacific Typhoon Names

Hurricanes

List	Contributing nation
List	Cambodia	China	North Korea (DPRK)	Hong Kong	Japan	Laos	Macau	Malaysia	Micronesia	Philippines	South Korea (ROK)	Thailand	United States	Vietnam
1	Damrey	Haikui	Kirogi	Kai-tak	Tembin	Bolaven	Sanba	Jelawat	Ewiniar	Maliksi	Gaemi	Prapiroon	Maria	Son-Tinh
1	Ampil	Wukong	Jongdari	Shanshan	Yagi	Leepi	Bebinca	Rumbia	Soulik	Cimaron	Jebi	Mangkhut	Barijat	Trami
2	Kong-rey	Yutu	Toraji	Man-yi	Usagi	Pabuk	Wutip	Sepat	Mun	Danas	Nari	Wipha	Francisco	Lekima
2	Krosa	Bailu	Podul	Lingling	Kajiki	Faxai	Peipah	Tapah	Mitag	Hagibis	Neoguri	Bualoi	Matmo	Halong
3	Nakri	Fengshen	Kalmaegi	Fung-wong	Kammuri	Phanfone	Vongfong	Nuri	Sinlaku	Hagupit	Jangmi	Mekkhala	Higos	Bavi
3	Maysak	Haishen	Noul	Dolphin	Kujira	Chan-hom	Linfa	Nangka	Saudel	Molave	Goni	Atsani	Etau	Vamco
4	Krovanh	Dujuan	Surigae	Choi-wan	Koguma	Champi	In-fa	Cempaka	Nepartak	Lupit	Mirinae	Nida	Omais	Conson
4	Chanthu	Dianmu	Mindulle	Lionrock	Kompasu	Namtheun	Malou		Rai	Malakas	Megi	Chaba	Aere	Songda
5			Meari	Ma-on	Tokage		Muifa	Merbok	Nanmadol	Talas	Noru	Kulap	Roke	Sonca
5	Nesat	Haitang	Nalgae	Banyan	Hato	Pakhar	Sanvu	Mawar	Guchol	Talim	Doksuri	Khanun	Lan	Saola

These names are used one after another. Unlike the other basins, the Western Pacific season lasts all year, so the names are not grouped by year. Each nation contributes 10 names that are used on the rotating lists.

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What if a storm crosses basins?

Hurricanes

You may wonder: what happens if a storm crosses into a different basin? Does it get a new number or a new name? Before 2000, the answer was yes to both. When Hurricane Cesar crossed from the Atlantic to the Pacific in 1996, it was given a new number and name: Douglas.

However, to mitigate the confusion that may arise when mentioning a storm with two different names, this policy was discarded in 2000. Now, if a storm were to cross over into a different basin, it would receive a new identification number but would retain the same name. The first storm that crossed basins after this policy was enacted was Hurricane Otto of 2016. Below is an image of Otto crossing from the Atlantic into the Pacific.

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Name Retirement

Hurricanes

Will there ever be another storm named Katrina? While names are repeated every six years, what happens to those names that end up with storms so devastating that naming another cyclone with it would cause confusion? There is a process that occurs in which names can be removed from the rotating lists: name retirement. If a storm is so deadly or costly that the future use of its name on a different storm would be inappropriate, the offending name is retired from the list at an annual meeting of the World Meteorological Organization and another name is selected to replace it. Any nation impacted by a severe hurricane can lobby the WMO to have the hurricane's name retired.

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An Invest Forms

Hurricanes

In the Atlantic, a tropical wave touches down from the coast of Africa. What now?

Forecasters at the National Hurricane Center (NHC) work day in and day out to provide up-to-date information on tropical cyclone development and forecasts. If a disturbance in the Atlantic seems promising for development, it gets tagged as an investigative area or invest and is assigned a number between 90 and 99 (this is rotated during the season, much like name lists) and a suffix depending on the basin the invest is in (in the Atlantic, this suffix is “L”).

How do these meteorologists know how likely a storm is to develop? A healthy looking wave off the coast of Africa may succumb to dry air and completely disappear within 24 hours, while a disorganized mess can, in favorable conditions, rapidly intensify into a powerful hurricane. The answer lies in the models.

Hurricane specialists have access to valuable models, such as the GFS and the ECMWF (Euro), that allow them to track storms and monitor forecasts with great precision. Without these models, tropical analysis would not be as advanced as it is today. For instance, the European model (ECMWF) was the first to predict Hurricane Sandy’s unusual turn toward New England, a forecast that undoubtedly saved lives. As technology improves, there is no doubt that our abilities to forecast storms will get even better.

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Models Aren’t Always Perfect!

Hurricanes

However, it is important to note that models do not always get it right! Many times they spin up storms that end up failing to develop, or they may even fail to predict cyclogenesis altogether. In fact, here’s a nice tidbit of two GFS model runs a mere 6 hours from each other. The first forecasted a major hurricane devastating much of the east coast, while the second forecasted a system wiping out Florida and the entire Gulf coast. The system, it turns out, never actually developed. Nonetheless, models bring us more hits than misses, and despite their imperfection, we are all the better because of them.

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Studying Models

Hurricanes

Spaghetti models are often used to give an indication of where a tropical cyclone is headed. These spaghetti plots are employed in the method of ensemble forecasting, which runs several forecast models, each beginning with slightly different weather information. The closer these ensembles, the greater the confidence one would have in the storm’s path. On TV, you may sometimes see a cone of uncertainty, or a region where the center of a storm may pass over in the future. In the past few decades, cyclone forecasting has vastly improved; today, the average error for a projected hurricane three days in advance is 278 km — a large improvement from the 708 km error of the 1970s. Hurricane intensity, however, has shown little improvement since the early 1990s, and is still the area that challenges forecasters most.

MC Question on 9/10

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Hurricanes

Once an invest or disturbance attains a low level circulation, adequate winds, and convection around its center, it becomes a tropical depression and is assigned a number by the NHC. This number starts at 1 and does not rotate (unlike the invest numbers, which can only be between 90 and 99). For instance, the fifth tropical depression of the season would be known as Tropical Depression Five.

If the storm reaches sustained winds of 39 mph or above, the storm is designated as a Tropical Storm and is given a name from the rotating name list for that year. That storm, however, is still identified using the number it received upon becoming a depression. In addition, some disturbances jump straight to tropical storm status without ever existing as a tropical depression. In that case, a number is still assigned to the storm (for instance, if the sixth depression of the season becomes a tropical storm without ever being a tropical depression, it would still be given the number 6, and the next depression that forms would be known as Tropical Depression Seven).

A Tropical Depression Forms

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Hurricanes

On late August 2016, a tropical wave emerged into the Atlantic ocean from the coast of Africa. Noticing that this storm was plausible for further development, the NHC designated the disturbance as Invest 99L. This disturbance would fail to achieve tropical depression status for nearly two weeks as it aimed itself toward Florida.

Unfortunately, at the time, NHC policy prevented them from issuing a hurricane or tropical storm watch or warning until after a tropical cyclone had formed. For invests and disturbances that could develop right before landfall, no warnings could be given! For the 2017 season, however, this was changed.

In addition to tropical depressions, disturbances near land that had a potential to develop was also given numbers from the list given to depressions. This was first implemented in June 2017, when a disturbance near the Lesser Antilles was assigned the number 2 (since it was the 2nd disturbance to get numbered) and became Potential Tropical Cyclone 2.

Potential Tropical Cyclones

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Hurricanes

A special type of cyclone called an extratropical cyclone has cold air at its core and derives its energy from the release of potential energy when cold and warm air masses interact. These storms always have one or more fronts connected to them, and can occur over land or ocean. An extratropical cyclone can have winds as weak as a tropical depression, or as strong as a hurricane. Examples of extratropical cyclones include blizzards, Nor'easters, and ordinary low pressure systems that give the continents at mid-latitudes much of their precipitation.

However, if an extratropical cyclone stays over warm water for a prolonged period of time, it may begin to develop tropical characteristics. During this transition period, the storm is known as a subtropical cyclone. Because the core of a subtropical system is colder than a fully tropical one, they can be found in areas where tropical development is less common (such as over colder waters).

Unlike tropical cyclones, subtropical cyclones often have a cloud-free center of circulation, with storms miles away from the center. Subtropical systems tend to have weaker winds and produce less rain. Because of this, a subtropical system must become fully tropical for it to achieve hurricane winds — a subtropical hurricane does not exist! Beginning in 2002, subtropical storms were also given names and numbers from the lists used each hurricane season.

Subtropical Cyclones

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Hurricanes

It is often taken as common sense that tropical cyclones tend to weaken over land. However, in some cases, they don’t! For example, 2016’s Tropical Storm Julia formed in an extremely unusual location — not over any body of water, but rather over the state of Florida! Tropical Storm Erin in 2007 also stumped many meteorologists when it strengthened and developed an eye over Oklahoma.

How is this possible? This phenomenon can be attributed to the brown ocean effect, in which the land a storm is over mimics the moisture-rich environment of the ocean. This effect is named the way it is because storms that take advantage of it derive their energy from the evaporation of abundant soil moisture — a “brown ocean.”

Variations in climate, soil, and vegetation all play a role in determining how likely a tropical cyclone is to strengthen inland. While intensification over land is rare, it’s not impossible: if the atmosphere resembles a tropical atmosphere with minimal variation in temperature and the soil contains ample moisture, a system can still fuel up — even without its main source.

Brown Ocean Effect

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Hurricanes

When a storm is over the open waters, there are two phases that it goes through: diurnal maximum and diurnal minimum. Diurnal maximum occurs during the night, and convection (thunderstorm activity) associated with the storm is greatest. Diurnal minimum occurs during the day, when convection may temporarily taper off.

Why does this happen? Remember from Unit 3 that water has a higher specific heat capacity than land and air. In other words, it takes more heat to raise the temperature of water by one degree, requiring more time for it to warm and cool. During the day, the temperature of the air is greater than the temperature of the water, making it hard for air to rise (since the air near the surface of the water is cooler and already under the warmer air above it). Thus, convective storm formation is repressed. However, during the night, the water is warmer than the air above it, allowing warmer air parcels near the water’s surface to rise above the cooler air overhead. This movement of air allows tropical disturbances to fire off stronger convection during the nighttime hours, most notably right before the sun rises.

Diurnal Maximum and Diurnal Minimum

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The Madden-Julian Oscillation (MJO)

Hurricanes

An important global circulation that plays a role during hurricane season is the Madden-Julian Oscillation, or MJO. The MJO is an atmospheric pulse of clouds, rain, wind, and pressure in the tropics that propagates eastward around the globe over a period of 30 to 60 days. There are two phases of the MJO, the enhanced phase and the suppressed phase. These phases correspond with upward swingers in rainfall and drier conditions, respectively. The origins of a MJO pulse can be traced back to the Indian Ocean.

How does the MJO influence tropical activity? When the enhanced phase enters the Gulf or Atlantic, tropical activity tends to increase for a period of up to two weeks. The environment associated with an enhanced MJO, such as increased thunderstorm activity and lower wind shear, is extremely favorable for cyclone development; in fact, hurricanes are four times more likely to form during the enhanced MJO.

During the suppressed phase, air sinks across large swaths of the ocean basin, limiting thunderstorm growth. This results in a multi-week period lacking in tropical systems.

Do realize that the MJO is just one factor that affects the development of tropical systems. Other variables must also be considered to determine the level of tropical activity. Tropical systems can grow in the suppressed phase of the MJO, but they have a more difficult time intensifying. Because of the nature of MJOs, its presence often causes the Pacific and Atlantic basins to have opposite levels of activity at any point in time. In addition, the MJO is more active when neither El Niño nor La Niña events are present.

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The Madden-Julian Oscillation (MJO)

Hurricanes

The below image illustrates the effect that the MJO can have on global tropical activity. Notice that the Western Pacific, which is in an enhanced phase, is very active, while the Eastern Pacific and Atlantic, which are in a suppressed phase, are more quiet with regards to thunderstorm development. Once the MJO moves eastward, the levels of activity between the two regions will switch.

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Hurricanes

On the morning of October 22, 2015, Hurricane Patricia was a minor Category 1 hurricane riding along Mexico’s western coast. Just 24 hours later, Patricia was the strongest storm ever recorded in the Western Hemisphere, with sustained winds of up to 215 mph. This explosive strengthening was possible because extremely favorable conditions allowed Patricia to go through a process called rapid intensification, or RI.

According to the National Hurricane Center, rapid intensification involves an 30 kt (35 mph) or more increase in the maximum sustained winds of a tropical cyclone in a 24-h period. Rapid intensification can occur regardless of the storm strength. Because rapid intensification is often coupled with a rapid decrease in a storm's minimum pressure, RI is also known as rapid deepening (rapid deepening involves a 42+ mbar pressure drop in a day).

While favorable conditions may induce rapid intensification, the occurrence and timing of RI (and if the storm even undergoes it) is unpredictable. RI can happen with any storm at any location, but the most frequent instances of RI occur in lower Atlantic, Caribbean, and Gulf of Mexico.

Rapid Intensification (RI)

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Hurricanes

Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in powerful hurricanes, generally with winds greater than 185 km/h (115 mph). Cyclones that reach this intensity often have very small eyewalls, allowing outer rainbands to strengthen and organize into a separate outer eyewall. This outer eyewall then slowly moves inward and “chokes” the inner eyewall, robbing it of its needed moisture.

During eyewall replacement cycles, hurricanes noticeably weaken. They often cannot strengthen to their previous state until the outer eyewall completely replaces the inner eyewall, a process that may take quite a bit of time. However, if the outer eyewall successfully takes over, the storm may be able to once again strengthen.

Almost every intense hurricane undergoes at least one of these cycles during its existence. The discovery of this process discontinued a experiment known as Project Stormfury, in which scientists believed that seeding hurricanes with silver iodide could weaken them. What seemed like progress from human induced modification was actually the natural process of eyewall replacement!

Eyewall Replacement Cycles

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Hebert Boxes

Hurricanes

A Hebert Box is one of two regions of the tropical Atlantic Ocean that are useful as predictors of hurricanes that will strike South Florida, USA. If a tropical cyclone passes through the small geographic area of a Hebert Box, there is a strong likelihood that the storm will affect Florida.

The first Hebert Box is located east of Puerto Rico over the US Virgin Islands, between 15° and 20° north latitude and 60° to 65° west longitude. This box is most useful in the first half of the Atlantic hurricane season, from June to September. The second Hebert Box is located over the Cayman Islands between 15° and 20° north latitude and 80° to 85° west longitude. This box is most useful in the second half of the Atlantic hurricane season, from September to November.

Passage through a Hebert Box does not guarantee a

landfall in Florida, nor does missing the box mean that

Florida is safe. Several historical storms have passed

through a Hebert Box without affecting land (Earl 2010),

while others have struck Florida without passing through

either Hebert Box (Andrew 1992). However, if a cyclone

does enter a box, it provides forecasters with a message

to pay close attention.

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Storm Surge

Hurricanes

When a hurricane is approaching from the south, its highest winds are usually on its eastern side. The reason for this is that the winds that push the storm forward add to the winds on the east side and subtract from the west. But it’s not just the winds that are dangerous within a hurricane; huge waves, high seas, and flooding also cause much of the destruction. In fact, flooding makes up the majority of hurricane-related deaths during the past century.

Hurricanes often come with devastating storm surge, an abnormal rise of several meters in the ocean level, which inundates low-lying areas and destroys coastal structures. The storm surge is especially dangerous when it coincides with high tides. In addition, a hurricane will cause more storm surge in areas where the ocean floor slopes gradually. Shallow water coastlines are most vulnerable to storm surge damage. Storm surges are frequently considered the most devastating element of a hurricane!

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The Saffir-Simpson Scale

Hurricanes

What do we use to categorize hurricanes? To estimate the possible damage a hurricane’s sustained winds and storm surge could do to a coastal area, the Saffir-Simpson Scale was developed. This scale places hurricanes into five categories depending on its sustained winds. A hurricane that is classified as Category 3 or above (i.e. winds exceeding 110 mph) is a major hurricane. In the western Pacific, a typhoon with sustained winds of at least 130 knots or 150 mph is considered a super typhoon. The stronger a hurricane, the more likely it is to deliver catastrophic storm surge!

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The Saffir-Simpson Scale

Hurricanes

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Tornadoes

Hurricanes

While we will focus more on tornadoes in the next unit, they do contribute to the damage that may result from a hurricane. About 25% of the hurricanes that strike the United States produce tornadoes. Tornadoes tend to form in the right front (northeast) quadrant of the advancing hurricane.

It is important to never confuse a tornado with a hurricane, as tornadoes and hurricanes are very different! While they may both contain strong rotating winds that can cause damage, the similarities end there.

Also, storm surge is different from a tsunami. While storm surge is caused by winds blowing sea water into the coast, tsunamis arise from disturbances on the ocean floor, mostly due to earthquakes.

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The Record Breaking 2005 Atlantic Hurricane Season

Hurricanes

Up until 2005, the annual death toll from hurricanes in the United States averaged less than 50 people. Most of these fatalities were attributed to flooding, as the advanced warnings given by the National Weather Service often provided enough notice to allow residents to prepare before an incoming storm.

This average rose dramatically after the 2005 season — a record shattering season that used up all the letter names for the first time and produced a record 15 hurricanes, of which 7 were major. Powerful hurricanes — including Dennis, Emily, Rita, Stan, and Wilma — struck land during this season and often resulted in large death tolls. Hurricane Wilma, for instance, was the strongest storm ever recorded in the Atlantic basin, with an astounding pressure of 882 mb. Yet, despite the record breaking activity and intensity of the 2005 storms, one storm became the symbol for this devastating season: Hurricane Katrina.

Katrina was the costliest Atlantic hurricane on record, killing between 1,300 and 1,800 people in its path. What made Hurricane Katrina especially devastating was its impact on New Orleans; when the levees broke, parts of the city were inundated with water over 20 feet deep. In addition, Katrina’s storm surge wiped out even some of the sturdiest structures in its wake. The background on Slide 5 (in the table of contents) gives a glimpse of the damage wrought by this powerful hurricane.

Will 2005-level Atlantic activity ever happen again? We cannot tell for sure. But as population density continues to increase in vulnerable coastal areas, the need to prepare becomes ever more important.

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Hurricanes and Climate Change

Hurricanes

The record-shattering 2005 Atlantic Hurricane season may lead one to wonder: does climate change play a role in all this? Hurricanes do indeed fuel themselves from warmer waters, and a mere 0.6°C (1°F) increase in ocean temperature can increase a storm’s maximum winds by about 5 knots (6 mph). Studies have shown that water temperatures in 2005 were warmer than average, with half of that warming attributed to global greenhouse gas emissions. Thus, it can be hypothesized that the sheer intensity of some of the 2005 systems could have been worsened by the warming of the planet.

But can it be definitively stated that climate change will lead to stronger, more frequent, and more devastating hurricanes in the future? That claim may, in fact, be a reach.

Regarding hurricane strength, it is also important to note that warm water is not the only factor that plays a role in determining a storm’s intensity. If that were the case, powerful Category 5 storms would become ever more common with each hurricane season. But between the 2007 and 2016 Atlantic hurricane seasons, there was a Category 5 drought: no hurricane managed to attain Cat 5 status in the Atlantic until Hurricane Matthew of 2016. The 2013 Atlantic season was especially weak, with over 85% of the storms failing to reach hurricane status.

Frequency is also difficult to predict, as different models give different conclusions on its connection with climate change; in fact, many other factors may give reasons behind fluctuations in the number of storms each year. The same goes with the claim of more frequent landfalls — between Hurricane Wilma’s landfall in 2005 and Hurricane Harvey’s landfall in 2017, the U.S. saw 4,323 days of major hurricane drought! Thus, any trends due to climate change should be analyzed and integrated with reliable data on past activity before a conclusion can be made.

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Hurricane Watches and Warnings

Hurricanes

Here is a list of watches and warnings often associated with tropical cyclones:

A tropical storm watch is issued when tropical storm conditions with sustained winds from 39 -74 mph are possible in the area within the next 36 hours.
A tropical storm warning is issued when tropical storm conditions are expected in the area within the next 24 hours.
A hurricane watch is issued when hurricane conditions with sustained winds of 74 mph or greater are possible in the area within the next 36 hours.
A hurricane warning is issued when hurricane conditions are expected in the area within 24 hours.
A coastal flood watch is issued when the possibility exists for the inundation of land areas along the coast within the next 12 to 36 hours.
A coastal flood warning is issued when land areas along the coast are expected to become, or have become, inundated by seawater above the typical tide action.
A small craft advisory is a type of warning issued by the National Weather Service, most frequently in coastal areas. It is issued when winds have reached, or are expected to reach within 12 hours, a speed marginally less than that which is considered gale force (25-38 mph).

Since the winds of a hurricane can extend for large distances from the center, hurricane warnings tend to be issued for large swaths of coastal area, usually about 550 km (342 mi) in length.

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Hurricane Safety Tips

Hurricanes

Before a hurricane, have a disaster plan ready. Board up windows and bring in outdoor objects that could blow away. Make sure you know which county or parish you live in, and know where all the evacuation routes are. Prepare a disaster supplies kit for your home and car. Include a first aid kit, canned food, a can opener, bottled water, a battery-operated radio, a flashlight, protective clothing and written instructions on how to turn off electricity, gas, and water. Have a NOAA weather radio handy with plenty of batteries, so you can listen to storm advisories. Have some cash handy as well, as banks and ATMs tend to be closed after a hurricane. Make sure your car is filled with gasoline.

During a hurricane, stay away from low-lying and flood prone areas. Always stay indoors during a hurricane, because strong winds will blow things around. If you live in a mobile home, leave and to go to a shelter. If your home isn’t on higher ground, go to a shelter. If emergency managers tell you to evacuate, do so immediately.

After a hurricane, stay indoors until it is safe to come out. Check for injured or trapped people, without putting yourself in danger. Watch out for flooding which can happen after a hurricane. Do not attempt to drive in flooding water. Stay away from standing water. It may be electrically charged from underground or downed power lines. Don’t drink tap water until officials say it’s safe to do so.

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Historical Hurricanes

Hurricanes

Below are some articles on historical winter storms. It may be beneficial for you to study these prior to the competition. These articles will be updated as new events occur.

this slide will be updated upon the completion of the rest of the slideshow

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Tornadoes

“In the eye of the tornado, there's no

more high and low, no floor and sky.”

― Francis Alÿs, Belgian Artist

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Tornadoes

A tornado is a rapidly rotating column of air that extends down from a cumulonimbus cloud. They blow around a small area of intense low pressure with a circulation that touches the ground. Tornadoes can take many shapes, but the most common come in the form of funnels or wedges. A funnel cloud is a tornado that is beginning to form but has not reached the ground. Only about 30% of funnel clouds manage to touch the ground to become an active tornado.

In North America, tornadoes tend to rotate counterclockwise around their center of low pressure. The majority of tornadoes have wind speeds of less than 100 knots (or 115 mph), but an occasional few may have winds exceeding 220 knots (or 253 mph). The diameter of most tornadoes lies between 100 and 600 m, and these storms often last only a few minutes, with a path length of 7 km (or 4 mi). In addition, the majority of tornadoes tend to move from the southwest toward the northeast at speeds between 20 and 40 knots.

