Chapter 8

Weather:

Water Vapor and Air Masses

8.1

The state of the atmosphere at a certain time and place is the weather

The year-round weather typical of a certain place is its climate

The study of weather and of the atmospheric conditions that produce weather is meteorology

Three major factors work together to cause the earth’s variable weather:

• Heat energy
• Uneven distribution of heat energy
• Water vapor in the atmosphere

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Precipitation – water that falls to the earth, including rain, sleet, hail, and snow

Melting -the process of going from a solid to a liquid

Freezing -the process of going from a liquid to a solid

Evaporation - the process of going from a liquid to a gas

Condensation -the process of going form a gas to a liquid

When air becomes completely full of water vapor and cannot hold any more it is saturated

Humidity - the amount of water vapor in air

Relative humidity - the ratio of the actual humidity compared to the humidity of saturated air under the same conditions

Higher temperatures lower the relative humidity, whereas lower temperatures raise the humidity

When air cools, it becomes saturated (100% relative humidity) at the temperature called the dew point

Water vapor in the air that touches the cooler ground condenses into droplets of water called dew

Airborne water vapor cannot turn into liquid until it condenses around a small particle like dust, sea salt, soot, or smoke. The particle is called a condensation nucleus.

Frost – when water vapor crystallizes on the ground forming light, feathery deposits of ice crystals

8.2

Most clouds result from adiabatic cooling of moist air

Clouds are classified into ten basic categories based on shape and height

Low Altitude Clouds:

• Cumulus – “a heap” - white, billowy clouds that resemble piles of cotton puffs with flat bases.
• Cumulonimbus – “heap cloud” - thunderstorm clouds whose bases are at low altitude but can extend up to the troposphere
• Stratus – “to extend or to stretch” – these form a flat, gray layer of heavy clouds not far above the ground
• Stratocumulus – “extended heaps” - form a low, heavy layer of puffy gray clouds. These are the most frequently occurring clouds

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Mid Level Clouds

5. Nimbostratus – “extended cloud” - similar to stratus clouds but are much thicker, blocking out the sun or moon

6. Altocumulus – “ high heap”- similar to cumulus but appear smaller from the ground

7. Altostratus – “high and extended” - flat, sheet-like clouds that look similar to stratus clouds except they are lighter

High Altitude Clouds

8. Cirrus –“curl of hair”- the most common type of the high-level cloud; made completely of ice crystals

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9. Cirrocumulus – “heaps of curls” – the least common type of high level cloud

10. Cirrostratus – “extended curls” – thin, translucent clouds that spread like a sheet, similar to altostratus clouds

Lenticular Clouds – “lens shaped” - form above mountains and are almond shaped

Contrails “condensation trails” – an artificial cloud that forms from the vapor leaving an airplane engine

Fog – water vapor condensed in the layer of air near the ground – basically it is a stratus cloud near earth’s surface

Types of Fog:

• Radiation fog – on a clear night, the ground quickly radiates heat back into space and a thick layer of air near the ground may cool below the dew point, causing cloud droplets to form
1. Shallow fog – radiation fog that extends no more than 6 feet above the ground
2. Valley fog – radiation fog found in valleys due to cold mountain air flows down and cools the moist valley air
• Advection fog – forms when a warm, humid breeze blows over a cool surface (like a lake)

Sea fog – an advection fog that forms over the ocean

3. Upslope fog – found along the slopes of mountains and can cover a large area for several days

Smog – a mixture of smoke and fog

Photochemical smog – is a thick, brownish haze that results from complex molecules released into the air by cars, buses, trucks, lawn mowers, some factories, and even certain trees and other plants

8.3

Water Cycle – movement of water from the earth’s surface, into the air, and back to the surface

Rain – the most common form of precipitation

• Drops of liquid water falling from clouds to the earth
• Precipitation consisting of drops of water larger than 0.5 mm in diameter

Snow – In clouds with temperature below freezing, water vapor crystallizes around freezing nuclei forming ice crystals. These begin to grow as they absorb water vapor from the cloud. Soon they are large enough to be snowflakes and fall to the ground

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TEST ESSAY: Explain the two processes that form rain

1. Bergeron Process (cold-cloud process) – the snow making process underlined above, but as the snowflakes fall through the cloud, they melt in warmer air near the ground to form rain.

2. Collision-Coalescence Process larger than normal condensation nuclei in regions of the cloud above freezing cause the formation of “giant” cloud droplets, which grow larger by colliding with smaller droplets. When they become too big to remain suspended in the air, the droplets fall to the earth as rain.

In the tropics, rain seems to form by Collision-coalescence process

Drizzle – any precipitation less than 0.5mm in diameter

Freezing Rain – when supercooled raindrops touch a freezing surface, they turn to ice

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Raindrops from the Bergeron process sometimes encounter a deep layer of below-freezing air above the earth which causes the raindrops freeze and form tiny balls of ice called sleet

Dendrite - a leaf- or star-shaped pattern of a snowflake

Snow flurries – are brief periods of

light snowfall that result in little or no

Accumulation

Snow squall – a brief but intense

snowfall

Hail – layered balls of ice that form in strong thunderstorms

The layered ball of ice, is called a hailstone

Drought – when an area received abnormally low precipitation over a relatively long period

Agricultural drought – usually occurs when precipitation cannot supply enough moisture to the ground to support an area’s crops

Socioeconomic drought – based not on the physical water shortage but on the water shortage’s effect on people

8.4

Air mass – a large body of air with relatively uniform temperature, humidity, and pressure

The density of an air mass is affected by the temperature, humidity, and pressure

Maritime tropical air masses form over tropical and subtropical waters, thus they are warm and humid

Maritime polar air masses form over oceans of frigid regions, thus they bring cool, wet weather with them as they move south

Continental tropical air masses form over deserts and bring very hot dry weather

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Arctic air masses form over snow- and ice-covered portions of the Arctic, and they bring dry, frigid air southward to northern Canada

If an air mass remains stationary over a region for a long time, the region experiences air-mass weather

Warm front – when a warm air mass moves into a region occupied by a cold air mass

Cold front – when a cold air mass moves into the territory of a warm air mass

Stationary fronts – occurs when two air masses push against each other, but neither one advances

Occluded front – a combination of three air masses that form a Y-shaped front

Frontal cyclone – a low that forms along a front