Weather

Physical Science B

Unit 2

The Atmosphere

• Weather happens in troposphere

Atmosphere

• Air pressure – comes from the weight of air molecules pushing downward
• As altitude increases, air pressure decreases
• Altitude – the height from ground level
• Fewer air molecules pushing downward

Atmosphere

Atmosphere

Atmosphere

• As altitude increases, air density decreases
• Density - how closely packed molecules are
• Gravity pulls air molecules towards the Earth
• Decreases as you move away from the Earth’s surface
• Air molecules spread out more

Atmosphere and Weather Variables

• The layers of the atmosphere are based on how the temperature changes
• In the Troposphere, as altitude increases, temperature decreases

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Atmosphere

• Troposphere is heated when it comes in contact with the Earth’s surface (conduction)
• Very little of the sun’s energy is absorbed by the air
• Air low in the Troposphere is warmer because it is closer to the heat source (Earth’s surface)

Atmosphere

• Also, air density decreases as altitude increases
• Less molecules to collide with each other
• Harder to transfer heat

Atmosphere

Atmosphere

• So why is it hard to breathe at high altitudes?
• Decreased air pressure and density mean less oxygen is available
• Lungs have to work harder to supply oxygen to the body
• Can cause altitude sickness – shortness of breath, headaches, dizziness, brain or lung damage, death

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Tools: Wind

• Anemometer – Wind Speed
• km per hour
• mi per hour

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• Wind Vane – Wind Direction
• Cardinal units or numerical

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Tools: Atmospheric Pressure

• Barometer:
• Measures air pressure, millibars or inches of mercury

Tools: Atmospheric Pressure

• 1013.25 milllibars = 29.92 inches of mercury

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• How many inHg are in 1000 mbar?
• 1000 mbar x (29.92 inHg/1013.25 mbar) = 29.53 inHg
• How many inHg are in 990 mbar?
• 990 mbar x (29.92 inHg/1013.25 mbar) = 29.23 inHg
• How many mbar are in 30 inHg?
• 30 inHg (1013.25 mbar/29.92 inHg) = 1015.92 mbar
• How many mbar are in 32.5 inHg?
• 32.5 inHg (1013.25 mbar/29.92 inHg) = 1100.62 mbar

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Measuring �Upper-Atmospheric Conditions

• Radiosonde:
• Carried aloft by a

helium balloon.

• Measures upper-atm. conditions: Relative humidity, air pressure, air temperature.
• Information sent back to scientists via radiowaves.

Radar

• Radio Dish Array: Emits and receives

“bounced back” radio waves from larger

particles in the atmosphere.

• Doppler:
• Enhanced version of standard radar.
• Detects subtle shifts of movement in all the

particles in the atmosphere.

• Can determine the size of particles allowing
• scientists to “see” the particles approaching.

Satellites

• Carry cameras
• Upper level Cloud speed, direct.
• Hurricane observation
• Infrared reading of cloud tops

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Tools: Humidity

• Hair Hygrometer:
• Uses hair to measure humidity, hair expands and contracts in response to water vapor content in the air.

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• (Sling) Psychrometer:
• Difference between wet bulb and dry bulb used to determine relative humidity.

Cloud Formation

• Cloud Formation
• Cloud must be below the saturation point (Dew point temperature)
• Evaporation = condensation
• 100% Saturation

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Cloud Formation

• Parcel (block of air) cools to the saturation point
• Most commonly occurs because air is lifted from the surface higher into the atmosphere
• High altitude = lower temperature = closer to saturation point
• Condenses on a cloud condensation nuclei
• Why clouds occur at high altitude

Cloud Formation

• Air is in constant motion
• Constant evaporation and condensation
• More condensation = cloud formation
• More evaporation = cloud dissipation

Relative Humidity and Dew Point

• Humidity - the amount of water vapor in the air
• Relative Humidity – the percentage of water vapor the air is holding relative to how much it could possible hold
• Changes with temperature
• Warm air can hold more moisture than cold air