Tornadoes: A Few Facts

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Tornadoes

Major tornadoes usually go through a series of stages. The first of these stages is the dust-whirl stage, where dust swirling upward from the surface marks the low pressure circulation on the ground where the funnel often extends downward toward. Damage during this stage is normally light. The next stage is the organizing stage, when the tornado begins intensifying as the funnel begins to reach the ground. The mature stage that follows is the deadliest, as the tornado reaches its greatest severity. However, in the subsequent shrinking stage, the tornado begins shrinking in width and its funnel begins to tilt. Lastly, the decay stage occurs when the storm stretches out, often becoming contorted before it dissipates.

Not every tornado goes through every single stage; some go directly into decay after the organizing stage. However, if a tornado reaches the mature stage, its circulation often stays in contact with the ground until it dissipates.

The Tornado Life Cycle

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Tornado Occurrence and Distribution

Tornadoes

While tornadoes can occur anywhere around the world, they are most common in the United States. With an average of more than 1,000 tornadoes annually, the United States experiences more tornadoes than any other country (in comparison, the country with the second-most tornadoes, Canada, has an average of only 100 tornadoes per year). This spread is not evenly distributed among the states either; the greatest number of tornadoes occur in a belt of states known as Tornado Alley, located in the Central Plains that stretch from Texas to Nebraska. A separate belt where tornadoes are also common is the Dixie Alley, which encompasses Mississippi and Alabama.

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Tornado Occurrence and Distribution

Tornadoes

Why are tornadoes most common in the Central Plains? This is due to the favorable environment that this region often provides to the birth of severe thunderstorms that form tornadoes. During the springtime, the Plains are known to have a conditionally unstable atmosphere, with warm air at the surface is covered by cooler, drier air aloft. When the surface air is forced upward in the presence of strong vertical wind shear, large supercell thunderstorms are capable of forming. This is why tornadoes are most common during the spring (and least common during the winter, when warm surface air is normally absent). A chart that depicts tornado frequency by month is shown to the right.

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Tornado Occurrence and Distribution

Tornadoes

Tornadoes often exist where the contrast between warm and cold air masses is the greatest (see Unit 18 for more information on air masses). Because this contrast is greatest at different regions during the year, the frequency of tornadic activity shifts seasonally. During the winter, tornadoes are more likely to form over the southern Gulf states, while during the summer, tornadoes tend to form farther north, often including states touching the Canadian border. However, even with these patterns, no location is immune to tornadoes at any time!

About 70% of all tornadoes in the United States take place between March and July, with the greatest frequency in the month of May (an average of 9 per day!). The most violent tornadoes occur in April, when vertical wind shear is often present and horizontal and vertical temperature and moisture contrasts are greatest.

Tornadoes can occur at all times of the day, but they are most frequent in the late afternoon (between 3 PM and 7 PM), when surface air is the most unstable. On the other hand, tornadoes are least frequent in the early morning before sunrise, when the atmosphere is most stable.

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Tornado Occurrence and Distribution

Tornadoes

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Tornado Winds

Tornadoes

In the past, the winds of a tornado were often determined using observations of damage. However, we are able to measure wind speeds more accurately today using Doppler radar. Studies have shown that even the most powerful tornadoes seldom reach wind speeds above 220 knots, and most have wind speeds lower than 125 knots.

Many violent tornadoes (often those with winds exceeding 180 knots) contain smaller whirls that rotate within them. These tornadoes are called multi-suction tornadoes, and the smaller whirls are known as suction vortices. Suction vortices are typically only about 10 m or 30 ft in diameter, but they can spin very fast and cause large-scale damage.

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Tornado Winds

Tornadoes

If you look at the diagram below, a tornado that approaches from the southwest has its strongest winds on the southeast side. Because of the storm’s counterclockwise motion, its rotational speed and translational speed add on the side where the rotational and forward motion is in the same direction (consequently, the winds are weakest on the northwest side, as part of the rotational motion cancels out the forward translational motion). ask challenging question on this w/ diff dir.

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Tornadoes

The high winds of a tornado cause the most damage as walls of buildings collapse when blasted by the high winds and debris associated with the storm. In addition, as high winds blow over a roof, lower air pressure forms above the roof. As a result, the higher pressure inside the building lifts the roof high enough for the winds to carry it away.

It was once thought that opening the windows of a building would minimize the probability of a building exploding. This logic stems from the belief that opening the windows allows the pressures to equalize, preventing a pressure gradient from destroying the building. However, it is now known that opening windows actually increases the chances of a house getting destroyed. Why? Opening windows does not equalize the pressure; it instead increases the pressure on the wall opposite the window, making it more likely to collapse.

A Tornado’s Destruction

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Tornadoes

The best course of action to take when a tornado is on the ground is to seek shelter immediately. The best place to take shelter in a house is in the basement or in a storm shelter. If these options are not available, the safest place is usually in a small room on the lowest floor in the center of the building. Try to find a room without any windows. To add another level of safety, it is recommended that you cover yourself with a mattress or other form of protection. Employing a helmet will be helpful in protecting you from debris.

If you are at school, move to the hallway and lie flat with your head covered. If you are in a mobile home, leave immediately and seek other shelters. If none are available, lie flat on the ground in a depression or a ravine.

If you are in a car, do not outrun an oncoming tornado, as they often cover erratic paths with winds above 80 mph. If you are at a distance from the tornado, drive at a right angle to the tornado’s path. Avoid highway overpasses, as the tornado’s winds may be funneled through the structure.

Tornado Safety Measures

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Tornado Watches and Warnings

Tornadoes

A tornado watch is issued to alert the public that tornadoes may develop within a specific area during a certain time period, typically a few hours long. Volunteer spotters in each community look for tornadoes after watch is issued. If a tornado is spotted on the ground, either visually or on radar, a tornado warning is issued by the National Weather Service, often covering parts of one or several counties and lasting from 30 to 45 minutes. On rare occasions, a tornado emergency is issued when a strong tornado threatens a populated area. Sirens and radio/TV are used to broadcast warnings to the public, and newer cellphones are able to send alerts to the user automatically. While this method is not flawless, it does save many lives. Even though some warnings can be false alarms, and some tornadoes can touch down without a warning in place, tornado related deaths have decreased as a result of this procedure (with the exception of 2011, an exceptionally deadly year for tornadoes, which we will discuss in the historical tornadoes slide).

In the 1970s, the average lead time for a tornado warning was 3 minutes. Today, the lead time is often more than 20 minutes. With new technology and resources, meteorologists hope to incorporate data that could increase the lead time of tornado warnings to as long as 1-2 hours.

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The Fujita Scale

Tornadoes

In the late 1960s, Dr. Theodore Fujita developed a scale for classifying tornadoes according to their rotational wind speeds, which were estimated based on the damage caused by the storm. This scale was known as the Fujita scale. In 2007, a new scale (called the Enhanced Fujita scale) was adopted to provide a wide range of criteria in estimating a tornado’s winds by using a set of 28 damage indicators, of which included a variety of structures ranging from barns to homes to schools to even trees. This new scale also takes into account the building’s construction quality. A combination of these damage indicators and the degree of damage provides a range of probable wind speeds and an “EF” rating for the tornado. EF0 is the weakest category, while EF5 is the greatest.

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The Fujita Scale

Tornadoes

The damage in a tornado-ravaged area is often used to estimate the wind speed, and thus the category, of a tornado. Below is an indication of the damage associated with each category of a tornado on the Fujita scale, from EF0 to EF5.

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Tornadoes

Mobile homes are especially vulnerable during tornadoes. Roughly 45% of all tornado fatalities in the United States have occurred in mobile homes. Most of these fatalities are associated with the most violent of tornadoes, those on the EF4 or EF5 level.

The damage along a storm’s path can vary greatly, but the category that a tornado receives depends on the most severe damage observed. The damage used to estimate wind speed is based on an assumption that the structures involved in the analysis are well constructed.

Several of the most deadly tornadoes have been EF5’s hitting populated areas, such as the May 22, 2011 tornado that ripped through the city of Joplin, Missouri. However, while these individual tornadoes are dangerous, it is important to consider the threat posed by multiple tornadoes associated with the same system: a tornado outbreak.

A Tornado’s Aftermath

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Tornadoes

As aforementioned, individual tornadoes can take lives and damage property. But the worst tornadoes are those that occur in a family, associated with the same thunderstorm. Tornado families are often the result of a single long-lived supercell, which at times can produce several tornadoes that last over 2 hours and travel over 100 km.

When a large number of tornadoes develop in association with a particular weather system, it is referred to as a tornado outbreak. Outbreaks (such as the April 25-28, 2011 outbreak) can last over several days, as long as no more than a few hours elapse without a tornado sighting in the area of the outbreak. The good thing about tornado outbreaks is that, unlike individual tornadoes, they can be easily predicted using forecast software. This is because severe tornado outbreaks require large areas of conditionally unstable air and extremely strong vertical shear.

Tornado Outbreaks

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Tornado Formation

Tornadoes

Tornadoes develop in conditionally unstable atmospheres, often within supercell thunderstorms in strong vertical wind shear. A supercell is a thunderstorm with a single rotating updraft that can exist for others. These tornadoes that form within supercell thunderstorms are called supercell tornadoes. Below is a simplified diagram of a supercell tornado. As shown, warm, humid air is drawn into the supercell, spinning counterclockwise as it rises. Near the top of the storm, strong winds push the rising air to the northeast. After it gets chilled by evaporative cooling, the air descends as a strong downdraft called the forward-flank downdraft. The separation of the downdraft and the updraft allows the updraft to continue for long periods of time without a downdraft suppressing it.

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Tornado Formation

Tornadoes

Most tornadoes form from thunderstorms that form in the presence of warm, moist air from the Gulf of Mexico and cool, dry air from Canada. When these two air masses meet, they create instability in the atmosphere. A change in wind direction and an increase in wind speed with increasing height (vertical wind shear) creates an invisible, horizontal spinning effect in the lower atmosphere. This horizontal tube of spinning air is called a vortex tube. Rising air within the updraft tilts the rotating air from horizontal to vertical. An area of rotation now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

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Tornado Formation

Tornadoes

If the strong updraft of a developing thunderstorm tilts the rotating tube upwards and draws it into the storm, it becomes a rotating air column inside the storm. The rising, spinning air is now part of the storm’s structure called the mesocyclone, an area of lower pressure 5-10 km across. A mesocyclone is a rotating vortex of air within a supercell thunderstorm. They do not always produce tornadoes. However, they can circulate some of the precipitation in a thunderstorm counterclockwise around the updraft. This rotating precipitation may show up on Doppler radar as a shape of a hook, often called a hook echo.

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Tornado Formation

Tornadoes

At this point, the updraft, swirling precipitation, and the surrounding air may interact to produce the rear-flank downdraft, whose strength is driven by the amount of precipitation induced cooling in the upper levels of the storm. Warmer, weaker rear-flank downdrafts are more conducive for tornado formation that stronger, cooler downdrafts. When the rear-flank downdraft strikes the ground, it interacts with the forward-flank downdraft to initiate tornadogenesis, the formation of a tornado. At the surface, the winds of the rear-flank downdraft wraps around the updraft at the center of the mesocyclone, allowing for additional spin that can be lifted into the mesocyclone.

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Tornado Formation

Tornadoes

The lower half of the updraft begins to rise more slowly than the updraft higher up. This causes the column of air to shrink horizontally and stretch vertically. If this vertica stretching continues, the rapidly rotating air column may shrink into a narrow column of rapidly rotating air called a tornado vortex. As air rushes upward and spins around the low-pressure core of the vortex, the air expands, cools, and condenses into a visible cloud known as the funnel cloud. As the air beneath the funnel is drawn in, the air cools rapidly and its moisture condenses, allowing the funnel to descend toward the surface. Debris and dirt cause the tornado to obtain its dark and ominous appearance.

As shown in the figure to the right, tornadoes

usually develop in supercells near the right

rear section of the storm. In the case of the

northeastward moving storm shown, tornadoes

will tend to develop on the southwestern side.

The next slide summarizes the basics covered

by the previous few slides.

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Tornado Formation

Tornadoes

Didn’t fully understand all of the information in the previous five slides? That’s okay! Especially if you aren’t competing at the higher level, you don’t need to know this topic in this much detail! The video below effectively summarizes the basics of tornado formation:

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Tornado Formation

Tornadoes

Many atmospheric situations may actually suppress tornado formation. For instance, if the precipitation in the cloud is swept too far from the updraft or if too much precipitation wraps around the mesocyclone, the interactions needed for the formation of the rear-flank downdraft are inhibited, and a tornado is not likely to form. Neither will a tornado form if the air in the rear-flank downdraft is too cold. Only 25% of supercells with mesocyclones manage to produce a tornado.

The first sign that a supercell may produce a tornado is the sight of rotating clouds at the base of the storm. If the area of rotating clouds lowers, it becomes a wall cloud. A wall cloud, as shown below, is usually situated in the southwest portion of the storm. A rotating wall cloud usually develops before tornadoes or funnel clouds.

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Tornadoes

A rain wrapped tornado is especially dangerous. This is because rain wrapped storms are not visible due to the heavy rain that often surrounds it. Its invisibility is often associated with the rear-flank downdraft, which grabs the surrounding precipitation and gets it caught up in the rotating winds of a tornado. As a result, storms like these (“invisible” tornadoes) can be silent killers when they strike. Take a look at the rain wrapped tornado to the left. Can you spot it? You may have to look at your computer screen at different angles (or maybe even adjust the brightness) in order to fully discern the outline of this rain wrapped tornado.

While supercells are often the sources of tornadoes, not all supercells produce these violent storms. In addition, neither do all tornadoes come from rotating supercell thunderstorms either. In the next few slides, we will be talking about tornadoes that form without the mesocyclone of a supercell thunderstorm.

Tornado Formation

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Non-supercell Tornadoes

Tornadoes

Tornadoes that do not occur in association with a mid-level mesocyclone of a supercell are called nonsupercell tornadoes. Some nonsupercell tornadoes extend from the base of a thunderstorm, while others begin on the ground and build upwards in the absence of a condensation funnel. Some examples of nonsupercell tornadoes are given below.

A gustnado is a short-lived, relatively weak whirlwind that forms along a gust front. A gust front is the surge of very gusty winds at the leading edge of a thunderstorm's outflow of air, where the cool downdraft of the thunderstorm forces warm, humid air upwards. Gustnadoes are not tornadoes. They do not connect with any cloud-base rotation. But because gustnadoes often have a spinning dust cloud at ground level, they are sometimes wrongly reported as tornadoes.

A rather weak, short-lived tornado that occurs

with rapidly building cumulus congestus clouds. is often called a landspout.

Landspouts tend to form over northeastern

Colorado and other parts of the high plains.

Their name originates from the fact that they

look similar to a waterspout. A depiction of

their formation process is shown to the right.

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Tornadoes

A waterspout is a rotating column of air that forms over water. They are most common along the Gulf Coast. Waterspouts can sometimes move inland, becoming tornadoes causing damage and injuries.

Waterspouts that are not associated with supercells and that form over water are often referred to as fair weather waterspouts. These fair weather waterspouts tend to be smaller than the average tornado, and they have diameters typically between 3 and 100 meters. These storms are also less intense, with rotating winds of less than 45 knots.

Fair weather waterspouts also tend to move more slowly than tornadoes and only last about 10-15 minutes (with a few exceptions). They form in a way similar to landspouts, with conditionally unstable air in the presence of cumulus cloud development. Contrary to popular belief, a waterspout does not draw water up into its core.

Waterspouts

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Observing Tornadoes

Tornadoes

In recent years, most of our knowledge regarding tornadoes has been gathered through the use of Doppler radar. In addition to measuring precipitation intensity, Doppler radar can also measure the speed at which precipitation is moving horizontally toward or away from the radar antenna. These wind measurements are displayed in color on a map, giving an indication of how winds are changing within a storm. Tornadoes have a unique appearance on Doppler radar, called the tornado vortex signature (TVS), which shows up as a region of rapidly changing wind directions within a mesocyclone, as shown below. Similarly, the tornado debris signature (TDS), often just called the debris ball, is an area of high reflectivity on the radar due to the presence of debris in the air.

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Observing Tornadoes

Tornadoes

When Doppler radar shows precipitation intensity (reflectivity) inside a supercell thunderstorm, a signature of a mesocyclone or tornado may appear on the radar as a hook-shaped attachment, or hook echo. This hook becomes visible as precipitation and debris swirl around the mesocyclone in a counterclockwise direction. It is important to note that not all tornadoes show a hook echo or debris ball on radar.

Another system called Doppler lidar can help cover the areas that Doppler radar cannot. For instance, Doppler radars often do not have high enough resolutions to measure the wind speeds of most small tornadoes. Lidars measure the change in frequency of falling precipitation, cloud particles, and dust, using a shorter wavelength of radiation that provides a higher resolution than Doppler radar.

The network of more than 150 Doppler radar units around the United States is called NEXRAD. The NEXRAD system consists of computers that run algorithms that detect severe weather phenomena. This will allow meteorologists to make better predictions regarding severe weather and improve warning times. A new upgrade to Doppler radar, called the polarimetric radar, transmits both a horizontal and vertical radar pulse. This allows forecasters to distinguish between very heavy rain and hail. An image depicting this new upgrade is shown on the next slide.

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Tornadoes

While radar information is undoubtedly valuable, much of the research done on tornadoes today are done in the field. Many scientists volunteer to directly observe tornadoes on the ground, working to increase the knowledge we have on these magnificent storms by watching them in person. Several field studies have sent instruments directly into tornadoes to obtain valuable data on the inner workings of these powerful forces of nature. While storm chasing can be safe if done right, it is still a risky activity. On May 31, 2013, three research-based storm chasers, experts in the field, were killed in a violent tornado in El Reno, Oklahoma, as the tornado they were researching suddenly changed direction, accelerated, and expanded while heading toward their direction. However, these were the first known deaths in the history of storm chasing. (For more information, visit https://youtu.be/TBjr-nvA2Jg)

Storm Chasing and Field Research

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Historical Tornadoes

Tornadoes

Below are some articles on historical winter storms. It may be beneficial for you to study these prior to the competition. These articles will be updated as new events occur.

this slide will be updated upon the completion of the rest of the slideshow

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Wind and Air Pressure

“All flight is based upon producing air pressure; all flight energy consists in overcoming air pressure.”

― Otto Lilienthal, German Aviator

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Air Pressure

Wind and Air Pressure

Why does the wind blow? Air moves in response to horizontal differences in pressure, moving from high pressure regions to low pressure regions. In the atmosphere, the wind blows in an attempt to equalize imbalances in air pressure. In this unit, we will analyze the reasons behind why atmospheric pressure varies and the forces that drive the motions of the atmosphere aloft.

Air pressure is the mass of air above a given level. As we climb up a mountain, for instance, there are fewer air molecules above us and, as a result, air pressure is lower. This shows us that atmospheric pressure always decreases with increasing height. In addition, most of the atmosphere is crowded close to the earth’s surface, so air pressure decreases most rapidly at lower altitudes.

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Air Pressure

Wind and Air Pressure

One’s elevation above the earth’s surface can influence the vertical air pressure. But what causes the air pressure to change in the horizontal? And why does air pressure change at the surface? To answer this question, we will use the model shown below. Suppose that both columns are located at the same elevation, have the same air temperature, and the same surface air pressure. This also means that there is the same number of molecules (and mass of air) above both cities.

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Air Pressure

Wind and Air Pressure

Now suppose that the air above City 1 cools while the air above City 2 warms. Since the width of the air column, the number of molecules above each city, and the air pressure at the surface do not change, the colder (more dense) air above City 1 causes the column to shrink while the warm (less dense) air above City 2 causes the column to rise. We can conclude from this that it takes a shorter column of cold air to exert the same surface pressure as a taller column of warm air.

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Air Pressure

Wind and Air Pressure

Atmospheric pressure decreases more rapidly with height in the cold column of air than in the warm column (you would pass through more molecules by moving up air column 1 than by moving up air column 2, given that you move up the same distance). The pressure does not decrease as rapidly with height in the warm column because you climb above fewer molecules in the same vertical distance.

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Air Pressure

Wind and Air Pressure

Now let’s consider the pressure at a certain altitude above both cities: where the “H” and “L” are located on the far right diagram. Notice that at the same altitude, there are more molecules above the “H” than above the “L”. This shows that the air pressure is higher at point “H” than at point “L” — as a result, warm air aloft is normally associated with high atmospheric pressure, while cold air aloft is normally associated with low atmospheric pressure.

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Air Pressure

Wind and Air Pressure

As mentioned on the first slide of this unit, air in the atmosphere moves from areas of high pressure to areas of low pressure (the force that causes the air to move this way is the pressure gradient force). Thus, at all levels, air would want to move from City 2 to City 1. As the air leaves City 2, the surface air pressure above City 2 decreases. On the other hand, the addition of air above City 1 causes the surface air pressure to increase.

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Air Pressure

Wind and Air Pressure

So the surface pressure at City 1 rises while the surface pressure at City 2 falls. What happens now? Like before, air wants to move from high pressure regions to low pressure regions. Because of this, air at the surface will move from City 1 toward City 2. As the surface air leaves City 1, the air above slowly sinks to replace the air that moved away. As the surface air enters City 2, it slowly rises to replace the air that flowed out aloft. Thus, a complete cycle of air circulation forms from the heating and cooling of air columns, as shown to the right.

In summary, the heating and cooling of columns of atmospheric air can establish horizontal variations in air pressure both aloft and at the surface. These horizontal differences give us the phenomenon that we have become all too accustomed to in our daily lives: wind.

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Daily Pressure Variations

Wind and Air Pressure

Pressure changes created by the warming of the upper atmosphere can induce daily changes in temperature. In the tropics, maximum pressures occur around 10 AM and 10 PM, while minimum pressures occur at 4 AM and 4 PM. The largest pressure difference between these fluctuations occurs at the equator. This diurnal fluctuation of pressure appears to be due primarily to the absorption of solar energy by ozone in the upper atmosphere and by water vapor in the lower atmosphere.

In the middle latitudes, surface pressure changes are primarily the result of large high and low pressure areas that move toward or away from a region. Generally, when an area of high pressure approaches an area, surface pressure usually rises. When it moves away, pressure usually falls. The same occurs with an approaching low, just with the opposite effect.

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The Gas Law

Wind and Air Pressure

Air temperature, air pressure, and air density are all related. If one of these variables changes, the others do so as well. The relationship between these three variables is given by the equation

p = T × ρ × C

where p is the pressure in millibars (mb), T is the temperature in Kelvins, ρ is the density in kg/m³, and C is a constant equal to approximately 2.87 hJ/kg•K.

Using this formula, we can calculate the standard (average) sea-level pressure. Assuming that the average global temperature at sea level is 15°C (or 288 K) and the average air density at sea level is

1.226 kg/m³, the standard sea-level air pressure is approximately

p = T × ρ × C

p ≈ (288 K) × (1.226 kg/m³) × (2.87 hJ/kg•K)

p ≈ 1013 mb

This is close to the standard atmospheric pressure of 1013.25 mb, which we will touch upon on the next slide. (SI unit note: if you are using pressure in pascals instead of millibars, the value of C would instead be 287 J/kg•K)

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Wind and Air Pressure

In Unit 5, we mentioned that instruments that detect and measure pressure changes are called barometers. A common unit used to measure pressure on weather maps is the millibar (mb). The standard atmospheric pressure at sea level is 1013.25 mb. Some other units:

1013.25 mb = 29.92 inHg = 760 mmHg (torr) = 14.7 psi = 1 atm = 101325 Pa = 101.325 kPa

In this case, inHg and mmHg represent the units “inches of mercury” and “millimeters of mercury.” Why? This is because the height of mercury in a mercury barometer corresponds to these values.