Relative Humidity and Dew Point

• If an area has 100% relative humidity, it can not hold any more water vapor
• If the temperature in the area increases, it will be able to hold more water vapor
• Relative humidity would no longer be 100%

Relative Humidity and Dew Point

• Calculating Relative Humidity
• Dry Bulb and Wet Bulb temperature are taken using a psychrometer
• Wet bulb temperature should never be higher than dry bulb

Relative Humidity and Dew Point

• Subtract wet bulb temperature from dry bulb
• Use the difference and the dry bulb temp to find Relative Humidity on an RH table

Relative Humidity and Dew Point

• Dew point – the temperature the air has to cool to reach 100% Relative Humidity
• Air is saturated with water
• At the dew point, water vapor in the air condenses
• If the temperature is cold enough, water vapor sublimates (gas 🡪 solid)

Relative Humidity and Dew Point

• When the dew point is reached:
• At ground level, dew forms
• Higher in the atmosphere, clouds form
• Precipitation occurs when water droplets in the cloud become too heavy

Type of precipitation is determined by temperature

Air Masses

Aim: How do air masses affect our weather?

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What are air masses?

• Air mass = a large body of air throughout which temperature and moisture are similar
• Air remains stationary (or moves slowly) when there are only small differences in air pressure
• Air takes on the characteristic temperature and humidity of the region it is over
• Humidity – the amount of water vapor in the air

Types of Air Masses

• Classified according to their source regions (where they originated from)
• Source regions determine the temperature and the humidity of the air masses
• Two letter symbols for humidity and temperature

How do we name air masses?

m = marine/maritime (wet)

c = continent (dry)

� T = Tropical (warm)

P = Polar (cold)

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• Notice the first letters are lowercase, while the second letters are uppercase

1^st

�

2^nd

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Air Masses of North America:

Typical Air Masses of North America

• Cold & Dry ( cP ) from Canada
• Warm & Wet ( mT ) from the Gulf

Weather Fronts

Aim: How do scientists compare the weather patterns of cold fronts with those of warm fronts?

• When two unlike air masses meet, density differences usually keep the air masses separate.
• A cool air mass is dense and does not mix with the less-dense air of a warm air mass.
• More dense = sinks
• Less dense = rises

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• This is how a front forms…
• A boundary forms between the two air masses

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So…how do we define fronts?

• Boundary lines between air masses are called…
• Fronts = the front edge of the air mass moving into our area
• When a front passes �over an area, it �means a change in �the weather.

Did you know…

• Fronts do not exist in the tropics because no air masses that have significant temperature differences exist there.

Types of fronts

• For a front to form, one air mass must collide with another air mass.
• The type of front that forms is determined by how the air masses move in relationship to each other.
• Four types of fronts:
1. Cold Front
2. Warm Front
3. Stationary Front
4. Occluded Front

Cold Front

Forms when a cold air mass pushes under a warm air mass, forcing the warm air to rise.

The moving cold air (more dense) lifts the warm air (less dense).

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Cold Front Weather

• *Heavy rain and often quick, violent thunderstorms.
• *Thunderstorms can form as the moisture in the warm air mass rises, cools, and condenses.
• As the front moves
• through*Cool, fair
• weather is �likely to follow a cold
• front

Warm Front

• When a warm air mass overtakes a cold air mass, a warm front forms.
• The less dense warm air rises over the cooler air.

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Warm Front Weather

• * As the warm air mass rises, it condenses into a broad area of clouds.
• *Gentle rain or �light snow, for a �prolonged period of �time
• *Followed by �warmer, milder �weather

Stationary Front

• Sometimes, when two air masses meet, the air moves parallel to the front and neither air mass is displaced.
• *Forms when warm and cold air meet and neither air mass has the force to move �the other.
• *They remain stationary, or �“standing still.”

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Stationary Front Weather

• *Clouds and fog form, and it may rain or snow.
• *Can bring many days of clouds and light precipitation

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Occluded Front

• *Three fronts come together
• *Forms when a warm air mass gets caught between two cold air masses
• *Warm air mass rises as the cool air masses push and meet in the middle.

Occluded Front Weather

• Temperature drops as the warm air mass is occluded, or “cut off,” from the ground and pushed upward

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• Can bring strong winds, heavy precipitation, and severe thunderstorms.

Weather Report

Q: The cold front below is moving faster than the warm front. What usually happens to the warm air that is between the two fronts surfaces? �

Q: The cold frontal interface below is moving faster than the warm frontal interface. What usually happens to the warm air that is between the two frontal surfaces? �

A)  The warm air is forced under both front.

B)  The warm air is forced under the cold front

but over the warm front.

C)  The warm air is forced under the cold front

but under the warm front

D)  The warm air is forced over both frontal inter.

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Q: The cold frontal interface below is moving faster than the warm frontal interface. What usually happens to the warm air that is between the two frontal surfaces? �

A)  The warm air is forced under both frontal interfaces.

B)  The warm air is forced under the cold frontal interface

but over the warm frontal interface.

C)  The warm air is forced under the cold frontal interface

but under the warm frontal interface.

D)  The warm air is forced over both frontal interfaces.

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Cites A, B, C, and D on the weather map below are affected by a low pressure system (cyclone). �

Q: Which city would have the most unstable atmospheric conditions and the greatest chance of precipitation?

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Q: Identify the type of front pictured below:

Cold Front

Q: Identify the type of front pictured below:

Stationary Front

Q: Identify the type of front pictured below:

Occluded Front

Q: Identify the type of front pictured below:

Warm Front

Pressure Systems

Aim: What is the difference between low and high pressure systems?

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Pressure Systems

• An area of the Earth's atmosphere that has a particularly high or low �pressure compared to �the air around it
• Weather of an area �is determined locally by �the pressure system.

High Pressure Systems

• Regions of sinking air are called high pressure systems/regions or anticyclones
• Associated with clear skies and fair (dry) weather
• Represented on weather maps by a blue H.

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HAPPY HIGH

High Pressure Systems

• In the Northern Hemisphere,
• High pressure regions = diverging air masses.
• Winds rotate in a clockwise manner.
• Wind blows away from the center.
• Cool air sinks due to its high density causing clear and dry weather

Low Pressure Systems

• Regions of rising air are called low pressure systems/regionsor cyclones
• Associated with clouds, rain, and stormy weather
• Represented on weather maps by a red L.

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LOUSY LOW

Low Pressure Systems

• In the Northern Hemisphere,
• Low pressure regions = converging air masses
• Winds rotate in a counterclockwise manner.
• Wind blows toward the center.
• Warm air rises up due to its low density & cools to create clouds.

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What is the normal range of �barometric pressures?

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• The actual barometric pressure must be between
• 960.0 mb and 1040.0 mb (refer to the ESRT)

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• Symbols 000 – 400
• Actually mean high pressure readings H
• 1000.0 mb – 1040.0 mb

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• Symbols 680 – 999
• Actually mean low pressure readings L
• 968.0 mb – 999.9 mb

• High�Outwards�Clockwise
• Low�Inwards�Counter�Clockwise

What type of pressure system is represented by this hurricane?

These numbers represent different barometric pressures.

1. In your notes, connect the points of equal pressure with isobars.
2. Then, identify the center of the high and low pressure system by drawing a �blue H and a red L.
3. Finally, use arrows to indicate the direction of wind flow within each pressure system.

Pressure systems on an actual weather map. What do you notice?