The SI unit of pressure is the pascal (Pa), the force of 1 N of force acting on a surface area of 1 square meter. 100 pascals equals 1 millibar. Because of this, one millibar is equal to one hectopascal (hPa), i.e.

1 hPa = 1 mb

See Unit 5 for a discussion on barometers and how they work.

Pressure Measurements

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Rising and Falling Barometers

Wind and Air Pressure

A steady rise in atmospheric pressure, or a rising barometer, usually indicates clearing or fair weather, while a steady drop in atmospheric pressure, or a falling barometer, typically signals the approach of a storm system.

Several adjustments must be taken to read a barometer, as they can be extremely sensitive to changes in temperature and other factors. The station pressure is a barometer reading after it is corrected for temperature, gravity, and instrument error.

Altitude corrections are also done so that barometer readings can be compared between different elevations. This is especially important because pressure readings are more sensitive vertically than horizontally. The adjustment after elevation is considered is the sea-level pressure.

Near the earth’s surface, atmospheric pressure decreases on average by about 10 mb for every 100 m increase in elevation. This fact allows us to easily adjust measured pressures to sea level (a 952 mb reading 600 m above sea level would have a sea-level pressure of 952 + 60 = 1012 mb, for example). This process allows us to see horizontal variations in sea-level pressure.

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Isobars and Surface Maps

Wind and Air Pressure

Isobars are lines connecting points of equal pressure. On weather maps and diagrams, they are drawn at intervals of 2 or 4 mb, with 1000 mb as the base value. This allows us to read (and interpolate) data regarding atmospheric pressures in different areas. The bottom chart shown below (with isobars) is a sea-level pressure chart or a surface map. When weather data are plotted on the map, it becomes a surface weather map.

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Isobars and Surface Maps

Wind and Air Pressure

The figure below shows isobars drawn on a surface map of the entire country, with each individual observation taken into account. The version on the right is smoothed out to account for the errors in measurement that may result from data taken at higher altitudes. This type of sea-level pressure chart is known as a constant height chart because it represents the atmospheric pressure at a constant elevation (in this case, sea level). The same type of chart can be drawn to show the horizontal variations in pressure at any altitude in the atmosphere.

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Isobaric Charts

Wind and Air Pressure

Another type of chart commonly used in meteorology is the constant pressure chart, or isobaric chart. These charts are constructed to show height variations along an equal pressure (isobaric) surface. Constant pressure charts are easy to use because the height variables they show are easier to deal with in meteorological questions than the variables of pressure.

The figure to the right shows a isobaric chart. The tan surface halfway between the tropopause and sea level is known as a isobaric surface because the pressure is always the same on that surface (in this case, 500 mb). This means that there is an equal number of molecules above every point on the surface. However, it is important to note that the flat isobaric surface is a special case where there are no horizontal variations in either pressure of altitude. We’ll look at a more realistic surface on the next slide.

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Isobaric Charts

Wind and Air Pressure

If the air temperature changes in any portion of the column, the air density and pressure would also change along with it. This gives a more realistic constant pressure surface, as shown in the figure to the right. Notice that the colder air to the north allows for the isobaric surface to be below the average height at that location, while the warmer air to the south allows for the isobaric surface to be above the average height at that location.

In other words, when the air aloft is warm, constant pressure surfaces are typically found at higher elevations than normal, and when the air aloft is cold, constant pressure surfaces are typically found at lower elevations than normal. High heights on an isobaric chart correspond to higher-than-normal pressures at any given altitude, and low heights on an isobaric chart correspond to lower-than-normal pressures.

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Isobaric Charts

Wind and Air Pressure

The dark, gray horizontal lines on the isobaric surface to the right are contour lines. These lines connect points of equal elevation. In this case, each line tells us the altitude above sea level at which we can obtain a pressure reading of 500 mb. When the contour lines are high in elevation, the air is warmer and higher pressures may be present in the region. When the contour lines are low, the air is cooler and lower pressures may be present. Contour lines that are crowded together indicate that the pressure surface is changing rapidly, while the absence of many contour lines indicate that there is little horizontal temperature change.

Many isobaric surfaces are used in meteorology. While the example used in this slideshow incorporates a 500 mb isobaric surface, surfaces at other pressures are also used. The 700 mb and 850 mb isobaric surfaces are also rather common in meteorology.

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Ridges and Troughs

Wind and Air Pressure

Cold air aloft is normally associated with low heights or low pressures, and warm air aloft is normally associated with high heights or high pressures. As a result, in the Northern Hemisphere, contour lines and isobars usually increase from north to south as the air gets warmer. However, these lines are not perfectly straight; they bend and turn, indicating the presence of warmer ridges (elongated highs) and cooler troughs (elongated lows). A ridge is an elongated area of relatively high pressure extending from the center of a high-pressure region, while a trough is an elongated area of relatively low pressure extending from the center of a region of low pressure. The figure below shows how ridges and troughs can be identified.

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Ridges and Troughs

Wind and Air Pressure

The surface map below shows areas of high and low pressures, with the arrows representing wind direction. The large blue H’s on the map represent centers of high pressure, often known as anticyclones. The large L’s represent centers of low pressure, known as depressions or mid-latitude cyclonic storms. The chart on the right displays the contour lines on a constant 500 mb surface (with a contour interval of 60 m). The dashed red lines on the second map are lines of equal temperature, or isotherms. The contour lines, as shown, tend to parallel the isotherms. As expected, they also decrease in value from south to north.

Notice from the graph that winds blow across isobars but parallel to the contour lines. To understand why, we will need to look at the forces that affect the winds.

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Wind and Air Pressure

Our understanding of how the winds work can be traced to the scientific theories developed by Isaac Newton. Newton’s three laws of motion govern the movement of everything on Earth.

Newton’s Second Law states that an object will always accelerate in the direction of the net force acting on it. Thus, to determine the direction that the wind will blow, we must first identify the forces that affect the horizontal movement of air. These forces are as follows:

Pressure Gradient Force
Coriolis Force
Centripetal Force
Friction

The Forces That Influence the Winds

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Pressure Gradient Force

Wind and Air Pressure

The figure below shows a region of higher pressure on the left and a region of lower pressure on the right. The isobars on the map show how the horizontal pressure is changing. The pressure gradient can be calculated by computing the amount of pressure change that occurs over a given distance:

Pressure Gradient = Difference in Pressure ÷ Distance

If the isobars were closer together, the distance in the above equation would decrease, and the pressure gradient would be steeper (larger change in pressure per unit distance). If the isobars were farther apart, the difference in pressure would change less per unit distance, and the gradient would be more gentle.

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Pressure Gradient Force

Wind and Air Pressure

When differences in horizontal air pressure exist, there is a net force acting on the air. This force is called the pressure gradient force (PGF) and is directed from higher toward lower pressures at right angles to the isobars. The steeper the pressure gradient present, the stronger the pressure gradient force.

The pressure gradient force is the force that causes the wind to blow. Because closely spaced isobars on a weather map indicate steep pressure gradients, the PGF is greater, and the winds in the area are stronger. On the other hand, widely spaced isobars are associated with gentle pressure gradients, which indicate a weaker PGF and consequently weaker winds.

If the PGF were the only force acting on the air, winds would always blow from higher pressures to lower pressures. But this is not the case; when air begins to move, it is often deflected from its path. This is due to another force known as the Coriolis Force.

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Coriolis Force

Wind and Air Pressure

The Coriolis force (or Coriolis effect) is an apparent force that is caused by the rotation of the earth. This effect is present because all free-moving objects seem to deflect from a straight-line path because the earth rotates under them. The Coriolis force causes the wind to deflect to the right of its intended path in the Northern Hemisphere and to the left of its intended path in the Southern Hemisphere.

Imagine looking southward at a satellite in polar circular orbit, moving from north to south. If the earth were not rotating, the satellite would appear to move directly from north to south from your perspective on Earth. But since the earth is rotating eastward, so are you! Because of this, the satellite would appear to move southwest instead of directly south, a deflection to the right from where you are standing.

Q: What is the magnitude of the Coriolis force at the equator?

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Coriolis Force

Wind and Air Pressure

The magnitude of the Coriolis force varies with the speed of the moving object — the faster an object is moving, the greater the Coriolis force acting on it, and the greater the deflection off its straight-line path. In other words, the stronger the wind speed, the greater the deflection!

The Coriolis force increases for all wind speeds from a value of zero at the equator to a maximum at the poles.

In summary, to an observer on Earth, objects moving in any direction are deflected to the right of their intended path in the Northern Hemisphere and to the left of their intended path in the Southern Hemisphere. The amount of deflection depends on

the rotation of the earth
the latitude
the object’s speed

Since the Coriolis force acts at right angles to the wind, it only influences wind direction and never wind speed.

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Geostrophic Winds and Buys Ballot’s Law

Wind and Air Pressure

Above the level of friction, air at rest will accelerate until it flows parallel to the isobars at a constant speed. This outcome is only possible when the PGF balances the Coriolis force, producing a net force of zero (thus, by Newton’s First Law, the air would maintain a constant velocity). Wind blowing under these conditions is called geostrophic wind. This only occurs when the isobars (or contours) are straight and evenly spaced, and the wind speed is constant. This rarely happens because isobars are rarely straight or evenly spaced.

Geostrophic wind blows in the Northern Hemisphere with lower pressure to its left and higher pressure to its right. This exemplifies Buys Ballot’s Law, a law that states that if a person stands with their back to the wind, the atmospheric pressure will be low to the left and high to the right.

Geostrophic wind direction can be determined by studying the orientation of the isobars, and its speed can be estimated from the spacing of the isobars. If we know the isobar or contour patterns on an upper-level chart, we also know the direction and relative speed of the geostrophic wind. Similarly, if we know the geostrophic wind direction and speed, we can estimate isobar orientation and spacing in the area.

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Centripetal Force

Wind and Air Pressure

In the Northern Hemisphere, winds blow counterclockwise around lows and clockwise around highs. Because lows are also known as cyclones, the counterclockwise flow of air around them is often called cyclonic flow. Similarly, the clockwise flow of air around a high, or anticyclone, is called anticyclonic flow. But for air to move in a circle, it must seemingly defy the Coriolis force that only deflects it in one direction. Why is this the case?

Notice in the below diagram that around a low pressure area, the pressure gradient force accelerates the air parcel inward toward the center until its movement parallels the isobars. This movement cannot be geostrophic because the isobars are not straight, but rather curved. A wind that blows at a constant speed parallel to curved isobars above the level of frictional influence is called gradient wind.

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Centripetal Force

Wind and Air Pressure

Because the air is moving in a circle, it is always changing direction and, as a result, always accelerating. This acceleration is called the centripetal acceleration and is directed inward toward the center of the circle. By Newton’s Second Law, which claims that an object can only accelerate when there is a net force acting on it, there must be a force present to induce the air’s motion: an inward directed force called the centripetal force. Higher wind speeds and lower circle radii are associated with larger centripetal forces. When the winds are strong and blow in a tight circle (as with tornadoes and hurricanes), centripetal forces are large and can be quite important in mapping the air’s motion.

As shown in the figure on the previous slide, the centripetal force of wind around a low or high pressure center is caused by an imbalance between the Coriolis force and the pressure gradient force. The difference between these two forces is the magnitude of the centripetal force pointing toward the circular path’s center. In the Southern Hemisphere, the leftward deflection of moving objects by the Coriolis force causes wind to blow clockwise around areas of low pressure and counterclockwise around areas of high pressure.

Near the equator where Coriolis force is negligible, winds may blow around storms with the centripetal force being almost as large as the pressure gradient force. In this case, the wind is considered cyclostrophic.

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Winds on Upper Level Charts

Wind and Air Pressure

Take a look at the 500 mb map shown below. The winds parallel the contour lines in a west to east direction, and the contours increase in elevation from north to south (as air is warmer to the south and colder to the north). When horizontal temperature contrasts are large, so are the height gradients; as a result, contour lines are closer together and winds are stronger. When horizontal temperature contrasts are small, the height gradient is smaller, the contour lines are more spread apart, and the winds are weaker. In general, the north-south temperature contrast is greater in winter than in summer — as a result, winds aloft tend to be stronger in winter than in the other seasons.

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Winds on Upper Level Charts

Wind and Air Pressure

The wind is geostrophic where it blows in a straight path parallel to evenly spaced lines, and it is gradient where it blows parallel to curved contour lines. Where the wind flows in large meanders (that may occasionally blow from north to south, as it does directly off the west coast), the wind-flow pattern is called meridional. Where the winds blow in a west-to-east direction (as shown over the eastern half of the US), the wind-flow pattern is called zonal. If the flow is zonal, clouds and storms tend to move rapidly from west to east. But if the flow is meridional, surface storms tend to move more slowly, giving major storm systems a chance to intensify. (Winds aloft in the Southern Hemisphere also blow from west to east.)

Q: Why does a flight from New York to San Francisco take 30 minutes longer than a flight the other way around?

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Wind and Air Pressure

Surface winds tend to blow slower than winds aloft. This is because moving air on the ground has to deal with an obstacle air above does not: friction.

The frictional drag of the ground slows the wind down. Because friction decreases the farther you move from the earth’s surface, wind speeds also steadily increase with altitude. The atmospheric layer that is influenced by friction is called the friction layer and reaches upward to an altitude of about 1,000 m (3,300 ft).

At the surface, friction reduces wind speed, which thus reduces the Coriolis force. This causes the wind to blow across the isobars at an angle of about 30°. However, this angle depends on surface friction, wind speed, and the height above the surface. The rougher the surface that the wind blows over, the larger the angle. Because surface winds cross isobars (unlike winds aloft) at an angle, we can make a slight modification to our previous rendition of Buys Ballot’s Law: if you stand with your back to the wind and turn clockwise by about 30°, the center of lowest pressure will be to your left.

Friction and Surface Winds

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Friction and Surface Winds

Wind and Air Pressure

Previously, we talked about how air moves around areas of high or low pressure in a circular fashion, parallel to the isobars that surround them (e.g. cyclonic flow). However, as we learned on the last slide, friction prevents the air from moving parallel to the isobars at the surface. In this case, the pressure gradient force must be balanced by the sum of the frictional and Coriolis force, and not just the Coriolis force alone. Because of this, surface winds blow counterclockwise and into a low and clockwise and out of a high. (In the Southern Hemisphere, air also moves into lows and out of highs, but the direction in which they move around the system is flipped: clockwise around lows and counterclockwise around highs.)

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Convergence and Divergence

Wind and Air Pressure

We now know that surface winds blow into low pressure systems. But after the air converges toward the center (i.e. comes together), where does it go? Since the only direction this air can move is upward, it slowly rises and diverges (spreads apart) above the surface low. The pressure, however, does not change — unless the rate of convergence and divergence are not balanced.

If upper-level divergence exceeds upper-level convergence (i.e. more air is removed at the top than is taken in at the surface), the air pressure at the center of the low will decrease, and isobars around the low will become even more tightly packed. This increases the pressure gradient and, in turn, the surface winds.

Similarly, surface winds diverge, or move outwards, away from from high-pressure centers. To replace this air, air aloft must converge and descend into the high-pressure area, as shown below. The pressure does not change if convergence and divergence are in balance.

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Convergence and Divergence

Wind and Air Pressure

Generally, the air that rises above a low or descends above a high is small compared to the horizontal winds that enter a low or leave a high at the surface. Because the vertical motions are smaller, it makes sense that high pressure systems don’t often naturally attain higher and higher pressures (and vice versa).

One last thing that should be mentioned in this unit: we learned that air moves in response to pressure differences, moving from higher pressures to lower pressures. We also know that pressure decreases as we increase in elevation, with higher pressures at the surface and lower pressures aloft. Why, then, doesn’t the air blow upward toward space?

The answer is gravity. Air does not rush off into space because the pressure gradient force directed upwards is almost always balanced by the force of gravity. An exact balance between these two forces is known as a state of hydrostatic equilibrium. When air is in hydrostatic equilibrium, there is no net vertical force acting on it, so air does not accelerate vertically!

It is important to note that there are instances where hydrostatic equilibrium is not achieved. These situations are relatively rare, but when they do happen, they are often associated with violent thunderstorms and tornadoes — where the air shows appreciable vertical acceleration.

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Wind and Global

Weather Patterns

“If you reveal your secrets to the wind,

you should not blame the wind for

revealing them to the trees.”

― Khalil Gibran, Lebanese-American Author

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Wind and Global Weather Patterns

In the previous unit, we covered the basics of wind and how it is possible. In this unit, we will be building on the information from the last unit by talking about wind on two scales, the local scale and the global scale. Local scale winds incorporate winds that can be found on a smaller level, often occurring in specific places and affecting certain regions. Global scale winds occur worldwide, circulating the entire atmosphere of our planet and affecting everyone on it. Later on, we will also talk about global weather patterns that play a role in regulating the weather around us.

Two characteristics are necessary to specify wind: speed and direction. You will soon see that winds are often given names associated with a coordinate direction, such as easterlies and northwesterlies. It is important to know that winds are named from the direction from which they originate, not the direction they blow toward! So if you see the term prevailing westerlies, it means that the wind is blowing from the west, toward the east.

A Preview: Local and Global Systems

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Wind and Global Weather Patterns

Small Scale Winds

Circulations of all sizes exist within the atmosphere. The term “eddies” is used to describe spinning masses of air in the atmosphere. To make things simple, meteorologists arrange circulations according to their size, forming a hierarchy of motion from tiny gusts to giant storms called the scales of motion.

The four scales of motion, from smallest to biggest, are the

microscale - consists of eddies with diameters of a few meters or less; they form by convection or by the wind blowing past obstructions, and they do not last long (only about a few minutes at best).
mesoscale - winds with diameters from a few kilometers to about a hundred kilometers; these city-wide eddies can last minutes to days, and they include local winds and thunderstorms, tornadoes, and small tropical storms.
synoptic scale - these circulations are large enough to see on a weather map (such as circulations around high and low pressure areas). These wind patterns can dominate regions of hundreds to thousands of square kilometers and often last for periods ranging from days to weeks.
planetary (global) scale - these wind patterns cover the entire earth. Sometimes, the synoptic and global scales are combined and referred to as the macroscale.

A figure that summarizes the scales of motion can be found on the next slide.

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Wind and Global Weather Patterns

Small Scale Winds

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Wind and Global Weather Patterns

Friction on the Microscale

On a microscopic level, friction arises as atoms and molecules of two surfaces seem to adhere before snapping apart. Friction is not limited to solids; they also apply to fluids (liquids and gases). The friction of fluid flow is called viscosity. The viscosity associated with the slowing of a fluid due to random motions of the gas molecules is called molecular viscosity. The viscosity associated with the turbulence of an eddy itself is called eddy viscosity. These eddies are formed through contact with obstacles during movement, and the fluctuations of air speed and direction that result create wind gusts. Eddy motions created by obstructions are known as mechanical turbulence, and create a drag on airflow greater than that of molecular viscosity.

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Wind and Global Weather Patterns

Friction on the Microscale

Because friction tends to decrease as altitude increases (there are fewer impediments in the way of airflow the higher you go), wind speeds tend to increase with height. As we covered in the previous unit, the atmospheric layer near the surface that is influenced by friction is known as the friction layer, or planetary boundary layer. The top of this layer can usually be found at around 1,000 m (3,300 ft), with slight variations between regions due to the roughness of terrain and relative frequency of obstacles to airflow.

Surface heating and instability can cause turbulence at higher altitudes. As the

earth’s surface heats, thermals rise and convection cells form (see Unit 3). This

vertical movement creates thermal turbulence, which increases with greater

surface heating and instability. This is why thermal turbulence is lowest in the

early morning, when the air is most stable. As surface heating increases into

the afternoon, instability is induced and thermal turbulence intensifies.

Vertical mixing during the middle of the day links surface air with the faster

moving air aloft. This is why surface winds are usually stronger in the afternoon.

At night, convection is reduced, and the interactions between surface air and

air aloft is at a minimum. Because of this, wind near the ground is less affected

by the faster winds above, so it blows more slowly.

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Wind and Global Weather Patterns

In summary, the friction of air flow (viscosity) is a result of the exchange of air molecules moving at different speeds. This exchange is brought about by molecular viscosity and, to a larger extent, eddy viscosity. Because of this, the depth of mixing and frictional influence depend on the following three factors:

surface heating - produces a steep lapse rate and strong thermal turbulence
strong wind speeds - produces strong mechanical turbulent motions
rough terrain or landscape - produces strong mechanical turbulence

When all three factors are present at the same time, the frictional effect of the ground is transferred upward to considerable heights, and the wind at the surface tends to be strong and gusty.

Friction on the Microscale

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Wind and Global Weather Patterns

Eddies

When the wind encounters a solid obstacle, a whirl of air called an eddy forms on the object’s leeward side (side facing away from the direction of the wind). The size and shape of the resultant eddy depends on the size and shape of the obstacle and the speed of the wind. Light winds and smaller obstacles may produce smaller eddies, while rough surfaces, large impediments, and fast wind speeds may result in larger eddies. As shown in the figure below, wind moving over mountain ranges in a stable environment may create roll eddies, or rotors, that are associated with violent vertical motions.

Turbulent eddies form both at the surface and aloft. Turbulence aloft can occur suddenly and unexpectedly, especially where the wind changes its speed or direction abruptly. Such a change, as mentioned previously, is known as wind shear. Strong vertical wind shear, as explained in Unit 15, can create large supercells that spawn tornadoes. On the other hand, it can also inhibit tropical cyclone development, as strong vertical changes in wind speed and direction can prevent tropical systems from effectively aligning their centers.

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Wind and Global Weather Patterns

Strong winds can blow down trees, overturn mobile homes, and even move railroad cars. Even automobiles can be moved large distances if the winds are strong enough.

A car driving on a high bridge will experience higher wind speeds than a car driving below it, as the frictional influence provided by the ground is greatly reduced up on the bridge.

Winds blowing over mountains tend to be stronger than winds blowing at the same level on either side. In addition, wind tends to accelerate when it funnels through a narrow constriction, such as a highway overpass.

Strong winds flowing past an obstruction can produce a reverse flow of air that strikes an object from the side opposite the general wind direction. These abrupt flows can occur without warning, and they can be especially problematic for trucks, campers, and trailers.

The Force of the Wind

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Wind and Global Weather Patterns

Microscale Winds

Microscale winds can play a big role in shaping the earth’s surface. These winds can blow loose particles of sand and produce structures such as sand dunes and sand ripples. They can also create snow ripples and snow dunes over a snow-covered landscape.

Microscale winds can also influence water. Waves that form from winds over the water’s surface are known as wind waves. The amount of energy transferred to the surface of water is dependent on three factors:

wind speed
wind duration
wind distance (or fetch)

Microscale winds can help waves grow taller. The figure below depicts the process by which waves can create small eddies of air that can reinforce the up and down motion of the water.

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Wind and Global Weather Patterns

Characterizing Wind Direction and Speed

As mentioned at the very beginning of this unit, winds are characterized by their speed and direction. Wind direction is given as the direction from which it is blowing — a north wind blows from the north to the south. However, near large bodies of water, wind direction may be expressed in a different fashion. Wind blowing from the water onto the land is referred to as an onshore wind, or sea breeze (as the wind comes from the sea). Wind blowing from land to water is referred to as an offshore wind, or a land breeze. This is also the case in hilly regions. Air moving uphill is known as an upslope wind or valley breeze, while air moving downhill is known as a downslope wind or mountain breeze.

The Beaufort Wind Scale can be used to estimate wind speeds from surface observations. The scale was created by the British naval commander Sir Francis Beaufort around 1806, and it can be found here.