Let’s practice…

1. At which location will a low-pressure storm center most likely form?

• A)  along a frontal surface between different air masses
• B)  near the middle of a cold air mass
• C)  on the leeward side of mountains
• D)  over a very dry, large, flat land area

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• At which location will a low-pressure storm center most likely form?
• A)  along a frontal surface between different air masses
• B)  near the middle of a cold air mass
• C)  on the leeward side of mountains
• D)  over a very dry, large, flat land area

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2. An air mass located over the central United States (~45°N) will most likely move toward the _______.�(Hint: Take a look at the global winds chart in the ESRT)

A) Southwest

B) Southeast

C) Northeast

D) Northwest

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2. An air mass located over the central United States (~45°N) will most likely move toward the _______.�(Hint: Take a look at the global winds chart in the ESRT)

A) Southwest

B) Southeast

C) Northeast

D) Northwest

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3. How does air circulate within a cyclone (low pressure area) in the Northern Hemisphere?

• A)  counterclockwise and away from the center of the cyclone
• B)  clockwise and away from the center of the cyclone
• C)  counterclockwise and toward the center of the cyclone
• D)  clockwise and toward the center of the cyclone

3. How does air circulate within a cyclone (low pressure area) in the Northern Hemisphere?

• A)  counterclockwise and away from the center of the cyclone
• B)  clockwise and away from the center of the cyclone
• C)  counterclockwise and toward the center of the cyclone
• D)  clockwise and toward the center of the cyclone

4. Which map correctly shows the wind directions of the high-pressure and low-pressure systems?

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4. Which map correctly shows the wind directions of the high-pressure and low-pressure systems?

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5. A high-pressure center is generally characterized by

• A)  cool, wet weather C) warm, dry weather
• B)  cool, dry weather D) warm, wet weather

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5. A high-pressure center is generally characterized by

• A)  cool, wet weather C) warm, dry weather
• B)  cool, dry weather D) warm, wet weather

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Complete the t-chart in your notes by listing details about each pressure system

Remember…

• Differences in air pressure are caused by unequal heating of Earth’s surface
• Differences in air pressure at different locations on Earth create wind patterns
• Air moves from areas of high pressure to areas�of low pressure
• In general, worldwide movement of surface air �flows from the poles toward the equator

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Forecasting the Weather

Aim: How is weather data shown on weather maps?

Global Weather Monitoring

• Weather observers at stations around the world report weather conditions frequently, often several times per hour.
• They record:
• Barometric pressure
• Wind speed and direction
• Precipitation
• Temperature
• Humidity
• Cloud cover
• Visibility

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Weather Maps

• The data that weather stations collect are transferred onto weather maps.
• Allow meteorologists to understand the current weather and to predict future weather events.
• To communicate weather data on a weather map, meteorologists use symbols and colors.

What information is on the station model?

982

-15 \

.18

32

25

½

*

Cloud Cover

Clear Skies (0%)

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Overcast (100%)

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Others:

25%

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50%

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75%

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Wind Direction

N

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W E

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S

The wind is blowing from the:

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_______________

SOUTHWEST

Wind Speed

• Wind speed is indicated by little “flags” located on the station model.
• A single “feather” = 10 knots
• A half “feather” = 5 knots
• TOTAL: 15 knots

Abbreviating the pressure

• In order to save space, the air pressure has to be modified to fit on the station model.

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• To modify an air pressure reading, simply take the last three numbers and remove the decimal.

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Example: 1012.3 mb

989.8 mb

123

898

Un-abbreviating the pressure

• To change back to the correct pressure, add a 9 in front if the number is more than 500 and place a decimal one place in from the right.

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Example: 872

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• To change back to the correct pressure, add a 10 in front if the number is less than 500 and place a decimal one place in from the right.

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Example: 261

987.2 mb

1026.1 mb

Let’s practice…

Which station model represents an atmospheric pressure of 1,009.2 millibars and a temperature of 75°F?

Draw a station model with the following data:

• Temperature: 50°F
• Dewpoint: 25°F
• Cloud cover: clear skies
• Wind direction: NE
• Wind speed: 25 knots
• Barometric Pressure: 1004.0 mb

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50

25

040

Severe Weather

Severe Weather

What is the difference?