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Wind and Global Weather Patterns

Prevailing Winds

In many locations, the wind blows more frequently from one direction than from any other. The prevailing wind is the name given to the wind direction most often observed during a given time period. Prevailing winds can greatly affect the climate of a region; for instance, prevailing onshore winds may bring moisture and cooler temperatures to a region, while prevailing offshore winds may bring warmer, drier conditions.

Prevailing winds can also affect the architecture and design of a region. In the northeast US, for example, the prevailing wind is northwest in winter and southwest in summer. Thus, windows in these regions commonly face the southwest to provide ventilation, with very few facing the northwest to minimize the effect of cold, winter winds.

The prevailing wind can be represented by a wind rose, as shown to the

right. The wind rose indicates the percentage of time the wind blows from

different coordinate directions, with the length of each directional line

representing the percentage of time the wind blew in each direction. In

this specific example provided on the right, the most common prevailing

wind is the northwest wind, while the least common prevailing wind is

the northeast wind.

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Wind and Global Weather Patterns

For centuries, small windmills have helped civilizations perform daily tasks, from pumping water to sawing wood to providing energy. However, the energy crisis of the 1970s helped bring wind turbines into the public eye: a apparatus that harnessed the wind to run generators and provide electricity.

Wind energy is a promising source of renewable energy. It is clean, non-polluting, and (unlike solar) is not restricted to daytime use. However, they can be costly to build, and many note that their presence can make landscapes unaesthetic. Wind turbines also kill countless birds every year, a concern that has provoked wind turbine companies to investigate bird behavior. Due to this, many turbines now shut down during nesting season.

Nonetheless, as the technology available to us improves, wind energy will become a competitive substitute to the fossil fuels that currently claim much of the energy industry. In many states, wind farms (clusters of 50 or more wind turbines) have become a common sight. Not only will the presence of this new method provide us with valuable ways to harness the power of nature, it also mitigates many of the issues brought up in Unit 4.

Wind Power

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Wind and Global Weather Patterns

Wind Measurements

As we discussed in Unit 5, a wind vane can be used to measure wind direction (the long arrow is allowed to move freely about a vertical post and always points in the direction of the wind), while an anemometer can be used to measure wind speed (the hemispherical cups allow wind pressure from one side to spin the cups about its shaft). An aerovane or skyvane is an instrument that can measure both wind speed and direction. It consists of a bladed propeller that rotates at a rate proportional to the wind speed, with a shape that allows the blades to face the wind at all times. These wind instruments should be exposed to freely flowing air at a height of at least 10 m (30 ft) to ensure accurate measurements, as structures such as buildings and trees can alter the surrounding wind conditions. An aerovane is shown in the image below.

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Wind and Global Weather Patterns

Wind Measurements

There are many other ways to obtain wind data. A simple way is to use a weather balloon or pilot balloon. This balloon can be manually tracked as it drifts with the wind, and it can bring with it a radiosonde that can provide a vertical profile of the atmosphere (with characteristics such as temperature, pressure, and humidity). The observation of winds using a radiosonde balloon is called a rawinsonde observation.

A device very similar to radar called lidar (or light detection and ranging) uses infrared or visible light to collect wind information. It does so by reflecting light from particles in the air and using the data to measure the movement of these particles.

Similarly, Doppler radar been been used to obtain a vertical profile of wind speeds up to an altitude of

16 km. Such a profiler is called a wind sounding, and the radar acts as a wind profiler. Doppler radar is able to accomplish this task by translating the energy reflected back from atmospheric eddies into a vertical picture of wind speed and direction up to 16 km (10 mi).

Lastly, in remote regions where available equipment is scarce, satellites can provide wind observations. Geostationary satellites, for example, can read the movement of clouds to determine wind speeds aloft,

and they can observe sea surface roughness to estimate the strength of winds at the surface.

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Thermal Circulations

Look at the figure to the right, where the atmosphere is warmed to the

South and cooled to the North. The air temperature changes produce a

thermal circulation, in which warm air rises and colder air sinks. The

regions of surface high and low atmospheric pressure created by the

atmosphere as it cools or warms are called thermal (cold-core) highs and

thermal (warm-core) lows. These systems tend to be shallow and weaken

with height. Note that, in the figure below, the thermal pressure is lowest

at the center of a warm thermal low, but the higher temperatures above

the low produce isobars that are farther apart, which result in a high

higher up. The opposite works for the thermal high, where the cold dense

air produces crowded isobars that result in a low farther up.

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Wind and Global Weather Patterns

Sea Breezes and Land Breezes

On a warm summer day along the coast, the differential heating of land and sea leads to the development of local winds called sea breezes. As air above the land surface is heated by radiation from the Sun, it expands and begins to rise, being lighter than the surrounding air. To replace the rising air, cooler air is drawn in from above the surface of the sea. This is the sea breeze, and can offer a pleasant cooling influence on hot summer afternoons. On the other hand, a phenomenon known as a land breeze occurs at night when the land cools faster than the sea. In this case, it is air above the warmer surface water that is heated and rises, pulling in air from the cooler land surface.

Sea breezes are best developed where a large temperature difference between water and land exists. The leading edge of a sea breeze is called the sea breeze front; as the front moves inland, a rapid drop in temperature may occur right behind it. When there is a sharp contrast in air temperature across the frontal boundary, cumulus clouds may form along the sea breeze front. If the air is also conditionally unstable, thunderstorms may form. This is why a trip to the beach may be filled with heavy showers, but the conditions at the beach itself may be fair.

When cool, dense, stable marine air encounters an obstacle, the heavy air tends to flow around the object rather than over it. When the opposing breezes meet on the other side of the obstruction, they form a sea breeze convergence zone. The convergence of two moist wind systems, with daytime convection, can produce cloudy conditions and showery weather. See the next slide for diagrams regarding sea and land breezes, as well as the effects of a sea breeze convergence zone.

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Sea Breezes and Land Breezes

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Wind and Global Weather Patterns

Sea Breezes and Land Breezes

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Local Winds and Water

Local winds will frequently change speed and direction as they cross a large

body of water. In fact, they blow faster and from a slightly different direction

at the center of the lake. Why is this the case?

As the air moves from rough land to the smooth lake, its speed increases

because friction lessens. This increase in speed subsequently increases the

Coriolis force, which deflects the wind toward the right in the Northern

Hemisphere. When the air leaves the lake and re-enters land, its speed slows,

and the Coriolis force weakens. This brings the wind closer to its initial direction.

The increase in speed as the wind blows onto the water forces surface air to diverge, causing air above it to slowly sink. This inhibits the formation of clouds. On the other hand, as the wind leaves the water and re-enters land, it slows and is forced to converge (this causes the air to move upwards). This upward motion encourages cloud formation when the surface heating is sufficient. This is why clouds tend to form on the downwind side of the lake rather than the upwind side.

Strong winds can cause the water to slosh back and forth rhythmically. These water waves that oscillate due to the presence of wind are known as seiches.

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Wind and Global Weather Patterns

Monsoon Winds

A monsoon wind system is one that changes direction seasonally, blowing from one direction in the summer and from the other direction during winter. This reversal is commonly found in eastern and southern Asia. In many ways, a monsoon is similar to a large scale sea breeze.

During the winter in a monsoon region, the air over the continent becomes much colder than the air over the ocean. This causes a pressure difference (see the slide on thermal circulations) that causes surface winds to blow from land to sea. Thus, the winter monsoon is associated with a dry season with clear skies.

During the summer, the continents become much warmer than the air above water. As a result, winds blow from sea to land, allowing moisture-filled winds to enter the continent. This moist air converges with a drier westerly flow and rises, producing heavy showers and thunderstorms. Thus, the summer monsoon is associated with a wet season where heavy rain is common.

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Monsoon Winds

Monsoon winds can be found anywhere where large contrasts in temperature develop between oceans and continents. However, they are most pronounced in southeast Asia. In addition, the strength of a monsoon is associated with the Southern Oscillation (which we will cover later in this unit), which is itself dependent on a cycle of ocean warming called El Niño. During an El Niño, the region commonly known for monsoon induced weather will experience below average rainfall, as sinking air caused by the El Niño inhibits cloud formation and convection. We will discuss El Niño in greater detail when we cover global systems.

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Wind and Global Weather Patterns

Mountain and Valley Breezes

Mountain and valley breezes develop along mountain slopes. During the day, sunlight warms the valley and the air in contact with it. The heated air, which is now less dense, moves upward along the valley as a valley breeze. During the night, the mountain slopes cool quickly, chilling the air in contact with them. This cooler air sinks downward along the valley slope, producing a mountain breeze. In many locations, upslope winds begin early in the morning, reach a peak speed of about 6 knots by midday, and reverse direction by late evening. Mountain breezes, on the other hand, reach their peak intensity during the morning hours, usually just before sunrise. In the Northern Hemisphere, valley breezes are most present on the southern slope of a valley, as sunlight directed there is most intense. If upslope valley breezes are moist and well developed, they may produce cumulus clouds above the mountain’s summit. Because of this, cloudiness, showers, and thunderstorms are most common over mountains during the warmest part of the day.

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Wind and Global Weather Patterns

Katabatic Winds

A katabatic wind is a downslope wind, but the term is usually reserved for winds that rush down slopes faster than a mountain breeze. Katabatic winds tend to form when an elevated plateau is surrounded by mountains, with an opening present that slopes rapidly downhill. When winter snows accumulate on the plateau, the air above becomes extremely cold, allowing a high pressure system to force at the surface. If the horizontal pressure gradient is strong along the slopes of the plateau, cold air can run downhill like water over a waterfall. This sudden, forceful movement of air can be quite dangerous.

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Wind and Global Weather Patterns

Chinook Winds

The chinook wind is a warm, dry wind that descends the eastern slope of the Rocky Mountains. The region of the chinook is relatively narrow, and extends from northeastern New Mexico into Canada. Similar winds are called a foehn in the European Alps and a zonda in Argentina. When a chinook blows through an area, temperatures rise sharply (sometimes 20°C or 36°F is a mere hour), and a sharp drop in relative humidity occurs. Because of this unique characteristic, chinooks are sometimes referred to as “snow eaters,” as their presence over heavy snow cover can melt and evaporate a foot of snow in less than a day!

Chinooks occur when strong westerly winds aloft flow over a north-south oriented mountain range. These ranges can produce troughs of low pressure on their eastern edges, forcing air downslope as it blows over. As the air descends, it is compressed and warms at the dry adiabatic rate (10°C/km) — thus, the warmth provided by a chinook is due to compressional heating.

The presence of clouds and precipitation on the mountain’s windward side can enhance the chinook. This is because the release of latent heat associated with these clouds may add to the compressional heating that the air experiences as it rushes downhill. In situations like these, the air is warmer on the bottom of the downward side than it is at the bottom of the upward side.

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Wind and Global Weather Patterns

Chinook Winds

A chinook wall cloud, as shown to the right, is a sign of an incoming chinook.

This cloud usually remains stationary as air rises, condenses, and then rapidly

descends the leeward sides. The result: foothill communities often experience

strong winds, but, during the wintertime, they may also receive a temporary

relief from the cold grasps of winter.

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Wind and Global Weather Patterns

Santa Ana Winds

A warm, dry wind that blows from the east or northeast into southern California is the Santa Ana wind. The wind often blows with great speeds, often up to 90 knots in the Santa Ana Canyon where they originate. These warm, dry winds form as a region of high pressure builds over the Great Basin. The clockwise rotation of air around this anticyclone forces air downslope, allowing it to heat up through compressional heating.

Santa Ana winds are known to cause forest fires as they rush through canyon passes.

A similar downslope wind called a California norther can also produce extremely high temperatures in the northern half of California’s Central Valley. Like with the Santa Ana, compressional heating warms the air of the California norther as travels downslope.

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Wind and Global Weather Patterns

Desert Winds

Winds of all sizes develop over the deserts. Huge dust storms form in dry regions, where strong winds are able to lift fine dust and spread them throughout the atmosphere (an example is a SAL outbreak over Africa, as mentioned in Unit 14). An example of a storm composed of dust or sand is the haboob. A haboob forms as cold downdrafts along the leading edge of a thunderstorm lifts dust and sand into a huge, tumbling dark that may extend horizontally for over 150 km and rise vertically to the base of the thunderstorm. Haboobs are most common in the African Sudan and the desert southwest of the US.

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Desert Winds

Dust devils, spinning vortices that can be seen on hot days, are common in dry areas. Dust devils generally form on clear, hot days over a dry surface where most of the sunlight goes into heating the surface rather than evaporating water from vegetation. The atmosphere directly above the hot surface becomes unstable, convection sets in, and the heat air rises. Wind then flows into this region, rotating the rising air. Dust devils can spin both clockwise and counterclockwise, and are often small and short-lived.

Dust devils are not tornadoes! Unlike tornadoes, the circulation of a dust devil always begins at the surface.

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Desert Winds

Winds originating over the Sahara Desert are given local names as they move through different regions. If a storm system is located west of Africa or southern Spain, a hot, dry, dusty easterly or southeasterly wind called a leste blows over Morocco and out into the Atlantic. If the wind crosses the Mediterranean, it becomes a leveche as it enters southern Spain. On the other hand, a sirocco is a warm, dry air from Africa that picks up moisture across the Mediterranean. A khamsin is dry, hot southerly wind that blows over Egypt, the Red Sea, and Saudi Arabia. In Israel, this wind is called the sharav. This wind is very hot and can raise air temperatures to 50°C or 122°F.

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Wind and Global Weather Patterns

Other Local Winds

A smattering of different wind patterns on our planet:

A chinook is a westerly wind off the eastern side of the Rocky Mountains.
A santa ana is an easterly towards Southern California.
A scirocco is a southerly from North Africa to southern Europe.
A mistral is a northwesterly from central France to the Mediterranean.
A marin is a southeasterly from Mediterranean to France.
A bora is a northeasterly from eastern Europe to Italy.
A gregale is a northeasterly from Greece.
An etesian is a northwesterly from Greece.
A libeccio is a southwesterly towards Italy.
A Texas norther is a cold wind that brings winter conditions into Texas (called a norte if it spreads into Central America).
A Boulder wind is a strong down-mountain wind that is common in Boulder, Colorado during the winter. They have been recorded at speeds of over 100 knots.

See the next slide for more local winds of interest around the world.

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Wind and Global Weather Patterns

Other Local Winds

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Wind and Global Weather Patterns

In the first section of this unit we talked about the local winds that affect different regions, winds that vary largely from day to day and from season to season. These local winds are merely a part of a even larger circulation — if we were to average local winds throughout a long period of time, we would receive a large picture: the general circulation of the atmosphere.

It is important to remember that general circulation only represents the average air flow around the world, and that actual winds may differ by region at any point in time (like the difference between climate and weather). The underlying cause of the general circulation is the unequal heating of the earth’s surface. Because the energy from the sun is not equally distributed between the equator and the poles, the atmosphere transfers warm air poleward and cool air equatorward. In the following few slides, we will look at some models that simplify the daily phenomenon of general circulation.

General Circulation: From Local to Global

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Wind and Global Weather Patterns

The Single-Cell Model

The first model we will look at is the single-cell model. With this model, we make three assumptions:

The earth’s surface is uniformly covered with water so that differential heating between land and water is negligible.
The sun is always directly over the equator, so the effects of the seasons are negligible.
The earth does not rotate, so we only need to consider the pressure gradient force.

With these three assumptions, the general circulation of the atmosphere on the side of the earth facing the sun would look a little bit like the figure shown below on the right. This

circulation of air is called the Hadley cell. The Hadley cell is a thermally

direct cell because it is driven by energy from the sun as warm air rises

and cold air sinks. Heating at the equator produces a surface low pressure

while cooling at the poles produces a surface high pressure. Due to this

pressure gradient, cold polar air blows equatorward at the surface.

The opposite occurs aloft. Higher up in the atmosphere, the pressure is

higher at the equator than at the poles (see Unit 16). Thus, air aloft at

the equator travels poleward, completing the cycle.

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Wind and Global Weather Patterns

The Three-Cell Model

The Earth, however, is not stationary. If we break the third assumption and allow the earth to spin, the single cell becomes three cells. These three cells work together to distribute energy.

From the equator to latitude 30°, the circulation is called the

Hadley cell. Over the equator, the air is warm, horizontal

pressure gradients are weak, and winds are light. This region

is known as the doldrums. At this location, warm air rises to

form giant cumulus and cumulonimbus clouds called

convective hot towers. This rise of air drives the Hadley cell

by pushing rising air toward the tropopause (which acts as

a cap to vertically growing clouds, as mentioned in Unit 2).

At this level, the pressure gradient forces the air polewards,

and the Coriolis force deflects this poleward flow toward the

right in the Northern Hemisphere and to the left in the

Southern Hemisphere. This produces westerly winds aloft

in both hemispheres.

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Wind and Global Weather Patterns

The Three-Cell Model

As air moves poleward from the tropics, it cools and begins to converge as it reaches the middle latitudes. This causes the air pressure at the surface to rise. The result from this increase in air pressure is a weather phenomenon called a subtropical high, usually forming at a latitude of 30°. Because of this, the air above the highs slowly descend and warms by compression. As a result, regions near the latitude 30° tend to be generally clear and warm (this is why all the major deserts on Earth are located near 30°).

Over the ocean, the weak pressure gradient produces weak winds.

When ships travelling from Europe to the New World got stuck in

this region, the weak winds slowed down the trip drastically, forcing

sailors to eat their own horses as food supplies dwindled. Because

of this, this region is called the horse latitudes.

At the surface, some of the surface air moves from the horse

latitudes back to the equator. The Coriolis effect deflects these

winds in a way such that they blow from the NE in the Northern

Hemisphere and from the SE in the Southern Hemisphere. These

winds are called trade winds, and the boundary at which they

converge is known as the intertropical convergence zone (ITCZ),

where air rises and continues its cycle through the cells.

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The Three-Cell Model

At the horse latitudes, however, not all of the surface air moves equatorward. Some air moves toward the poles and deflects toward to the east, producing a wind pattern known as the prevailing westerlies in both hemispheres. As the air travels poleward, it encounters cold air moving down from the poles. The boundary between the mild poleward moving air and the cold equatorward moving air is known as the polar front, and a zone of low pressure called the subpolar low forms in this area. At this subpolar low,

surface air converges and rises, and storms and clouds develop.

Some of the rising air returns at high levels to the horse latitudes,

where it sinks back to the surface near the subtropical high.

This cell, where cool air rises and warm air sinks, is called the

Ferrel cell. Surface air in the Ferrel cell moves from the horse

latitudes toward the polar front.

Behind the polar front, the Coriolis effect deflects toward the west.

This results in an east-to-west wind called a polar easterly. In this

region, rising air moves toward the poles, and the Coriolis effect

deflects the air into a westerly wind at high levels. After the air

aloft reaches the poles, it sinks and flows back toward the polar

front. This produces a cell known as the polar cell.

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The Three-Cell Model

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Wind and Global Weather Patterns

The Three-Cell Model in the Real World

How does this model compare with the real world? When we observe the real world, we may notice that there are pressure systems that appear to persist throughout the year. These systems are known as semipermanent highs and lows because they move very little during the course of a year.

In January, there are four semipermanent pressure systems in the Northern Hemisphere. Between latitudes 25°N and 35°N sits the Bermuda high (see Unit 14 for how this affects Atlantic cyclone steering). The Bermuda High’s counterpart, the Pacific high, sits in the Pacific Ocean. We can find trade winds to the south of these anticyclones and prevailing westerlies to their north.

Where we would expect the polar front, there are two semipermanent lows. The Icelandic low covers Iceland and southern Greenland, while the Aleutian low sits over the Gulf of Alaska near the Aleutian Islands. These zones form in regions where storms converge after traveling eastward, especially during the wintertime.

In January, the shallow Siberian high and Canadian high are not semipermanent, as they are only present due to the intense cooling of the land. As summer approaches, the land warms, and these cool shallow highs disappear. In some cases, a surface low replaces the high in the summertime — these lows are called thermal lows. An example is the monsoon low that forms over India as the Asian continent warms.

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The Three-Cell Model in the Real World

Compare the January and July maps to the right. The strong subpolar lows present in January are hardly recognizable during July. The subtropical highs, however, remain dominant year round.

Because the sun is over the Northern Hemisphere in July and the Southern Hemisphere in January, the zone of maximum surface heating shifts seasonally. As a result, the major pressure systems, wind belts, and ITCZ shift as well. They shift toward the north in July and toward the south in January. The below figure shows a situation where the general circulation has been displaced southward.

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Wind and Global Weather Patterns

General Circulation and Precipitation Patterns

The position of the major features of the general circulation and their latitudinal displacement play a large role in influencing the precipitation patterns in those regions. For instance, we would expect abundant rainfall where the air rises and very little in the regions where air sinks. Because of this, areas of high rainfall exist near the ITCZ and between 40° and 55° latitude, where air is forced upward by the polar front. On the hand, regions at 30° latitude tend to have lower rainfall levels.

The region between the doldrums and the horse latitudes are influenced by both the ITCZ and the subtropical high. In the summer, the subtropical high moves poleward and the ITCZ enters this area, bringing with it ample rainfall. In the winter, the subtropical high moves back toward the equator and brings clear, dry weather with it.

This works the same way off the Pacific coast. The Pacific high drifts northward toward the California coast during the summer, bringing to the region dry and fair weather conditions. During the winter, it moves back down, giving the polar front a chance to enter the region. Due to this, summers in California tend to be dry while winters in California tend to be wet.

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Wind and Global Weather Patterns

General Circulation and Precipitation Patterns

Note below that the clockwise circulation of winds around the Bermuda high brings warm, tropical air northward from the Atlantic and Gulf into the eastern seaboard of the United States. This humid air can rise and condense into looming thunderstorm clouds. Thus, the anticyclonic motion of these oceanic highs gives dry summer conditions to California and wet conditions to states such as Georgia.

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Wind and Global Weather Patterns

Pressure Patterns Aloft

The charts to the right are the average global 500 mb pressure charts for the months of January and July. Notice that on the January map, both the Icelandic and Aleutian lows aloft are located to the west of their surface counterparts (as shown 3 slides ago). On the July map, notice that the subtropical high pressure areas of the Northern Hemisphere appear as belts of high pressure that circle the globe below 30°N latitude. In both hemispheres, the air is warmer over low latitudes and colder over high latitudes. This creates a horizontal pressure gradient that causes the winds to blow from west to east, especially in the middle and high latitudes. This gradient is steeper in January than in July, resulting in stronger winds aloft during the former month. The westerly winds aloft do not extend all the way to the equator; they are easterly on the equatorward side of the upper-level subtropical highs.

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Pressure Patterns Aloft

The speed of wind is directly related to the pressure gradient and inversely related to the air density. As we rise above the 500 mb level, the decrease in air density increases wind strength. In addition, the north-south temperature gradient causes the horizontal pressure gradient to increase in speed up to the tropopause.

Above the tropopause, the temperature gradients reverse, reducing the strength of the westerly winds. Thus, the strongest winds concentrate into narrow bands at the tropopause, creating rivers of fast-flowing air that play a major role in dictating the weather conditions that we experience. These fast flowing rivers are called jet streams, and we will study them in greater depth in the next few slides.

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Wind and Global Weather Patterns

Jet Streams

Jet streams in the atmosphere are swiftly flowing air currents that can be found near the tropopause (10 to 15 km elevation). They are thousands of kilometers long, a few hundred kilometers wide, and only a few kilometers thick. Wind speeds at the central core of a jet stream often exceed 100 knots and sometimes even exceed 200 knots. As shown below to the left, there are two jet streams, both located in tropopause gaps. The jet stream located near 30° latitude and 13 km above the surface is known as the subtropical jet stream, while the jet stream located near 60° latitude and 10 km above the surface is known as the polar front jet stream or polar jet stream. Both are found in the tropopause, so they can also be referred to as tropopause jets. Jet streams often flow in a wavy pattern, and the polar jet stream may occasionally split into a northern and southern branch. The figure to the right shows how the jet stream may appear.

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Jet Streams

Jet streams play a major role in transferring heat across our planet. Notice that some sections of the jet stream are meridional in nature. This orientation is important: in the Northern Hemisphere, when the jet stream air flows southward, it brings cold air toward the equator; in contrast, when the jet stream air flows northward, it brings warm air toward the poles. The looping nature of the jet stream is also responsible for the formation of mid-latitude cyclones, which we will look at in Unit 19.

The cause of jet streams lies in the energy imbalance between high and low latitudes. In the next few slides, we will analyze how they form.

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Jet Stream Formation

Remember that the polar front separates cold polar air from warm subtropical air. The temperature difference between these two sides is extremely great, and air that crosses this front undergoes a rapid change in temperature (and subsequently a rapid change in pressure). This sudden change in pressure along the front sets up the steep pressure gradient responsible for the winds that create the jet stream.

The temperature contrast along the polar front is strongest during the winter and weakest during the summer. This is why the winds blow stronger and the jet stream is located further south in the winter and why the jet is weaker and found farther north in the summer.

The subtropical jet stream tends to form along the poleward side of the Hadley cell. Here, warm air carried poleward by the Hadley cell produces sharp temperature contrasts along a boundary called the subtropical front. In the vicinity of the subtropical front, sharp contrasts in temperature produce sharp contrasts in pressure and strong winds.

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Other Jet Streams

There is another jet stream that forms in the summer near the tropopause above Southeast Asia, India, and Africa. Here the altitude of the summer tropopause and the jet stream is near 15 km, and the winds are easterly because it is on the equatorial side of the upper-level subtropical high. Because of this, this jet stream is known as the tropical easterly jet stream. It’s formation appears to be related to the warming of the air over large elevated land masses, such as Tibet. During the summer, the air above this region is warmer than the air above the ocean to the south, producing a temperature gradient that results in strong easterly winds near 15°N.

Not all jet streams form at the tropopause. During the dark polar winter, there is a jet stream that forms near the top of the stratosphere. This jet stream is known as the stratospheric polar night jet stream. The stratospheric jet disappears during the summer, as the greater sunlight hours that the poles receive increases stratospheric temperatures over the poles at a greater rate than at the same altitude at locations closer to the equator.

Jet streams from near the earth’s surface as well. These low-level jets tend to form at night above a temperature inversion and is sometimes referred to as a nocturnal jet stream for this reason.

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Other Jet Streams

During the summer, strong southerly winds carry moist air from the Gulf of Mexico toward the Central Plains. This moisture combines with converging, rising air of a low-level jet to enhance thunderstorm formation. Because of this, on moist, summer nights when the low-level jet is present, it is common to have nighttime thunderstorms over the plains.

This concludes our analysis on the jet stream. In the final section of this unit, we will look at global weather patterns caused by atmosphere-ocean interactions.

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Wind and Global Weather Patterns

Surface Ocean Currents

As wind blows over the oceans, it causes the surface water to drift along with it. The moving water gradually piles up, creating pressure differences within the water itself. This leads to further motion several hundreds of meters down into the water. Due to this, the general wind flow across the globe is responsible for starting major ocean currents moving.

Due to drag, oceans currents move a lot slower than the prevailing wind. Typically, these currents range in speed from several kilometers per day to several kilometers per hour. In addition, major ocean currents do not follow wind patterns exactly. Instead, they spiral in semi-closed circular whirls called gyres. Surface water tends to move in a circular pattern as winds blow outward, away from the center of the subtropical highs.

Some important ocean currents include the Gulf Stream, which carries a vast quantity of warm tropical water into higher latitudes, and the Labrador Current, which brings cold water down to the east seaboard of the United States. The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and the west coast of Europe. The consensus is that the presence of the Gulf Stream allows the climate of Western and Northern Europe to be warmer than it otherwise would have been without this movement of warm water. The next slide shows a map of important global ocean currents.

Distinct temperature gradients exist along the boundaries of surface ocean currents. The boundary separating two masses of water with contrasting temperatures and densities is an oceanic front.

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Surface Ocean Currents

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El Niño and La Niña

Wind and Global Weather Patterns

Every few years, a warm episode occurs in the eastern tropical Pacific. This extremely warm episode, which occurs at irregular intervals of two to seven years and covers a large area of the Pacific Ocean, is known as El Niño. The term “El Niño” is Spanish for “The Little Boy”. It was named this because fishermen in the Pacific noticed that this period of warm water occurred near Christmas. This climate pattern can change the weather of the United States, particularly in California and the southern states. Usually, El Niño brings more rain and higher temperatures. El Niño may also bring warmer than normal winter temperatures to the eastern part of the United States.

A similar phenomenon, La Niña, works in the opposite way. During La Niña, the waters in the same area along the equator gets colder than usual. This, too, affects weather around the globe and in the U.S. According to scientists, La Niña cycles generally create a more active hurricane season in the Atlantic.

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El Niño and La Niña

Wind and Global Weather Patterns

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El Niño and La Niña

Wind and Global Weather Patterns

Why does the ocean become so warm over the eastern Pacific during an El Niño? Normally, in the tropical Pacific Ocean, the trades are persistent winds that blow westward from a region of higher pressure over the eastern Pacific toward a region of lower pressure centered near Indonesia. The trades create upwelling (or rising of cold water to the surface after old surface water drifts away) that brings cold water to the surface. As this water moves westward, it is heated by sunlight in the atmosphere. Thus, the surface water along the equator is usually cool in the east and warm in the west. In addition to this, the dragging of surface water by the trades raises sea level in the western Pacific and lowers it in the eastern Pacific.

Every few years, however, the surface atmospheric pressure patterns break down, and air pressure rises over the western Pacific and falls in the eastern Pacific. This weakens the trades, and the strong pressure reversals that result, east winds are replaced by west winds. Warm surface water in the Pacific is then blown eastward in a surge known as a Kelvin wave. Toward the end of the warming period, the atmospheric pressure over the eastern Pacific rises again, whereas the pressure over the western Pacific falls.

This seesaw pattern of pressure reversals between the western and eastern Pacific is known as the Southern Oscillation. Because these reversals and El Niño/La Niña are essentially simultaneous, the entire phenomenon is grouped together as the El Niño/Southern Oscillation, or ENSO.

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El Niño and La Niña

Wind and Global Weather Patterns

During strong ENSO events, the easterly trades may become westerly. As these trades push eastwards, they drag surface water with them, raising sea levels in the eastern Pacific and vice versa for the western Pacific. The eastward-moving water heats up from the sun and becomes a lot warmer than normal in the eastern Pacific. The warm tropical water fuels the atmosphere with additional warmth and moisture, which results in storms and rainfall. The frequency of typhoons and hurricanes in the Pacific increases, while hurricanes in the Atlantic tend to decrease in frequency (the winds aloft associated with El Niño / ENSO positive tend to disrupt thunderstorm organization in this basin). In addition, as previously mentioned, El Niño weakens the tendency for monsoon conditions over India.

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Wind and Global Weather Patterns

What if the opposite happens? During an ENSO negative phase (La Niña), the trades instead are exceptionally strong. This confines the warm water and rainy weather to the western Pacific, providing cooler temperature to the eastern Pacific. Atlantic storms, in this situation, are more likely to organize into tropical systems.

The below figure depicts the typical winter weather patterns that are associated with El Niño and La Niña.

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Wind and Global Weather Patterns

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Wind and Global Weather Patterns

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Other Ocean-Atmosphere Interactions

Wind and Global Weather Patterns

A long term Pacific Ocean temperature fluctuation, called the Pacific Decadal Oscillation (PDO), occurs every 20 to 30 years. During the warm (positive) phase, warm surface water can be found along the west coast of North America, whereas cooler than normal temperatures prevail over the central North Pacific. At the same time, the Aleutian low in the Gulf of Alaska strengthens, allowing Pacific storms to move into Alaska and California. This results in warmer and drier winters throughout northwestern North America, drier winters over the Great Lakes region, and cooler and wetter conditions in the southern United States.

The cool (negative) phase is associated with cooler than normal surface water along the west coast of North America and an area of warmer than normal surface water extending from Japan to the north Pacific. Winters during the cool phase tend to be cooler and wetter than average over northwestern North America, wetter over the Great Lakes, and warmer and drier in the southern United States.

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Other Ocean-Atmosphere Interactions

Wind and Global Weather Patterns

A reversal of pressure called the North Atlantic Oscillation (NAO) exists over the Atlantic. This has an effect on the weather in Europe and along the east coast of North America. During the positive phase of the NAO, strong westerlies produced by an increased pressure gradient — as pressure decreases near the Icelandic low and increases near the Bermuda high — direct strong storms on a more northerly track into northern Europe, where winters tend to wet and mild. Meanwhile, conditions in the eastern United States tend to be wet and mild, and conditions in northern Canada and Greenland are usually cold and dry.

During the negative phase of the NAO, pressure in the vicinity of the Icelandic low rises while pressure near the Bermuda high drops. This results in weaker westerlies that steer fewer and weaker winter storms across the Atlantic. With the negative phase, southern Europe tends to experience wet weather, while winters in Northern Europe end up cold and dry, along with winters over the eastern coast of North America. Greenland and northern Canada usually experience mild winters during the negative phase.

Lastly, a related oscillation called the Arctic Oscillation (AO) is associated with atmospheric pressure changes between the Arctic and regions to the south. These changes are due to changes in upper level westerly winds. During the positive warm phase of the AO, strong pressure differences prevent cold arctic air from invading the United States, resulting in warmer than normal winters. The positive phase also gives northern Europe wet, mild weather. During the negative cold phase, smaller pressure differences between the Arctic and its surroundings produce weather westerly winds aloft. This allows cold arctic air to penetrate farther south, producing colder than normal winters over the United States, northern Europe, and northern Asia. The effects on Greenland, however, are opposite (negative = warmer, positive = colder).

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Other Ocean-Atmosphere Interactions

Wind and Global Weather Patterns

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Other Ocean-Atmosphere Interactions

Wind and Global Weather Patterns

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Fronts and Air Masses

“If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them.”

― Henry David Thoreau, Transcendentalist

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Air Masses

Fronts and Air Masses

An air mass is an extremely large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given altitude. Regions where air masses originate are called source regions. In order for a huge mass of air to develop uniform characteristics, its source region should be generally flat and of uniform composition with light surface winds. The longer the air remains stagnant over its source region or the longer the path over which the air moves, the more likely it will acquire properties of the surface below. Ideal source regions are usually dominated by surface high pressure.

Air masses are classified according to their temperature and humidity, both of which remain fairly uniform in any horizontal direction. Air masses that originate in polar latitudes are designated by the letter “P” (for polar), and those that originate in warm tropical regions are designated by the letter “T” (for tropical). If the source region is land, the air mass will be dry and labeled with the letter “c” (for continental). If the source region is water, the air mass will be moist and labeled with the letter “m” (for maritime). The lowercase letters precede the uppercase letters. In winter, an extremely cold air mass from the arctic may be designated “cA” for continental arctic. See the table and diagram on the next slide for a better indication of what these air masses are and where they come from.

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Air Masses

Fronts and Air Masses

A brief introduction:

Polar air masses are located poleward of 60° north and south.
Tropical air masses are located within about 25° from the equator.
Continental air masses are dry and are located over large land masses.
Marine air masses are located over the oceans.
Continental Polar (cP) air masses are cold, dry, and stable.
Continental Tropical (cT) air masses are hot and dry, with stable air aloft and unstable air at the surface.
Maritime Polar (mP) air masses are cool, moist, and unstable.
Maritime Tropical (mT) air masses are warm, moist, and usually unstable.
Continental Arctic (cA) air masses are extremely cold, dry, and stable.

An air mass that moves to a warmer region will be warmed from below, which produces instability at low levels. This is associated with good visibility, cumuliform clouds, and showers of rain or snow. If, on the other hand, winds aloft blow an air mass to a colder region, it will be chilled by the ground below. This leads to very little vertical mixing, resulting in poor visibilities.

In the next few slides, we will look at each of these air masses in depth.

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Continental Polar (cP) Air Masses

Fronts and Air Masses

The cold weather that enters southern Canada and the United States in the winter is associated with a continental polar or continental arctic air mass. These air masses originate over the ice and snow covered regions of the arctic, where long, clear nights allow for strong radiational cooling at the surface. Air in contact with the surface becomes quite cold and stable and, since little moisture is added to the air, it is also quite dry (with dewpoints often less than -30°C). These air masses break off and travel southward under the influence of air flow aloft. The Texas Norther mentioned in the previous unit is associated with this air mass. Cumulus clouds are seldom associated with this air mass because it is so dry.

When this cold, dry air mass moves over a relatively warm body of water, such as the Great Lakes, heavy snow showers called lake-effect snow often form on the eastern shores (as mentioned in Unit 13).

Fair weather accompanying cP and cA air is due to the stable nature of the atmosphere aloft. Sinking air develops a large dome of high pressure, and warmer air lies above colder surface air. Thus, a strong upper-level subsidence inversion often forms, which may worsen atmospheric visibility for a few days.

cP air that moves into the US in summer has properties similar to air that arrives in the winter. A summertime cP usually brings relief from oppressive heat in the central and eastern states as cooler air lowers the air temperatures in those regions. Daytime heating warms the lower layers, producing surface instability. Water vapor from rising air may create a sky dotted with fair weather cumulus clouds.

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Continental Polar (cP) Air Masses

Fronts and Air Masses

In summary, polar and arctic air masses are responsible for bitter cold weather that can cover large swaths of North America. When the air mass originates over the Canadian Northwest Territories, frigid air can bring record breaking low temperatures. Below shows the surface weather map for the US during the frigid winter of 1983-1984, frequently dubbed as the “Siberian Express.” Here, a continental polar air mass covers most of the plains, if not the entire United States.

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Maritime Polar (mP) Air Masses

Fronts and Air Masses

During the winter, cP and cA originating over Asia and frozen polar regions is carried eastward and southward over the Pacific Ocean by the circulation around the Aleutian low. The ocean water modifies these cold air masses by adding warmth and moisture to them. During its trek across the ocean, these air masses gradually turn into maritime polar air masses.

When the mP air mass reaches the Pacific Coast, it is cool, moist, and conditionally unstable. As the air moves inland, coastal mountains force it to rise, and much of the water vapor associated with it condenses into rain-producing clouds. In the colder air aloft, the rain changes into snow, with heavy amounts accumulating in the mountain regions. After moving over the mountain ranges, it loses much of its moisture (see diagram below) and becomes a dry, stable Pacific air that brings fair weather and temperatures that are cool (but not as cold as air from cA and cP air masses) to regions east of the Rockies.

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Maritime Polar (mP) Air Masses

Fronts and Air Masses

Along the East Coast, mP air originates in the North Atlantic as continental polar air moves southward some distance off the Atlantic coast. Steered by northeasterly winds, mP air then swings southwestward toward the northeastern states. Because the water of the North Atlantic is very cold and the air mass travels only a short distance, wintertime Atlantic mP air masses are usually much colder than their Pacific counterparts. Because the prevailing winds aloft are westerly, Atlantic mP air masses are also much less common. The figure to the right shows a late winter or early spring weather pattern that brings mP air from the Atlantic into New England and the middle Atlantic states.

The storms that form along the stationary front shown are referred to as Hatteras lows, and some may even swing northeastward along the coast to become nor’easters (as introduced in Unit 13).

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Maritime Tropical (mT) Air Masses

Fronts and Air Masses

The wintertime source region for Pacific maritime tropical air masses is the subtropical east Pacific Ocean, as shown in the figure below. These air masses are very warm and moist by the time they arrive along the West Coast. In the winter, the warm air produces precipitation usually in the form of rain, even at high elevations. The movement of maritime tropical air led to the formation of the Pineapple Express, as described back in Unit 11.

The warm, humid subtropical air that influences much of the weather east of the Rockies originates over the Gulf of Mexico and the Caribbean Sea. In winter, cold polar air tends to dominate the continental weather scene, so maritime tropical air is usually confined to the Gulf and extreme southern states.

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Maritime Tropical (mT) Air Masses

Fronts and Air Masses

In the summer, the circulation of air around the Bermuda High pumps warm, humid mP air northward off the Gulf of Mexico and Atlantic into the eastern half of the United States. As this humid air moves inland, it warms even more, rises, and frequently condenses into cumuliform clouds that produce afternoon thunderstorms. This thunderstorm activity tends to die off as nighttime approaches.

During the summer of the other half of the United States, humid subtropical air originating over the southeastern Pacific and Gulf of California normally remains south of California. Occasionally, an upper level southerly flow will spread this humid air northward into the southwestern United States, most often Arizona, Nevada, and southern California (an example is shown to the right). This moist, conditionally unstable air usually produces altocumulus and cirrocumulus clouds; however, if provided with a mountain barrier to rise up on, shower producing clouds aren’t infrequent.

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Continental Tropical (cT) Air Masses

Fronts and Air Masses

The only real source for hot, dry continental tropical air masses in North America is found in northern Mexico and southwestern United States during the summer. Here, the air mass is hot, dry, and unconditionally stable at low levels. Air must rise over 3000 m before condensation can occur here.

Generally, the skies associated with cT air masses are generally clear, the weather is hot, and rainfall is practically nonexistent where cT masses prevail. If this air mass moves outside its source region and into another (such as the Great Plains), a severe drought may ensue.

This concludes our discussion of air masses. In the next section of this unit, we will look at fronts.

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Fronts

Fronts and Air Masses

A front is the transition zone between two air masses of different densities. Since density differences are most often caused by temperature contrasts, fronts usually separate air masses with contrasting temperatures (and occasionally different humidities). The upward extension of a front is referred to as a frontal surface or frontal zone.

To the right is a figure that illustrates the vertical extent of two frontal zones: the polar front and the arctic front. The polar front boundary, which extends upward to over 5 km, separates warm, humid air to the south from cold polar air to the north. The arctic front, which separates cold from extremely cold arctic air, is much more shallow than the polar front and only extends upward to an altitude of about one or two kilometers. Even as we look at fronts on flat surface weather maps, it’s important to remember that these fronts have both a horizontal and vertical extent.

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Fronts

Fronts and Air Masses

What do fronts look like on a surface map? Below is a weather map showing four different fronts. These fronts are associated with lower pressures and separate differing air masses. We will be examining the properties of each of these fronts for the rest of this unit.

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Cold Fronts

Fronts and Air Masses

A cold front represents a zone where cold, dry stable polar air is replacing warm, moist conditionally unstable subtropical air. The front is represented by a solid blue line with triangles facing the direction of movement.

The following criteria are used to locate a front on a surface weather map.

sharp temperature changes over a relatively short distance
changes in the air’s moisture content (as shown by marked changes in the dew point)
shifts in wind direction
pressure and pressure changes
clouds and precipitation patterns

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Cold Fronts

Fronts and Air Masses

Take a look at the close up of the map shown two slides ago.

Notice that the isobar kinks as it crosses the front, forming an

elongated area of low pressure that accounts for the wind shift.

This elongated area of low pressure is a trough.

How do you locate a cold front? The lowest pressure usually occurs

just as the cold front passes a station. As you move toward the

front, the pressure drops, and as you move away from it, the

pressure rises. This can be shown with the lines to the right of the

individual weather symbols (we will cover this later): behind the

front, the little lines slope upward (pressure rise), while ahead of

the front, the little lines slope downward (pressure fall).

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Cold Fronts

Fronts and Air Masses

Along a cold front, cold dense air wedges under warm air, forcing it upward. As this moist, unstable air rises, it condenses into a series of vertically growing cumuliform clouds. Ahead of the front, cirrostratus and cirrus clouds can be seen (these are blown off by the strong upper-level westerly winds). At the front itself, a narrow band of thunderstorms produces heavy showers with gusty winds. Behind the front, the air cools quickly: the winds shift from southwesterly to northwesterly, pressure rises, and precipitation ends. The skies clear as the air dries out.

Fast moving cold fronts, as shown below, have very steep leading edges. This is due to friction, which slows the airflow near the ground. Cold fronts that move slower have much more gentle slopes.

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Cold Fronts (Squall Lines)

Fronts and Air Masses

Occasionally, out ahead of a fast-moving front, a line of active showers and thunderstorms called a squall line may develop parallel to and ahead of the advancing front. These squall lines produce heavy precipitation and strong gusty winds. You may see the following symbol used to depict a squall line on a weather map:

Below shows what a squall line would look like on Doppler Radar:

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The survival of a cold front depends on the temperature contrast at the leading edge of the front. If the temperature contrast lessens, a front may weaken and dissipate. Such a process is called frontolysis.

On the other hand, an increase in temperature contrasts across a front can cause it to strengthen and regenerate into a more vigorous frontal system. This process is called frontogenesis.

The figure below shows a weakening cold front over land intensifying into a stronger cold front over the Atlantic. This frontogenesis was fostered by the presence of the warm Gulf Stream, which increased the temperature contrast at the edge of the cold front.

Cold Fronts

Fronts and Air Masses

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Cold fronts usually move toward the south, southeast, or east. But in some cases, they will move southwestward. These cold fronts from the east are known as back door cold front. As the front passes, westerly surface winds usually shift to easterly or northeasterly and temperatures drop as mP air flows in off the Atlantic Ocean. An example of a back door cold front is shown in the figure. Behind this front, the weather is cold and damp with drizzle; meanwhile, ahead of the front, temperatures are much warmer. However, once the front crosses, this weather would change rapidly to become more winterlike.

In the situation provided, the Appalachian Mountains act as a block to the front’s forward progress. This prevents the back door cold front from continuing west, and the cold, damp air is trapped behind the eastern side of the mountains. This process is called cold air damming, and the trapped cold front becomes a stationary front (which we will look at very soon).

Cold Fronts

Fronts and Air Masses

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The following weather conditions are often associated with a cold front in the Northern Hemisphere.

Cold Fronts

Fronts and Air Masses

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Warm Fronts

Fronts and Air Masses

A warm front represents a zone where warm, moist, subtropical mT air from the Gulf of Mexico replaces retreating cold mP air from the North Atlantic. The front is represented by a solid red line with semicircles facing the direction of movement.

The average speed of a warm front is about 10 knots, or about half that of an average cold front. Warm fronts tend to travel faster during the day than during the night.

When the forward surface edge of the warm front passes a station, the wind shifts, the temperature rises, and the overall weather conditions improve.

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Warm Fronts

Fronts and Air Masses

The map to the right shows a close up view of a warm front. When the

warmer, less dense air rides up and over the colder more dense air — a

process known as overrunning — clouds and precipitation occurs well in

advance of the front’s surface boundary. The slope of the leading edge of

a warm front is a lot more gentle than that of a cold front.

As shown in the figure below, the warm air overriding the cold air creates

a stable atmosphere. This temperature inversion, called a frontal inversion,

occurs in the region where the warm air overrides the cold air. Also notice

that the winds shift clockwise with altitude, with southeasterly surface

winds becoming more and more southwesterly and westerly with height.