• Severe Weather Watch: severe conditions are possible in your area
• Be cautious and watch out.
• Severe Weather Warning: severe weather conditions are currently occurring, or are expected to occur, in your area.
• It’s time to take action!
• i.e. A tornado has been spotted close to your area

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THUNDERSTORMS

They are heavy rainstorms accompanied by thunder and lightning from Cumulonimbus clouds

A line of thunderstorms is called a squall

• Form when warm air is forced upward at a cold front

Sudden change in air pressure

• More common during humid afternoons (spring and summer)
• Strong upward and downward winds
• Updraft
• Downdraft - can cause wind shear
• Change in wind speed and/or direction with height, can cause turning

Thunderstorm Life Cycle

The typical lifecycle of a thunderstorm consists of three stages: towering, mature cumulus, and dissipating stage

• Towering cumulus cloud indicates lifting air.

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• Little if any rain

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• Lasts about 10 minutes.
• Occasional lightning

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Mature Stage

• The air continues to rise up to the tropopause, forcing the air to be spread out.

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• Hail, heavy rain, lightning, strong winds likely.
• Lasts an average of 10 to 20 minutes .

Dissipating Stage

• Thunderstorm dissipates and is dominated by a downdraft.
• Rainfall decreases in intensity.

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• Three important ingredients:
• Moisture
• Instability
• Lifting mechanism

Thunderstorm hazards

Thunderstorms can create many hazards including flooding, hail, high winds, and lightning.

In many years, more people die from lightning than any other weather-related cause.

TORNADOES

• Most violent of all storms.
• By definition, a tornado is a rapidly rotating extremely low pressure funnel that hangs down from a column of air extending from a thunderstorm to the ground.

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Tornados

• Low, heavy cumulonimbus clouds
• Spring and summer (late afternoon)
• High humidity
• Form along squall lines
• Cold air meets warm air
• Associated with thunderstorms
• Variations in wind speed with height causes rotation in updrafts
• Brief but deadly
• Waterspout- tornado that forms

over water

Tornado characteristics

• Wind speeds 40 mph to 110 mph

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• Often travel a few miles before dissipating.
• Rotate counterclockwise in NH. Opposite in SH

Tornado formation

Before thunderstorms develop, vertical wind shear creates an invisible, horizontal spinning effect in the lower atmosphere

Updraft within the thunderstorm tilts the rotating tube from horizontal to vertical.

An area of rotation of few miles wide is formed within the storm. Most tornadoes form within this area of strong rotation

Tornado scales

Types of Tornadoes

Hurricanes

• Occur between June and November
• Typically over 300 miles across
• Atlantic = Hurricane
• Pacific/Indian Ocean = Typhoon
• Bring destruction but needed rainfall to S and SE Asia
• May last over a week

Hurricane Formation

• Begin over WARM water as a LOW pressure system (Tropical Depression)
• Grow larger = Tropical Storm → Hurricane
• Hurricane when winds are 74 mph or greater
• Energy comes from water water/humid air
• Air rises and produces clouds
• Winds spiral in toward LOW pressure
• Center = low pressure and warm temperatures
• Faster winds towards center

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Hurricane Formation

HURRICANES

• Characterized by rotating bands of precipitating clouds.
• Lowest pressure is in the center of the storm (cloudless eye)
• After the eye of the storm, winds reverse in the opposite direction
• Spin due to the Coriolis Effect
• Hurricanes spin counterclockwise in the Northern Hemisphere and clockwise in the Southern
• Storm surge causes most of the damage.

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Hurricanes

• High winds can get to over 150 mi/hr
• When a hurricane passes over land or cold ocean water, it loses its energy source of evaporating water and slowly dissipates.
• Greatest threat is when a hurricane hits land.
• In the U.S. the southern and eastern coasts are most vulnerable.

Saffir-Simpson Scale�Categorizes hurricanes