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Warm Fronts

Fronts and Air Masses

A close up of the figure on last slide:

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Warm Fronts

Fronts and Air Masses

Heading toward a warm front, cirrus clouds overhead gradually begin to thicken into a veil of cirrostratus clouds (whose ice crystals cast a halo around the sun, as mentioned in Unit 9). These high level clouds begin to evolve into altocumulus and altostratus clouds as we approach the warm front. Precipitation (if the atmosphere is moist enough and conditionally unstable) begins to fall as the clouds become nimbostratus clouds, and the atmospheric pressure begins to slowly fall. When we move past the warm front’s surface boundary, the temperature and the dew point begin to slowly rise, the pressure stops falling, and the precipitation ends. The changes associated with a warm front are more gradual than the sudden changes often associated with a cold front.

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The following weather conditions are often associated with a cold front in the Northern Hemisphere.

Warm Fronts

Fronts and Air Masses

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Drylines

Fronts and Air Masses

Drylines are not warm fronts or cold fronts; rather, they represent a narrow boundary where there is a steep horizontal change in moisture. Drylines separate moist air from dry air. Because dew points are known to drop rapidly along the dryline boundary, drylines are also known as dew-point fronts. They are most common in the western half of Texas, Oklahoma, and Kansas, especially during spring and early summer.

The dryline is represented with a brown line with semicircles, as shown below. These semicircles point toward the humid side of the line.

Using the map of the dryline over Texas on the right, the area to the

west contains dry, warm continental tropical air from the southwest.

To the east, humid, warm maritime tropical air can be found coming

from the Gulf of Mexico. On this humid side, temperatures are slightly

lower and the humidity and dew points are significantly higher.

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A stationary front represents a front where there is no movement. Surface winds tend to blow parallel to the front, but in opposite directions on either side of it. Upper-level winds often blow parallel to a stationary front. On a weather map, the stationary front is drawn in as an alternating red and blue line with both semicircles and triangles.

If the warmer air at a stationary front begins to move and replace the colder air, the front would then become a warm front. If, on the other hand, the cold air replaces the warm air on the other side, the front would then become a cold front. Only when neither the cold front or a warm front is moving will the front in question be considered stationary!

Stationary Fronts

Fronts and Air Masses

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An occluded front can be found if a cold front catches up and overtakes a warm front. On a surface weather map, an occluded front is represented by a solid purple line with alternating semicircles and triangles, as shown below.

There are two types of occlusion: cold occlusion and warm occlusion. Cold occlusion can be identified if the air behind an occluded front is colder than the air ahead of it. As a cold occluded from approaches, the weather sequence that follows is similar to that of a warm front, with high clouds lowering and thickening into middle and low clouds, with precipitation forming well in advance of the surface front. However, the frontal passage brings weather similar to that of a cold front: heavy, often showery precipitation with winds shifting to the west or northwest. After a period of wet weather, the sky begins to clear, atmospheric pressure rises, and the air turns colder. The most violent weather usually occurs when the cold front is just overtaking the warm front, at the point of occlusion where the greatest contrast in temperature exists.

Occluded Fronts

Fronts and Air Masses

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On the other hand, if the air in front of a warm front is colder than the air behind the cold front as it catches up and overtakes, the milder, lighter air behind the cold front will be unable to lift the colder, heavier air off the ground. As a result, the cold front rides “piggyback” along the sloping warm front. This produces warm occlusion, and the weather patterns associated with this phenomenon are similar to those of a warm front.

The primary difference between warm and cold occlusion fronts lies in the location of the upper-level front. In a warm occlusion situation, the upper-level cold front precedes the surface occluded front, while in a cold occlusion situation the upper warm front follows the surface occluded front. The next slide provides images that detail this difference.

Occluded Fronts

Fronts and Air Masses

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Occluded Fronts

Fronts and Air Masses

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Occluded Fronts

The following weather conditions are often associated with occluded fronts in North America.

Fronts and Air Masses

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An upper-air front is a front that is present aloft. This front may or may not extend down the surface. The figure below shows a north-to-south side view of an idealized upper-air front. The front forms when the tropopause dips downwards and folds under the polar jet stream. The small arrows in the figure show air motion associated with the upper front. On the north side of the front (and north of the jet stream), the air is slowly sinking, and ozone-rich air from the stratosphere is allowed to descend into the troposphere. To the south of the front (and south of the jet stream), the air slowly rises. These rising and descending air motions can aid in mid-latitude cyclone development.

Upper-Air Fronts

Fronts and Air Masses

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In Unit 16, we covered the basics of wind and its formation. We then used these basics and applied them to global wind patterns, global circulations, and weather systems in Unit 17.

You might be wondering: does this unit and the next follow the exact same format? If you are thinking this, you are absolutely correct! This unit acts as a foundation for the bigger concept that we will cover in the next unit: the mid-latitude cyclone.

The mid-latitude cyclone is made up of the frontal systems described in this chapter. The figure to the right shows how a cold front, warm front, and occluded front can come together in a mid-latitude cyclone. We will be looking closely at this phenomenon in the next unit!

(Note: if you doing the Division A competition, the information in the next unit will not be as important as the information in this one.)

A Preview: Mid-Latitude Cyclones

Fronts and Air Masses

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Mid-Latitude Cyclones

“The pessimist complains about the wind;

the optimist expects it to change;

the realist adjusts the sails.”

― William Arthur Ward, Motivational Writer

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The Polar Front Theory

Mid-Latitude Cyclones

Using surface observations, a group of scientists in Norway developed a model explaining the life cycle of an extratropical or middle-latitude cyclonic storm. An extratropical storm is a storm that forms at middle and high latitudes outside of the tropics. This model became known as the polar front theory, and it shows how a mid-latitude cyclone progresses through birth, growth, and decay. An important of the model involved the development of weather along the polar front. In this unit, we will look at how a mid-latitude cyclone forms along a polar front and how winds aloft influences the developing surface storm.

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The Polar Front Theory

Mid-Latitude Cyclones

According to this model, the development of a mid-latitude cyclone begins along the polar front. The polar front, as mentioned previously, is a semicontinuous global boundary separating cold polar air from warm subtropical air. Because the mid-latitude cyclone forms and moves along the polar front in a wavelike manner, the developing storm is called a wave cyclone. The stages of developing wave cyclones are shown in the previous slide.

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The Polar Front Theory

Mid-Latitude Cyclones

Under the right conditions, a wavelike kink forms on the front, as shown in part b of the diagram below. This wave is known as a frontal wave or an incipient cyclone. Here, the region of lowest pressure (called the central pressure) is located at the junction between the two fronts. As the cold air displaces the warm air upward along the cold front, a narrow band of precipitation forms (shaded green blow). This system is then steered by winds aloft, gradually becoming a fully developed open wave in 12 to 24 hours.

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The Polar Front Theory

Mid-Latitude Cyclones

The central pressure here is now much lower, and the tightly packed isobars that result create a stronger cyclonic flow, as the winds swirl counterclockwise and inward toward the low’s center. Precipitation forms in a wide band ahead of the warm front and along a narrow band of the cold front. The region of warm air between the cold and warm fronts is known as the warm sector. Here, the weather tends to be partly cloudy, but scattered showers may occasionally develop if the air is conditionally unstable.

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The Polar Front Theory

Mid-Latitude Cyclones

As the open wave moves eastward, its central pressure continues to fall, and the winds blow more vigorously as the wave quickly develops into a mature cyclone. The faster moving cold front constantly inches closer to the warm front, squeezing the warm sector into a smaller area (see part d in the diagram). Eventually, the cold front overtakes the warm front and the system becomes occluded. As this point, the storm is most intense, with clouds and precipitation covering a large area.

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The Polar Front Theory

Mid-Latitude Cyclones

The point of occlusion where the cold front, warm front, and occluded front all come together is known as the triple point. In this region, a new wave called a secondary low may occasionally form, move eastward, and intensify into a cyclonic storm. The center of the intense storm shown in (e) will begin to slowly dissipate when cold air lies on both sides of the occluded front. The warm sector here is far removed from the center of the storm. Without the supply of energy provided by rising, warm air, the old storm system dies out and gradually disappears, as shown in (f).

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A Family of Cyclones

Mid-Latitude Cyclones

In the figure below, a series of wave cyclones exist at various stages of development along the polar front in winter. Such a succession of storms is known as a family of cyclones. To the north of the front are cold anticyclones, and to the south is the Bermuda high. The polar front itself has developed into a series of loops, and at the apex (a peak in the front) of each loop is a cyclonic storm system. In the diagram, Low 1 is just forming, Low 2 is an open wave, and Low 3 is dying out. The figure on the right shows visible satellite imagery of two mid-latitude cyclones at different stages of development.

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Where Do Mid-Latitude Cyclones Tend to Form?

Mid-Latitude Cyclones

Cyclogenesis is the term used to describe any development or strengthening of a mid-latitude cyclone. Areas where cyclogenesis is common include the Gulf of Mexico, the Atlantic Ocean east of the Carolinas, and the eastern slopes of high mountain ranges, such as the Rockies and the Sierra Nevada.

Storms that form on the leeward side (away from the wind) of a mountain are called lee-side lows, and their formation is called lee cyclogenesis. This is because westerly winds that cross a north-to-south mountain range tend to curve cyclonically on the eastern side, as shown below.

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Where Do Mid-Latitude Cyclones Tend to Form?

Mid-Latitude Cyclones

Another common location for cyclogenesis can be found near Cape Hatteras, North Carolina, where warm Gulf Stream water can supply moisture and warmth to the region south of a stationary front, increasing the contrast between air masses to a point where storms may suddenly spring up along the front. These cyclones are called northeasters or nor’easters, and they were introduced back in Unit 13.

Other types of mid-latitude cyclones include the Alberta Clipper and the

Colorado Low, both of which are shown on the map to the right.

When mid-latitude cyclones deepen rapidly, their growth can be referred to as

explosive cyclogenesis.

Some frontal waves form suddenly and grow into large, long-lasting storms,

while others may dissipate in a few days. Why is this the case? The key to the

development of a wave cyclone lies in the upper-wind flow in the region of the

high-level westerlies. To answer this question, we will have to look at how the

winds aloft influence surface pressure systems.

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The Vertical Structure of Mid-Latitude Cyclones

Mid-Latitude Cyclones

Developing surface mid-latitude cyclones are deep dynamic lows that usually intensify with height. Because of this, they appear on an upper-level chart as either a closed low or a trough.

At the surface, winds blow inward toward the low’s center. The piling up of air that results is called convergence, and it increases the air density directly above the air’s surface. This increase in mass causes pressures to slowly increase, which end up dissipating the surface low if prolonged.

On the other hand, if a closed high or ridge lies directly above a surface anticyclone, the spreading out of air at the surface, called divergence, will remove air from the column directly above the high. This removal of mass causes surface pressures to fall, which weakens the high pressure area.

Because of these properties, if upper level pressure systems were directly located over their surface counterparts, they wouldn’t last long. What, then, allows these systems to develop and intensify?

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The Vertical Structure of Mid-Latitude Cyclones

Mid-Latitude Cyclones

An idealized model of the vertical structure of a mid-latitude cyclone is shown to the right. The cold, dense air behind the cold front helps to maintain the surface anticyclone. Because in cold, dense air the atmospheric pressure decreases rapidly with height, the cold aloft is associated with low pressure, and the upper low is located behind the surface low. Directly above the surface low, the air spreads out and diverges, allowing the converging surface air to rise and flow out of the top of the air column just below the tropopause, which acts as a constraint to vertical motions.

When the upper level divergence is stronger than surface convergence (i.e. more air is taken out at the top than is brought in at the bottom), surface pressures drop and the low intensifies, or deepens. Similarly, when upper level divergence is less than surface convergence (i.e. more air flows in at the bottom than is removed at the top), surface pressures rise, and the system weakens.

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The Vertical Structure of Mid-Latitude Cyclones

Mid-Latitude Cyclones

The figure to the right can also explain the structure of the anticyclone. At the surface and aloft, warm air lies to the southwest of the surface high. This is because warm air aloft causes isobaric surfaces to spread farther apart, which results in warm air aloft being associated with higher pressures. As a result, the surface anticyclone tends to tilt toward the warmer air at higher altitudes.

Directly above the surface high, convergence occurs. This causes an accumulation of air above the the surface high, which allows the air to sink slowly and replace and diverging surface air. Hence, when upper-level convergence of air exceeds low-level divergence (i.e. inflow at the top is greater than outflow near the surface), surface pressures rise, and the anticyclone builds. On the other hand, when upper-level convergence of air is less than low-level divergence, the anticyclone weakens as surface pressures fall.

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The Vertical Structure of Mid-Latitude Cyclones

Mid-Latitude Cyclones

If you look at the wind direction at the 500 mb level, you will notice that they tend to steer surface systems in the same direction that the winds are moving. Because of this, the surface mid-latitude cyclone will move toward the northeast, while the surface anticyclone will move toward the southeast. These paths indicate the average movement of surface pressure systems in the eastern two-thirds of the United States.

In general, surface storms travel across the United States at about 16 knots in summer and about 27 knots in winter. Why is this winter velocity higher? This is because the upper-level flow at this time of year is stronger.

On the next slide, we will focus a little bit more on the topics of convergence and divergence.

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Convergence and Divergence, Redux

Mid-Latitude Cyclones

To recap, convergence is the piling up of air above a region, while divergence is the spreading out of air above a region. Convergence and divergence of air can result in changes in wind direction and speed.

On an upper-level chart, convergence occurs when contour lines move closer together, while divergence occurs when contour lines move farther apart. Convergence and divergence may also result from changes in wind speed. Speed convergence occurs when wind slows down as it moves along, whereas speed divergence occurs when the wind speeds up. The two figures below illustrate these aforementioned processes.

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Upper-Level Waves and Mid-Latitude Cyclones

Mid-Latitude Cyclones

If we show an upper-level chart that incorporates almost the entire Northern (or Southern) Hemisphere, as shown to the right, we would see waves that appear as a series of troughs and ridges that encircle the entire globe. The distance from trough to trough (or from ridge to ridge) is known as the wavelength. When this wavelength is many thousands of kilometers long, the wave is called a longwave. At any one moment, there are usually between 3 and 6 longwaves looping around the earth. The fewer the number of waves, the longer their wavelengths.

Longwaves are also known as Rossby waves. Embedded in longwaves are shortwaves, which are small disturbances or ripples that move with the wind flow. The shorter the wavelength of a particular wave, the faster it moved downstream. Shortwaves tend to move eastward at a speed proportional to the average wind flow near the 700 mb level, about 3 km above sea level. Longwaves, on the other hand, often remain stationary, moving eastward at less than 4° of longitude per day.

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Upper-Level Waves and Mid-Latitude Cyclones

Mid-Latitude Cyclones

While longwaves move eastward very slowly, shortwaves move fairly quickly around these longwaves. These shortwaves tend to increase in size when they approach a longwave trough and become smaller when they approach a ridge. Moreover, when a shortwave moves into a longwave trough, the trough tends to deepen.

Look at the below diagram. Where the contour lines are roughly parallel to the isotherms (dashed lines), the atmosphere is said to be barotropic. In a barotropic atmosphere, winds blow parallel to the isotherms. By comparison, where the isotherms cross the contour lines, temperature advection occurs and the atmosphere is said to be baroclinic.

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Upper-Level Waves and Mid-Latitude Cyclones

Mid-Latitude Cyclones

In the region of baroclinicity, winds cross the isotherms and produce temperature advection. Cold advection is the transport of cold air by the wind from a region of lower temperatures to a region of higher temperatures. In the region of cold advection, the air temperature normally decreases. On the other hand, warm advection occurs when wind from a warmer region is transported to a colder one. In the region of warm advection, the air temperature normally increases. For cold advection to occur, the wind must blow across the isotherms from colder to warmer regions, whereas for warm advection, the wind must blow across the isotherms from warmer to colder regions.

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Upper-Level Waves and Mid-Latitude Cyclones

Mid-Latitude Cyclones

Look at the right hand side of the below diagram. In the baroclinic region here, strong winds cross the isotherms and produce cold advection (represented by the blue arrows) on the trough’s west side. These winds also produce warm advection (represented by the red arrows) on the trough’s east side. Below the baroclinic zone lies the polar front, and above it flows the polar-front jet stream. The disturbed flow created by the shortwaves is now capable of intensifying surface mid-latitude cyclones. The theory explain how this happens is known as the baroclinic wave theory of developing cyclones.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

To understand how a wave cyclone develops, we have to examine the atmospheric conditions at the surface and aloft. Suppose that a portion of a longwave trough at the 500 mb level lies directly above a surface stationary front, as shown in figure a. Here, contour lines (solid lines) and isotherms (dashed lines) parallel each other and are close together. Colder air is located in the northern half of the map, while warmer air is located to the south. Winds blow at high velocities, which produce a sharp change in wind speed (a strong wind speed shear) from the surface up to this level. If a shortwave moves through this region and disturbs the flow (as shown in figure b), it sets up a kind of instability called baroclinic instability.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

When baroclinic instability arrives, horizontal and vertical air motions begin to enhance the formation of a cyclonic storm. For example, as the air flow aloft becomes disturbed, it begins to lend support for the intensification of surface pressure systems, as a region of converging air forms above position 1 and a region of diverging air forms above position 2 in figure b. The converging air aloft causes the surface air pressure to rise in the region marked H in figure b. Surface winds begin to blow away from the region of higher pressure, and the air aloft gradually sinks to replace it. Meanwhile, diverging air aloft causes the surface air pressure to decrease beneath position 2, in the region marked L on the surface map.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

This initiates rising air, as the surface winds blow in toward the region of lower pressure. As the converging surface air develops cyclonic spin, cold air flows southward and warm air moves northward. As shown in figure b, you can notice that the western half of the stationary front is now a cold front and the eastern half is now a warm front. Cold air moves in behind the cold front, while warm air slides up along the warm front. These regions of cold and warm advection occur all the way up to the 500 mb level.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

On the 500 mb chart in figure b, cold advection is occurring at position 1 (blue arrow) as the wind crosses the isotherms, bringing cold air into the trough. The cold advection makes the air more dense and lowers the height of the air column from the surface up to the 500 mb level. (Recall that, on a 500 mb chart, lower heights mean the same as lower pressures.) Consequently, the pressure in the trough lowers and the trough deepens. The deepening of the upper trough causes the contour lines to crowd closer together and the winds aloft to increase.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

Meanwhile, at position 2 (red arrow), warm advection is taking place, which has the effect of raising the height of a column of air; here, the 500 mb heights increase and a ridge builds. Therefore, the overall effect of differential temperature advection is to amplify the upper-level wave. As the trough aloft deepens, its curvature increases, which in turn increases the region of divergence above the developing surface storm. At this point, the surface mid-latitude cyclone rapidly develops as surface pressures fall.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

Regions of cold and warm advection are associated with vertical motions. Where there is cold advection, some of the cold, heavy air sinks; where there is warm advection, some of the warm, light air rises. Hence, due to advection, air must be sinking in the vicinity of position 1 and rising in the vicinity of position 2.

The sinking of cold air and the rising of warm air provide energy for a developing cyclone, as potential energy is transformed into kinetic energy. Further, if clouds form, condensation in the ascending air releases latent heat, which warms the air. The warmer air lowers the surface pressure, which strengthens the surface low even more.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

Eventually, the warm air curls around the north side of the low, and the system occludes (see figure c). Some storms may continue to deepen, but most do not as they move out from under the region of upper-level divergence. Additionally, at the surface the storm weakens as the supply of warm air is cut off and cold, dry air behind the cold front (called a dry slot) is drawn in toward the surface low.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

Sometimes, an upper-level pool of cold air (which has broken away from the main flow) lies almost directly above the surface low. Occasionally the upper low will break away entirely from the main flow, producing a cut-off low, which often appears as a single contour line on an upper-level chart. When the upper low lies directly above the surface low (as in figure c), the storm system is said to be vertically stacked.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

Usually the isotherms around the upper low parallel the contour lines, which indicates that no significant temperature advection is occurring. Without the necessary energy transformations, the surface system gradually dissipates. As its winds slacken and its central pressure gradually rises, the low is said to be filling. The upper-level low, however, may remain stationary for many days. If air is forced to ascend into this cold pocket, widespread clouds and precipitation may persist for some time, even if the surface storm system itself is long gone.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

In order for mid-latitude cyclones to develop and intensify, there must be upper-level diverging air above the surface storm. The polar jet stream provides these areas of divergence.

The region of strongest winds in the jet stream is known as the jet stream core, or jet streak. When the polar jet stream flows in a wavy west-to-east pattern, a jet streak tends to form in the trough of the jet, where pressure gradients are tight. The curving around the jet streak produces regions of strong convergence and divergence of air along the jet.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

Notice that the region of diverging air (marked D in figure a) draws warm surface air upward to the jet, which quickly sweeps the air downstream. Since the air above the mid-latitude cyclone is being removed more quickly than converging surface winds can supply air to the storm’s center, the central pressure of the storm drops rapidly. As surface pressure gradients increase, the wind speed increases. Above the high pressure area, a region of converging air (marked C in figure a) feeds air downward into the anticyclone. Thus, we find the polar jet stream removing air above the surface cyclone and supplying air to the surface anticyclone.

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Mid-Latitude Cyclone Development

Mid-Latitude Cyclones

As the jet stream steers the mid-latitude cyclone along, the surface cyclone occludes, and cold air surrounds the surface low (as shown in figure b). Since the surface low has moved out from under the pocket of diverging air aloft, the occluded storm gradually fills as surface air flows into the system.

Since the polar jet stream is strongest and moves farther south in the winter, mid-latitude cyclones are better developed and move more quickly during the colder months. During the summer when the polar jet shifts northward, developing mid-latitude storm activity shifts northward and occurs primarily over the Canadian province of Alberta and the Northwest Territories.

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Jet Streaks

Mid-Latitude Cyclones

The figure below on the right shows an area of maximum winds. The jet streak has winds of at least 50 knots and represents small segments within the meandering jet stream flow.

Jet streaks are important in the development of surface mid-latitude cyclones

because areas of convergence and divergence form at specific regions around

them. To understand why, look at the figure below on the left.

As air enters into the front of the streak (the entrance

region), it increases in speed. As it leaves the rear of

the streak (the exit region), it decreases in speed. As

the air enters the jet streak, in increases speed because

the contour lines are closer together, causing an increase in the pressure gradient

force. This larger force allows the air to swing to the north across the contours.

The air is temporarily geostrophic at the center of the jet streak, but as it leaves, the contours spread apart and the pressure gradient force decreases. At this point, the Coriolis force exceeds the pressure gradient force (opposite the entrance scenario), causing the air to cross the contour lines and swing toward the south. The result is strong convergence at point 1 and strong divergence at point 3 in the left side figure.

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A Short Summary

Mid-Latitude Cyclones

The previous slides provides a fairly good picture as to why some surface lows intensify into huge mid-latitude cyclones while others do not. For a surface cyclonic storm to intensify, there must be an upper-level counterpart — a trough of low pressure — that lies to the west of the surface low. As shortwaves disturb the flow aloft, they cause regions of differential temperature advection to appear, leading to an intensification of the upper-level trough. At the same time, the polar jet forms into waves and swings slightly south of the developing storm. When these conditions exist, zones of converging and diverging air, along with rising and sinking air, provide energy conversions for the storm’s growth. With this atmospheric situation, storms may form even where there are no pre-existing fronts. In regions where the upper-level flow is not disturbed by shortwaves or where no upper trough or jet stream exists, the necessary vertical and horizontal motions are insufficient to enhance cyclonic storm development, and we say that the surface storm does not have the proper upper-air support. The horizontal and vertical motions, cloud patterns, and weather that typically occur with a developing open-wave cyclone are summarized in the figure on the next slide.

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A Short Summary

Mid-Latitude Cyclones

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Conveyor Belt Model

Mid-Latitude Cyclones

The figure below represents a three-dimensional model of a developing mid-latitude cyclone. The model describes rising and sinking air as traveling along three main “conveyor belts.” According to this conveyor belt model, a warm air stream (or warm conveyor belt, shown by the orange arrow) originates at the surface in the warm sector ahead of the cold front. As the warm air stream moves northward, it slowly rises along the sloping warm front, up and over the cold air below. As the rising air cools, water vapor condenses, and clouds form well out ahead of the surface low and its surface warm front. Steady precipitation usually falls from these clouds in the form of rain or snow. Aloft, the warm air flow gradually turns toward the northeast, parallel to the upper-level winds.

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Conveyor Belt Model

Mid-Latitude Cyclones

Directly below the warm conveyor belt sits a cold, dry air stream known as the cold conveyor belt. This belt moves west ahead of the warm front, precipitation and surface moisture evaporates into the cold air, making it moist. As the cold, moist airstream moves close to the surface low, rising air gradually forces the cold conveyor belt upwards. As the cold, moist air sweeps northwest of the surface low, it often brings heavy winter snowfalls. The rising airstream usually turns counterclockwise, around the surface low, first heading south, then northeastward, when it gets caught in the upper air flow. It is the counterclockwise turning of the cold conveyor belt that produces the comma shaped cloud similar to the one to the right. Comma clouds, a cloud band shaped like a comma that can be found in mid-latitude cyclones, often indicate that a storm is still developing and intensifying.

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Conveyor Belt Model

Mid-Latitude Cyclones

The last conveyor belt is a dry one that forms in the cold, dry region of the upper troposphere. This belt is called the dry conveyor belt (shaded yellow in the below figure), and it slowly descends from the northwest behind the surface cold front, where it brings generally clear, dry weather. If a branch of the dry air sweeps into the storm, it produces a clear area called a dry slot, which appears to pinch off the comma cloud’s head from its tail. This feature can appear on the satellite images of storms that become more developed, such as the one shown on the previous slide.

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Vorticity

Mid-Latitude Cyclones

Divergence and convergence, as we’ve learned before, are due to changes in either wind speed or wind direction. The problem is that it is a difficult task to measure divergence or convergence with any degree of accuracy simply using upper-level wind information. Because of this, meteorologists must look for something else that can be measured and, at the same time, can be related to regions of converging or diverging air. This is where vorticity comes into play.

In meteorology, vorticity is a measure of the spin of small air parcels. When viewed from above, air that spins cyclonically (counterclockwise) has positive vorticity, while air that spins anticyclonically (clockwise) has negative vorticity. The faster something spins, the greater its vorticity.

Divergence aloft causes an increase in the cyclonic (positive) vorticity of surface cyclones, which usually results in cyclogenesis and upward air motions.

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Vorticity

Mid-Latitude Cyclones

Because the earth spins, it has vorticity. In the Northern Hemisphere, the earth’s vorticity is always positive because the earth spins counterclockwise about its vertical North Pole axis. The amount of earth vorticity imparted on any object depends on its latitude. On any object on the planet, the earth vorticity acting on it increases from zero at the equator to a maximum at the poles.

Moving air will also provide additional vorticity relative to Earth’s surface. This type of vorticity is known as relative vorticity, and it is a combination of two effects: the curving of the air flow (curvature) and the changing of the wind speed over a horizontal distance (shear). Air moving through a trough will increase vorticity positively (as it spins cyclonically), while air moving through a ridge will increase vorticity negatively (as it spins anticyclonically). In addition, if wind blows faster on one side of an air parcel than on the other, the air parcel will rotate and increase or decrease its relative vorticity. Reference the two diagrams on the right for a depiction of these two properties.

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Vorticity

Mid-Latitude Cyclones

The sum of the earth’s vorticity and the relative vorticity is called the absolute vorticity. With this information, we can look at how vorticity can be used to measure convergence and divergence.

At position 1 (in the ridge), the air parcel has only a slight cyclonic spin because the relative vorticity, due to curvature, is anticyclonic and subtracts from the earth’s vorticity. At position 2, the relative vorticity due to curvature is zero, which allows the earth’s vorticity to spin the parcel faster. At position 3, the parcel spins even faster as the curvature is cyclonic, which adds to the earth’s vorticity. At position 4, the parcel spins more slowly as the curvature is once again zero.

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Vorticity

Mid-Latitude Cyclones

We can see in the figure that as the parcel moves from position 1 in the ridge to position 3 in the trough, its absolute vorticity increases as it moves along. Within this region is typically found an area of upper-level convergence. As the parcel moves from position 3 in the trough to position 5 in the ridge, its absolute vorticity decreases as it moves along. Within this region is typically found an area of upper-level divergence. We can now summarize the information in the figure by stating that as a parcel of air moves with the upper-level flow, an increase in its absolute vorticity with respect to time is related to upper-level converging air, and a decrease in its absolute vorticity with respect to time is related to upper-level diverging air.

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Vorticity

Mid-Latitude Cyclones

Although clockwise flow around a high-pressure area produces negative relative vorticity, because of Earth’s positive vorticity, the absolute vorticity is normally positive everywhere on the map. However, there are regions, and indicated on a surface map, that may have an absolute vorticity that is noticeably greater. These areas are known as vorticity maximums, or simply a vort max. Similarly, a region with low absolute vorticity is known as a vorticity minimum.

A region of high absolute vorticity (vort max), as shown in the diagram below, has diverging air aloft, converging surface air, and ascending air motions on its downwind (eastern) side. On the upwind (western) side of a vort max, there is converging air aloft, diverging surface air, and descending air motions.

Water vapor imagery allows meteorologists to identify

vorticity centers. The infrared water vapor image to the

right shows regions of maximum vorticity as cyclonic swirls

of moisture are found off the Pacific Northwestern coast.

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Polar Lows

Mid-Latitude Cyclones

Storms that develop over polar water behind the main polar front are called polar lows. With diameters of 1000 to 500 km (or less), polar lows are generally smaller in size than mid-latitude cyclones (the wave cyclone that tends to form along the polar front). Some polar lows have a comma-shaped cloud band, while others have a tight spiral of convective clouds that swirls counterclockwise about a clear area, or eye, which resembles the eye of a tropical cyclone. These usually bring heavy precipitation in the form of snow.

Polar lows tend to form during the winter, from November through March. During this time, the sun is low on the horizon and absent for extended periods. This allows the air next to snow and ice covered surfaces to cool rapidly and become incredibly cold. As this frigid air sweeps off the winter ice that covers much

of the Arctic Ocean, it may come in contact with warmer air that is resting above a

relatively warm ocean current. Where these two masses of contrasting air meet, the

boundary separating them is called an arctic front. Along an arctic front, warmer air

rises while colder air slowly sinks beneath it. This establishes baroclinic instability. As

the warm air rises, some of its water vapor condenses, forming clouds and releasing

latent heat that warms the atmosphere. This lowers the surface air pressure. Meanwhile,

at the ocean surface there is a transfer of sensible heat from relatively warm water to

the cold air above. This drives convective updrafts directly from the surface. Polar lows

tend to dissipate rapidly when they move over land.

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Climate Zones

“The warmer the climate gets, the faster the climate zones are shifting. This could make it harder for plants and animals to adjust.”

― Irina Mahlstein, CIRES scientist

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Five Factors That Affect Climate

Climate Zones

The five factors that influence climate are latitude, air masses, elevation, proximity to water, and landforms.

Latitude: the further away you move away from the equator, the colder the temperatures.
Air Masses: air masses carry the conditions from which they originate. This affects moisture content and temperature.
Elevation: the higher a location’s elevation, the colder the temperatures.
Proximity to Water: large bodies of water have a moderating effect on temperature; locations near bodies of water (like Michigan) have cooler summers and milder winters.
Landforms: landforms can affect the patterns of precipitation. This is shown by the windward and leeward sides of a mountain.

In this unit, we will look at the five main climate zones on earth: Tropical, Dry, Moderate, Continental, and Polar.

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Tropical Wet Climates

Climate Zones

Tropical wet climates are hot and muggy all year round. Dense tropical rainforests grow in regions with this climate. Heavy rainfall occurs in frequent showers and thunderstorms throughout the year, averaging from about 70 to 100 inches per year. Tropical wet climates experience high temperatures that vary little during the year. Temperatures do not fall below 64°F (18°C), even during the coolest part of the year. There is a greater temperature difference between day and night than between summer and winter. Frost and freezing temperatures never occur, allowing plants to grow all year. These climates are usually found within 5-10 degrees north or south latitude of the equator.

Examples: Singapore, parts of Brazil, parts of South and Central America - Peru and east coasts of Costa Rica and Ecuador

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Tropical Wet and Dry Climates

Climate Zones

Tropical wet and dry climates, also known as the tropical savanna, are known for their mostly warm to hot temperatures. Temperatures fluctuate moderately during the day and throughout the year. Areas with this climate receive moderate rainfall during the year, with clear cut wet (summer) and dry (winter) seasons. These climates are generally located on the edges of Tropical Wet climates, throughout the tropics between 25 degrees north and south latitude.

Examples: Acapulco, parts of central Africa, parts of Central and South America, southern Asia, northern Australia.

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Semiarid Climates

Climate Zones

Semiarid or steppe climates are similar to arid climates but are more moderate, experiencing less of the extreme high or low temperatures. These areas typically surround the desert areas, separating them from the more humid climates beyond. Rainfall totals are slightly higher than in the arid climates. While precipitation totals are still relatively low, there may be a slight increase in precipitation during the summer months.

Examples: Denver, parts of Australia and Africa, parts of Russia.

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Arid Climates

Climate Zones

Arid climates are characterized by hot to very hot summers and mild or cold winters, depending on if the area is located in a subtropical or midlatitude region. Subtropical deserts are found on the western sides of continents about 25-30 degrees latitude. These areas have very hot summers and mild winters, with very little cloud cover and scarce rainfall throughout the year. Midlatitude deserts are found at the interior of continents and also have hot summers and scarce precipitation. The winters are cold with erratic precipitation, sometimes in the form of light snow.

Examples: Phoenix, parts of Mexico, most of (inner) Australia, large portions of northern Africa, central Asia.

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Mediterranean Climates

Climate Zones

Mediterranean or subtropical dry summer climates have warm to hot, dry summers and mild, rainy winters. These climates are typically found on the west side of continents roughly between 30 degrees and 45 degrees latitude. The closer to the coast the area is, the more moderate the temperatures, making for less contrast between summer and winter temperatures.

Examples: San Francisco, Los Angeles, Seattle, large parts of Italy, Athens (Greece), Madrid (Spain).

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Humid Subtropical Climates

Climate Zones

Humid subtropical climates are characterized by warm to hot summers and cool winters. Rainfall is evenly distributed throughout the year. Winter rainfall (or snowfall) is associated with large storm systems moving from west to east under the influence of the westerly winds. Most summer rainfall occurs during thunderstorms or during a tropical storm or hurricane. Humid subtropical climates are found in the interiors of continents, or on the southeast coasts of continents, between 25 degrees and 40 degrees latitude.

Examples: Atlanta, Houston, most of Florida, parts of China, Sydney (Australia).

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Marine West Coast Climates

Climate Zones

Marine west coast climates, also known as humid oceanic climates, are found only on the western sides of continents where the prevailing wind direction is from sea to land. The ocean influences the climate of these areas, reducing seasonal temperatures variations. Winters are cool to mild and summers are warm with moderate precipitation occurring throughout the year. Low clouds, fog, and drizzle are common while thunderstorms, extreme hot or cold temperatures, and droughts are rare.

Examples: England, New Zealand, parts of Alaska, most of France.

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Humid Continental Climates

Climate Zones

Humid continental climates are characterized by mild to warm summers and cold winters. The further inland you move, the greater the difference between temperatures during the warmest and coldest time of the year. Temperature differences for this climate can be as great as 45 to 60 degrees Fahrenheit (25 to 30 degrees Celsius) throughout the year. Inland locations may also have more precipitation during the summer months; however, precipitation is generally distributed evenly throughout the year. Winter temperatures are so low that snowfall can be substantial and snow cover can be persistent. Continuous snow cover lowers daytime temperature even more.

Examples: New York, Chicago, Michigan, parts of China.

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Subarctic Climates

Climate Zones

Subarctic climates have short, cool summers and long, bitterly cold winters. Freezes are common in these areas, even in midsummer. This climate receives most of its precipitation in the summer. Snow arrives early in the fall and lasts on the ground through the early summer. Subarctic climates are found mostly in the 50s north latitude, although it might occur as far north as 70 degrees latitude.

Examples: British Columbia, Quebec, and most of Canada, Anchorage, Siberia, parts of Norway and Russia.

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Tundra Climates

Climate Zones

Tundra climates are dry, with a brief, chilly summer and a bitterly cold winter. Continuous permafrost (permanently frozen ground) lies under much of the treeless tundra regions. These climates occur on the northern edges of the North American and Eurasian landmasses, and on nearby islands. They also exist along the outer fringes of Antarctica.

Examples: parts of Russia, Antarctica, and Canada.

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Ice Cap Climates

Climate Zones

Ice cap climates are the coldest on Earth. Even in summer, temperatures rarely rise above the freezing point. WInter temperatures are extremely low and winters are long and dark. Precipitation is rare, but almost always in the form of snow.

Examples: Greenland and most of Antarctica.

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Highlands Climates

Climate Zones

Highlands climates are not defined in the same sense as other climate types. These climates occur in mountainous regions where elevation plays a role in the weather and climate. Air temperature decreases with increasing elevation in the mountains, so each range of altitude has slightly different climate characteristics. For example, the climate at the base of a mountain might be humid subtropical, and the climate at the summit might be tundra. Vertical zonation also depends on the exposure of a slope or peak. Windward and leeward sides of mountains may have drastically different conditions due to the amount of precipitation received.

Examples: west coast of South America, parts of Mexico, US, and Canada.

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Climate Zones

Climate Zone Locations Around the World

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Climate Zones

Climate Graphs

A climate graph is a graph used to illustrate the average temperature and rainfall experienced at a particular place over the course of a year. The lines on a climate graph represent temperature, while the bars on a climate graph represent rainfall. Climate graphs may use different colored lines to represent both average high and low temperatures. An example of a climate graph is shown below.

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Michigan Weather

“If you don't like the weather in

Michigan, wait five minutes.”

― Anonymous

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Michigan Weather

The following unit deals with the topic of Michigan Weather, one that is present in the Division A competition Weather or Not here in Michigan. If you are not competing in this division (or if you are not from Michigan), please skip this unit (although it wouldn’t hurt to learn a bit about why Michigan’s weather is often considered weird!).

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Michigan Weather

In the winter, arctic air from the northwest is very cold and dry, but as it crosses Lake Michigan, it is warmed and picks up moisture from the warmer lake. When it reaches land, the moisture condenses, and snow creates heavy snowfalls inland. The vastness of Lake Michigan is a big influence on the cold air traveling across the relatively warmer lake waters, which evaporates a lot of moisture in the air and, with other processes, helps to produce lake effect snow. How far the snow goes inland is determined by the strength of the wind.

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Michigan Weather

Lake-effect snow is common in Michigan, especially on the west coast. Model data sometimes do not pick up on the influences of Lake Michigan, making it hard to forecast snowfall amounts.

Because of the presence of the Great Lakes, many regions in the state may be exposed to this phenomenon (as Michigan’s border is almost entirely surrounded by water). This is especially the case in a region known as the Keweenaw Peninsula, as shown below. What makes the Keweenaw Peninsula so vulnerable to lake-effect snow is its position in Lake Superior — it is surrounded by the lake on almost all sides! Thus, snow can pile in from the lakes on almost any side of the peninsula. It is no surprise that the Keweenaw region of Michigan is known for having some of the largest snowfall totals in the state!

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Michigan Weather

The average temperature in West Michigan is influenced by Lake Michigan; for example, in the wintertime, lake effect cloud cover over the region makes it less cold as it would be with a clear sky. The average temperatures are not as cold as other regions unless there is an arctic air mass affecting the area or daytime heat loss due to clear skies. In the winter the area can and does experience several days in a row that are cloudy; this helps to keep the temperature from falling as low as it could.

As mentioned extensively in the previous units, a city’s proximity to water greatly influences its temperatures. Areas closer to water tend to have milder summers and warmer winters than areas closer to land (a moderating effect). This is why a port city like Detroit may have warmer winter temperatures than an inland city like Ann Arbor!

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Michigan Weather

The presence of the Great Lakes also influences daytime and nighttime temperatures in Michigan. In the fall and winter, when the lake temperature is warmer than the land temperature, and there’s more moisture in the air, the temperature at night will not cool down as significantly. Because of this, you have warmer than normal average low temperatures during those times of the year. In the spring and summer, you have the opposite effect with a colder lake temperature and a warmer land temperature. A cool wind coming across the colder lake will keep the average high temperatures below normal.

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Michigan Weather

The weather in each season in West Michigan is influenced by Lake Michigan. For instance, in the summer, a tropical air mass from the Gulf of Mexico can cause a temperature inversion. This occurs when the warm air crosses over the colder Lake Michigan waters, warming the top layers of the water while the bottom layers remain cool, which occasionally traps the cool layer below trapping moisture and airborne pollutants from rising and being distributed in the air. The result of this is a temperature inversion which affects the weather with humid conditions in the summer. Increased summer sunshine warms the water on the lake’s surface, making it lighter than the colder water below. The release of the heat stored in the lakes moderates the climate near the shore in the fall and winter months.

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Michigan Weather

How does Michigan weather influence plant behavior? Sometimes, the influence of Lake Michigan allows the state to remain cool well into April. In these situations, this delays the leafing and blooming of plants, which protects plants, such as fruit trees, from late frosts. This allows plants from warmer climates to survive in West Michigan and is the reason for the presence of wine and grape vineyards in the area.

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Michigan Weather

A special wind system may form along the coastline of Lake Michigan in late spring and summer days when the temperature soars, called a lake breeze. It forms during the daylight because the lake waters do not warm as quickly as the surrounding land surfaces. Air cooled by contact with the cold lake waters is denser than that the surrounding lake, and it forms a high pressure cell over the lake. When the sun heats the land, the air above it warms and becomes less dense. Solar heating over the land produces lower pressure. The pressure gradient between the two pushes winds inland off the lake, known as lake breeze flow. When the lake breeze forms and brings the colder lake air onto the land, a boundary zone forms between the two air masses forms called a lake breeze front. A lake breeze front could cause enough instability to cause convective storms over land, yet another influence of Lake Michigan on West Michigan weather.

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Michigan Weather

Clouds influence the weather greatly and especially in West Michigan with the influence of Lake Michigan. There is a decrease in cloudiness in the summer because of the cooling and stabilizing effect of the relatively cool water. This decrease can extend 20 or so miles inland along the western shore of Lake Michigan. In the winter, however, there are significantly more lake effect clouds, producing a higher average of cloudy days. These cloudy days impact the temperature greatly by producing warmer temperatures at night while being cooler during the day.

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Michigan Weather

There are two primary reasons that cause the increase in speed of cold fronts over Lake Michigan. One is a change in the frontal temperature gradient when the cold front is over the lake, and the other is a change in surface roughness that may alter the effect of friction.

What about winds and global weather patterns? The dominant wind pattern of Michigan is the prevailing westerlies, and the state’s climate zone is humid continental. The presence of El Niño may also greatly influence Michigan’s weather: a strong El Niño gives Michigan a likely chance of warmer than normal winter temperatures and lower than normal snowfall (notice that warmer winter temperatures or low snowfall levels do not indicate that an El Niño is present!). Transitions from El Niño to La Niña may also bring hot temperatures to the state.

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Weather Forecasting

“The trouble with weather forecasting is that

it's right too often for us to ignore it and

wrong too often for us to rely on it.”

― Patrick L. Young, Entrepreneur

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Weather Forecasting

The process of weather forecasting involves predicting how the present state of the atmosphere will change. To successfully make a weather forecast, current weather conditions over a large area must be known.

Forecasters have access to many resources that illustrate present conditions at various atmospheric heights as well as vertical profiles (called soundings) of temperature, dewpoint, and winds. They also have access to visible and infrared satellite images, as well as Doppler radar information that can detect and monitor the intensity of precipitation. These observations are bundled together into a computer-based atmospheric model that uses mathematical relationships to predict the weather conditions that will occur in the future.

This is the final main topic of this event. At the end of this unit, we will be able to pull together everything we learned to make a weather forecast!

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Weather Forecasting

Surface and Upper-Air Data

A giant network of surface observation stations can be found all around the world, reporting weather conditions on the ground at least four times a day. Most airports observe weather conditions hourly, and hundreds of Automated Surface Observation Systems (ASOS) send data even more frequently. The ASOS system provides continuous data on wind, pressure, temperature, cloud-base height, and runway visibility and various airports.

To sample atmospheric conditions above ground level, radiosondes are launched at more than 800 locations around the world. These are collected twice a day, at 0000 UTC and 1200 UTC. Data on upper-level conditions may also be collected by aircraft as they travel their usual routes.

Satellite images also provide clear images of the atmosphere. Visible, enhanced infrared, and water vapor images provide a wealth of information that can be examined to quickly analyze rapidly-changing conditions. The benefit of satellites is that they can collect data from regions that would otherwise be inaccessible to people or planes.

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Doppler Radar

Weather Forecasting

A network of over 100 Doppler radar units dot the contiguous United States. These radars provide live coverage on the evolution of weather systems that produce precipitation. Because Doppler radar can track winds as well as precipitation, the network is valuable in presenting forecasters with ample time to issue warnings of destructive windstorms and tornadoes.

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Weather Forecasting Tools

Weather Forecasting

To help forecasters analyze many available charts and maps at once, a system called the Advanced Weather Interactive Processing System or AWIPS is employed by the National Weather Service (NWS). A second generation version of this software, AWIPS II, was adopted by the NWS in 2013. The AWIPS II system has data communications, storage, processing, and display capabilities to help each forecaster extract weather data from a wide variety of sources. Much of the information from ASOS and Doppler radar is processed by software according to predetermined formulas, or algorithms, before it is sent to the forecaster.

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Weather Forecasting Tools

Weather Forecasting

The meteogram is a chart that shows how one or more weather variables has changed at a station over a given period of time. An example of a meteogram is shown to the right.

A 2D vertical profile of temperature, dewpoint, and winds is known as a sounding. The analysis of a sounding can be especially helpful when making a short-range forecast that covers a relatively small area. Another resource used by meteorologists is the thickness chart, which allows meteorologists to analyze temperatures at different altitudes and how they are changing.

More information on a thickness chart is shown on the next slide.

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Thickness Chart

Weather Forecasting

A thickness chart shows the difference in height between two constant pressure surfaces. The vertical depth or thickness between any two pressure surfaces is related to the average air temperature between the two surfaces. As shown below, air pressure decreases more rapidly with height in cold air than in warm air.

The figure to the right displays two pressure surfaces, a 1000 mb and 500 mb

surface. The difference between these two pressure surfaces in this case is

called the 1000 mb to 500 mb thickness. This vertical distance, or thickness,

is greater in warm air than in cold air. Thus, warm air produces high thickness

while cool air produces low thickness.

When 1000 mb to 500 mb thickness lines are drawn on a chart, the

following map results. Regions of low thickness represent cooler conditions,

while regions of high thickness represent warmer conditions. A good rule

of thumb when analyzing these charts: the 5400 meter line acts as an

approximate dividing line between rain and snow. If precipitation is falling,

cities with a thickness greater than 5400 m would receive rain, while

cities with a thickness less than 5400 m would receive snow.

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Time Range of Forecasts

Weather Forecasting

Weather forecasts tend to be grouped according to how far into the future the forecast extends. A weather forecast for up to a few hours is called a very short range forecast, or nowcast.

When severe weather is likely to affect a region (or is actually occurring), the NWS issues short-range alerts in the form of weather watches and warnings. A watch indicates that atmospheric conditions favor hazardous weather conditions in a certain region during a specific time period, but that the actual location and timing of the occurrence is uncertain. A warning indicates that hazardous weather is either imminent or actually occurring within the specified forecast area. Advisories are issued to inform the public on less dangerous conditions, such as those caused by wind, dust, fog, snow, sleet, or freezing rain.

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Time Range of Forecasts

Weather Forecasting

Weather forecasts that range from about 12 hours to a few days are called short-range forecasts. The forecaster may incorporate a variety of techniques in making a short-range forecast, such as satellite imagery, Doppler radar, surface weather maps, and upper-air winds.

As these forecasts extend beyond 12 hours, a greater reliance on statistics, models, and mathematical equations is needed. A medium-range forecast is one that extends from about 3 days up to 8 days into the future. Medium range forecasts are almost entirely based on computer derived products, such as forecast charts and statistical forecasts. A forecast that extends beyond 3 days is often called an extended forecast. Formulas were used to forecast Hurricane Irma’s position and intensity in the Atlantic, as shown below.

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Time Range of Forecasts

Weather Forecasting

A forecast that extends beyond 8 days is known as a long-range forecast. Even though some models may provide forecasts for up to 16 days into the future, these predictions are rarely accurate. Because of this, conditions from beyond a week are often covered in outlooks that give an indication of a prevalent trend rather than a detailed forecast for each upcoming hour.

Seasonal outlooks are also issued every month. These outlooks cover three-month periods that extend out to roughly a year. Rather than depicting specific weather features, these outlooks show the odds that a given area might experience temperatures or precipitation that are above or below average. A typical 90 day outlook is shown below.

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TV Weathercasters

Weather Forecasting

How do meteorologists describe weather phenomena on TV? You may see them pointing to specific weather information on a giant map. But in many cases, this giant map doesn’t exist!

Frequently, weathercasters point to a green screen when forecasting the weather on TV. The image you see, be it the radar or any other weather system, is actually superimposed on the screen for viewers to see. In other words, a computer inserts the background needed (such as a map, chart, satellite photo, or other graphic) to the green area that it captures. You may ask: how does a forecaster know where to point then? Carefully placed monitors surrounding the forecaster allows them to see where they are pointing at!

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Forecasting Techniques

Weather Forecasting

Prior to the advent of computer models, forecasters often used skill and intuition to forecast the weather. Below are some common forecast methods employed by meteorologists.

The easiest weather forecast to make is a persistence forecast, which is simply a prediction that future weather will be the same as the present weather. For instance, if it is snowing today, a persistence forecast would call for snow through tomorrow. These types of forecasts are most accurate for time periods of several hours, and they become less and less accurate as time goes on. Persistence forecasts also tend to be more useful where the weather tends to change less dramatically.

Another type of forecast used is the steady-state or trend forecast. This forecast relies on the assumption that surface weather systems tend to move in the same direction and at approximately the same speed as they have been moving. For instance, if a cold front is moving east at 25 km per hour and is 75 km west of your house, you would expect that the front would arrive in 3 hours using the steady-state method.

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Forecasting Techniques

Weather Forecasting

Another method of weather forecasting is the analog method. This method relies on the fact that existing features on a weather chart may strongly resemble features associated with certain weather conditions in the past. In this case, the weather conditions present may seem familiar to a meteorologist, who may then use similar weather situations to provide a prediction.

Statistical forecasts involve weather elements based on the past

performance of computer models. These predictions (known as

Model Output Statistics, or MOS) are statistically weighted analog

forecast corrections included into the computer model output. For

example, a forecast of tomorrow’s maximum temperature for a city

may be derived from equations relating temperature with

humidity, cloud cover, wind direction, and air temperature.

A probability forecast involves the likelihood of an event occurring.

For example, a 60% chance of rain indicates that there is a 60%

chance that any random place in the forecast area will receive

measurable rainfall. The table to the right shows the wording

used by the NWS to describe the percentage probability of measurable precipitation.

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Forecasting Techniques

Weather Forecasting

In general, weather patterns are categorized into similar groups or “types” using criteria such as the position of the subtropical high, the upper-level flow, and the prevailing storm track. Predicting the weather by weather types employs the analog method; for instance, when the Pacific high is farther north and the wind flow aloft is meridional, a meteorologist may predict that surface lows will turn into deep systems.

A forecast based on the climate of a particular region is known as a climatological forecast. As an example, Los Angeles rarely receives rain during the summer, and climatological records show that rain occurs only about 1% of the time during this period. Thus, if we forecast that it will not rain in Los Angeles during the summer, the forecast has a 99% chance of being correct.

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Numerical Weather Prediction

Weather Forecasting

Because the atmosphere is so complex, today’s supercomputers analyze large quantities of data extremely quickly while carrying out trillions of calculations per second. This routine daily forecasting of weather by computers using mathematical equations has become known as numerical weather prediction.

Atmospheric models used by meteorologists use many equations that describe how atmospheric conditions will change with time. These equations are translated into complex software, and surface and upper-air observations of temperature, pressure, moisture, winds, and air density are fed into the equations at regular intervals. The process of integrating these data into numerical models is called data assimilation.

To determine how these meteorological variables will change, each equation is solved for a small increment of future time (for example, 5 minutes) for a large number of locations called grid points, each situated a given distance apart. The results of the equation are then re-entered back into the equation and solved again. This is done until a specific time is reached, usually 6, 12, 24, 36 hours into the future. The computer then analyzes the data and draws the projected positions of pressure systems with their isobars and contour lines. The final forecast chart is called a prognostic chart or simply prog.

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Numerical Weather Prediction

Weather Forecasting

The following is a model run of the GFS from 06z on Sept. 2, 2017. Notice that a new prognostic chart is generated every 6 hours.

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Numerical Weather Prediction

Weather Forecasting

Major meteorological centers around the world have their own computer models, including some that are tailored to a particular continent or region. The NOAA National Centers for Environmental Prediction (NCEP) runs several models in operational mode, meaning that they have been thoroughly tested and are reliable enough for everyday use. Other models are run in experimental mode, which indicates that the model may not yet be robust enough for operational mode.

Currently, the NCEP operational models include the North American Mesoscale (NAM) model, which is run every six hours and predicts conditions at three-hour intervals out to 84 hours, and the Global Forecast System (GFS) model, which is run every 12 hours and issues projections extending out to 384 hours. A new addition to the operational models, the High Resolution Rapid Refresh (HRRR) runs every hour and predicts weather in sharp detail at hourly time steps out to 15 hours. The HRRR nailed the precipitation forecast in the midst of Hurricane Harvey in 2017, predicting precipitation and radar patterns that essentially matched what actually happened.

The distance between grid points is called the resolution. While models with high resolution may provide detailed predictions, it does not mean that the model is more accurate.

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Numerical Weather Prediction

Weather Forecasting

Despite all the computer power that goes into modeling, forecasts sometimes go wrong. You may receive a forecast that indicates clear weather only to be greeted with a heavy downpour! Why does this happen?

For one, computer models have inherent flaws that limit the accuracy of weather forecasts. For example, models often idealize the real atmosphere, making assumptions that may be off from the actual result. In addition, a majority of models are not global in their coverage, and errors are able to creep in along the model’s boundaries. A model for the United States may not be able to predict with accuracy the systems that may be prevalent on the edges of its forecast area!

Some areas in the world also have very few weather observations, such as oceans and locations at high latitudes. The presence of satellites, however, has made this issue less prominent.

Lastly, models with lower resolutions may not be able to capture small thunderstorms (and other small systems) located in between the grid points. This is why a forecast of fair weather may still involve a rainstorm! Newer models, such as the HRRR described on the previous slide, seek to alleviate this issue by providing forecasts using higher resolutions.

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Numerical Weather Prediction

Weather Forecasting

Many computer models cannot adequately interpret many of the factors that influence surface weather, such as interactions between water, ice, surface friction, or local terrain. Even if this issue is fixed, there are countless small, unpredictable atmospheric fluctuations known as chaos that limit model accuracy.

To account for this chaos, meteorologists use a strategy

called ensemble forecasting to improve short and medium

range forecasts. The ensemble approach is based on

running several forecast models — different versions of

the same model, each beginning with different starting

conditions. This creates an ensemble of forecasts for a

range of small initial changes. Because these ensembles

are scattered on a forecast chart, they are often known as

a spaghetti plot. An example is shown to the right. In

general, the less agreement among the ensemble model

runs, the less predictable the weather!

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Weather Symbols

Weather Forecasting

Weather symbols are used on my weather maps as shorthand for the conditions at weather observing stations. An example of a weather symbol is shown below:

A detailed list of weather symbol notations is given on the next slide. You can also visit the link below for another resource regarding these symbols: http://goo.gl/ISRkGO.

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Weather Symbols

Weather Forecasting

For a full resolution view, visit the link below: https://docs.google.com/document/d/18_rtj7mT-wA0CBmeQZU8HulZwuOu3yF9iBmy4kahTBM/

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Weather Symbols

Weather Forecasting

The figure below details the flags associated

with different weather systems and warnings:

Isotherms are lines of constant or equal temperature. Isobars are lines of constant or equal pressure. When two isobars are close together, windy conditions are expected. Isohyets are lines of constant or equal rainfall totals over a given time period. Above is an isohyet map of Australia.

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Weather Forecasting Using Surface Charts

Weather Forecasting

How do meteorologists make short-range weather predictions with only the information available on a surface weather map? Can a forecast be made using such a chart? Definitely. In this final portion of this unit, we will be predicting weather using the surface chart below. Here, a single isobar is drawn around pressure systems to minimize clutter. The dashed lines represent the position of the weather 6 hours ago.

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Weather Forecasting

Determining System Direction

How do we determine the movement of weather systems by just looking at a surface map? Here are several rules of thumb used regarding direction forecasting:

for short-time intervals, mid-latitude cyclones and fronts tend to move in the same direction and at the same speed as they did six hours prior (unless evidence indicates otherwise).
low-pressure areas tend to move in a direction that parallels the isobars in the warm air (warm air sector) ahead of the cold front.
lows tend to move toward the region of greatest pressure drop, while highs tend to move toward the region of greatest rise.
surface pressure systems tend to move in the same direction as the wind at 5500 m (18000 ft), or the 500 mb level. The speed at which surface systems move is about half the speed of winds at this level.

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Weather Forecasting Using Surface Charts

Weather Forecasting

Let’s apply the rules on the previous slide to the map. It appears that, using rules 1 and 2, that the low pressure centered over the plains will move northeast. Isallobars — lines that connect points of equal pressure change — help to illustrate regions of rising and falling pressures. Using isallobars on the previous map, we can tell that the area of largest pressure drop (denoted by “F”) is to the northeast of the warm front. Thus, we can see that the low will move toward the northeast. Similarly, the rise in pressure northwest of the cold front (denoted by “R”) indicates that the high pressure is moving southeast in that direction.

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Weather Forecasting Using Surface Charts

Weather Forecasting

These changes in pressure (or pressure tendencies) also indicate how pressure systems are changing with time. The rapid fall in pressure in advance of the low indicates that the storm center is deepening, meaning closer isobars, greater pressure gradients, and stronger winds. A drop in pressure, on the other hand, around an anticyclone suggests that it is weakening. Using the diagram below and the map on Slide 733, the high above Montana is associated with rising pressures, so it is strengthening. Meanwhile, the high above South Carolina is either moving east or weakening, as it is associated with falling pressures.

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Weather Forecasting Using Surface Charts

Weather Forecasting

One can also use a 500 mb chart (if available) to determine how the systems move, as winds at this level tend to steer these systems along. Using the 500 mb chart below, we can confirm that the low above the plains is heading northeast.

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Everything Comes Together: Let’s Make a Forecast!

Weather Forecasting

Finally, we will be using everything we learned in the past 22 units to emulate the work of a meteorologist. Let’s make six short-range weather forecasts for six cities, assuming that steady-state conditions exist. We will be using the weather map below and the map on Slide 733 for our forecasts.

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City #1: A Forecast for Augusta, Georgia

Weather Forecasting

Observe that an anticyclone is moving away from Augusta, heading toward the east. Southerly winds on the western side of this system will bring warmer and more humid air to the region. As a result, afternoon temperatures will be warmer than those of the day before. As the warm front approaches from the west, cloud cover will increase, appearing first as cirrus, then thickening as the warm front continues getting closer. Barometric pressure should fall, and clouds and high humidity should keep minimum temperatures above freezing during the night. Using this information, a meteorologist may make the following forecast

for the city of Augusta (adapted from C. Donald Ahrens’s textbook Meteorology Today):

“Clear and cold this morning with moderating temperatures by afternoon. Increasing high clouds with skies becoming overcast by evening. Cloudy and not nearly as cold tonight and tomorrow morning. Winds will be light and out of the south or southeast. Barometric pressure will fall slowly.”

A more experienced meteorologist may notice that cold temperatures allow warm, moist air to chill as it moves across the cold surface. This would produce foggy conditions, which could be included in the forecast.

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City #2: A Forecast for Washington, D.C.

Weather Forecasting

Observe that the low-pressure area over the Central Plains is slowly approaching Washington, D.C. from the west. As a result, the clear weather, light southwesterly winds, and low temperatures of the current day will gradually give way to cloudier conditions, southeasterly winds, and higher temperatures. By the next morning, the band of precipitation (shaded in green) will arrive at the city.

But is this precipitation in the form of rain or snow? If we look closely, we may notice that regions south of D.C. are receiving snow. Thus, without the aid of temperature profiles, we could make an assumption that snow would fall, and that it could eventually turn into rain. A meteorologist could then make the following forecast for Washington, D.C. (again, adapted from C. Donald Ahrens’s textbook Meteorology Today):

“Increasing clouds today and continued cold. Snow beginning by early tomorrow morning, possibly changing to rain. Winds will be out of the southeast. Pressures will fall.”

If sleet falls, would our prediction be wrong? Not really. The difference between the presence of snow and sleet in this situation depends on the strength of the low-pressure area. If the low deepened more than expected, stronger southeasterly winds would blow in warm, moist air from the Atlantic. This air rides over the colder surface air, allowing snow to partially melt and refreeze during its fall, producing sleet. This warm air from the ocean would gradually raise temperatures, allowing the sleet to transition into rain.

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City #3: A Forecast for Chicago, Illinois

Weather Forecasting

The overrunning of warm air has produced a wide area of snow that appears to be heading straight for Chicago. Since colder air north of the low will be over Chicago, precipitation reaching the ground should be frozen. Assuming constant system speed, the snow should arrive to Chicago within 6 hours.

Snowfall should become heavy as the storm intensifies into the evening, and it should taper off around midnight as the system moves east. If it snows for a total of 12 hours — 6 light (1 in/3 hours) and 6 heavy (1 in/1 hour) — the total snow accumulation should be around 6-10 inches. As the center of the low moves eastward, winds will gradually shift from southeasterly to easterly, then northeasterly by evening. The winds will continue to shift throughout the following morning, becoming northerly and finally northwesterly. Skies should begin to clear at this time, as the storm moves away from the area. An example forecast could be:

“Cloudy and cold with light snow beginning by noon, becoming heavy by evening and ending by tomorrow morning. Total accumulations will range between 6 and 10 inches. Winds will be strong and gusty out of the east or northeast today becoming northerly tonight and northwesterly by tomorrow morning. Barometric pressure will fall sharply today and rise tomorrow.”

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City #4: A Forecast for Memphis, Tennessee

Weather Forecasting

Within 24 hours, both a warm front and a cold front should move past Memphis. The light rain from the current morning should saturate the cool air, creating a blanket of low clouds and fog by midday. As the warm front moves through during the afternoon, temperatures should rise as winds shift to the south or southwest. At night, clear to partly cloudy skies should allow the ground and air above to cool, offsetting any chance for rapid temperature rise. Falling pressures should level off in the warm air, then fall once again as the cold front approaches before midnight. This cold front would bring with it gusty northwesterly winds, showers, and the possibility of thunderstorms, rising pressures, and colder air. Using this information, a meteorologist may make the following forecast:

“Cloudy and cool with light rain, low clouds, and fog early today, becoming partly cloudy and warmer by late this afternoon. Clouds increasing with possible showers and thunderstorms later tonight and turning colder. Winds southeasterly this morning, becoming southerly or southwesterly this evening and shifting to northwesterly tonight. Pressures falling this morning, leveling off this afternoon, then falling again, but rising after midnight.”

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City #5: A Forecast for Dallas, Texas

Weather Forecasting

A cold front is expected to pass the area around noon the current day. Weather along the front is showery with a few developing thunderstorms; behind the front the air is clear but cold. By the next morning, the cold front will be far to the east and south of Dallas and an area of high pressures will be centered over southern Colorado. North or northwesterly winds on the east side of the high will bring cold arctic air into Texas, dropping temperatures as much as 40°F within a 24 hour period. With minimum temperatures well below freezing, Dallas should experience a cold wave. As such, a forecast for the region may go as follows:

“Increasing cloudiness and mild this morning with the possibility of showers and thunderstorms this afternoon. Clearing and turning much colder tonight and tomorrow. Winds will be southwesterly today, becoming gusty north or northwesterly this afternoon and tonight. Pressures falling this morning, then rising later today.”

The weather for the next day actually ends up being overcast, and temperatures are not as cold as predicted. What went wrong? In this situation, the cold front actually slowed and became stationary along the Gulf of Mexico. This allowed a wave of low pressure to form along the stationary front, bringing with it warm, moist Gulf air to slide over the cold surface air. As a result, clouds formed, and minimum temperatures did not fall as dramatically.

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City #6: A Forecast for Denver, Colorado

Weather Forecasting

The cold high-pressure area should move southeast toward Denver by the next morning. Sinking air aloft associated with this high-pressure area should keep the sky relatively clear, and weak pressure gradients would produce weak winds. The interaction between weak winds and dry air would allow for intense radiational cooling, perhaps resulting in minimum temperatures below 0°F. Let’s make a final forecast:

“Clear and cold through tomorrow. Northerly winds today becoming light and variable by tonight. Low temperatures tomorrow morning will be below zero. Barometric pressure will continue to rise.”

Suppose that temperatures never actually went below zero. How did this happen? This can be attributed to a downslope wind coming off the mountains west of Denver. This wind kept the air mixed and allowed temperatures to be warmer than predicted. A forecaster familiar with the Denver landscape would be able to take this into consideration.

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A Summary of Our Forecasts

Weather Forecasting

The figure below shows the surface weather systems 24 hours after the map we looked at earlier. Compare this to the forecasts provided in the past six slides. What went well, and what didn’t? And a bonus, challenging question: if you were analyzing the map below, would you place a warm front in the middle Atlantic region of the US? What evidence is present that suggests a warm front could be drawn, and why do you feel that it was included in this map?

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A More Challenging Forecast: Help is Needed

Weather Forecasting

In situations where surface weather features are extensively modified by large bodies of water and limited surface and upper-level data is available, a meteorologist may need help from other resources. One such resource is the 500 mb chart, which allows forecasters to view the steering patterns.

The 500 mb surface map is an example of one that allows meteorologists to better understand steering patterns in the atmosphere. Here, the shape of the flow around San Francisco is meridional, taking the shape of the Greek letter “omega” (Ω). The high associated with this (and its accompanying ridge) is thus known as an omega high. The omega high is a blocking high, in that it tends to persist in the same geographic location for many days. This blocking pattern also tends to keep the troughs in their respective positions. A figure illustrating this phenomenon can be found on the next slide.

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A More Challenging Forecast: Help is Needed

Weather Forecasting

Here is a standard omega block pattern. Because of the positioning of the high and low pressure systems, the jet stream is forced to orient itself in the shape of an “omega.” This phenomenon slows storms down and keeps weather conditions constant for a bit of time.

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A More Challenging Forecast: Help is Needed

Weather Forecasting

Lastly, meteorologists can employ computer models and satellite/upper-air assistance to complement their forecasts. A well-programmed model can outdo even the most experienced of forecasters, as these computers are able to solve thousands of complex equations at any single moment. While forecasters can make predictions through analogs or experience, the output of a computer model can almost always better represent the complexities of the atmosphere.

The use of satellites can provide meteorologists with a perspective that they would not be able to obtain on their own. By looking at how cloud masses move on satellite imagery, forecasters can predict the arrival time of storms. They can also use the structure and shape of storms on visible imagery to estimate storm intensity or duration.

As these resources continue to improve, weather forecasters and meteorologists are better able to predict the weather conditions of the future. While analyzing the weather and the beauty of Earth’s atmosphere is often done with curiosity and excitement, this process does serve a greater purpose: to provide society with valuable information that could save lives in the moments when the worst of Mother Nature’s fury comes to fruition. And we are all the better for it.

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One Last Thing...

Weather Forecasting

As this presentation comes to a close, I would like to bring one last tidbit for fun. While meteorologists do use computer models extensively, it is quite important to realize that, regardless of how good a model is, the propensity for error is very large the farther out you go. This is why forecasters rarely

use model information for conditions more than a week out.

To see what I mean, here is the most recent run of the GFS

model, a model often used to predict weather conditions for

the United States and the behavior of tropical cyclones, on

the day I am typing this. This model predicts that the

strongest hurricane ever recorded in the Atlantic basin will

make landfall in the northeast and completely obliterate

Washington D.C. To make things worse, the storm eventually

ends up right here in Michigan (although a bit weaker)!

It would a safe bet to assume that this scenario will not

happen. If the entire northeast is destroyed on Sept. 11, 2017

(the date of predicted landfall on this model run), I will be

the first to take back my words. But I’m fairly certain that I

won’t have to — at least, I really hope that this doesn’t end

up actually panning out! Fun fact: the GFS run from six hours

earlier destroys New York City instead — see Slide 726.

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Notable Scientists

in Meteorology

“The scientific man does not aim at an immediate

result. He does not expect that his advanced ideas will

be readily taken up. His work is like that of the planter —

for the future. His duty is to lay the foundation for

those who are to come, and point the way.”

― Nikola Tesla, Inventor

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A Bonus Unit: Notable Scientists in Meteorology

Notable Scientists in Meteorology

While it is certainly vital to understand the inner workings of the atmosphere and apply it when studying the fascinating subject of meteorology, it is also important to look at the individuals whose contributions have allowed us to understand our planet in ways never imagined before. Thus, this final complementary unit focuses on a few of the scientists involved in the field and the ideas and innovation they brought forth. This subject is explicitly included in the Division A Weather or Not event for Michigan, but if you’re doing Division B Meteorology or higher, it wouldn’t hurt to know about these scientists!

The individuals mentioned in this unit comprise only a mere sampling of the people, all throughout history, who have worked tirelessly to provide us with the resources we have today. As such, I may not be able to include every notable scientist on my first go when producing this presentation. If you feel that a person was left out of my presentation (or if a scientist that the Division A focuses on is missing), please contact me at ajzhou@umich.edu and provide me with the name of the scientist that you want me to include. I will be sure to add the information in when I have the time.

With that, it’s a wrap! Regardless of which division you’ll be participating in, I wish you the best of luck! It was definitely a learning experience for me as well during the two years I’ve worked on these slides, and I hope you enjoyed studying the world of meteorology as much as I did!