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Chapter 20: Weather

20.1 Air Masses & Weather

• Air mass: a large body of air in the lower troposphere that has similar characteristics throughout, such as temperature and humidity

• Characteristics depend on origin

Air Masses of North America

www.fas.org

Air Masseshttp://www.classzone.com/books/earth_science/terc/content/visualizations/e

s2001/es2001page01.cfm?chapter_no=20

Chapter 20: Weather

20.2 Fronts and Lows

• Front: the boundary that separates opposing air masses

• Air masses may differ in

–Temperature

–Humidity

–Both

•Less dense air is forced to rise over more dense air

•Forms a wedge as less dense air is uplifted.

• The lifting force causes condensation (clouds), and if enough moisture, precipitation

Kinds of fronts:

• Cold: boundary between advancing cold air mass and the warm air mass it is displacing

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•The greater the contrast between air masses, the more dramatic the weather.

•Precipitation coverage is usually a narrow band

•Cold fronts move relatively quickly

http://www.classzone.com/books/earth_science/terc/content/visualizations/es2002/es2002page01.cfm?chapter_no=20

Warm Front: warm air replaces retreating cold air

•Warm front weather is usually less dramatic than cold front weather.

•Precipitation coverage is more widespread

•Warm fronts move relatively slowly

http://www.classzone.com/books/earth_science/terc/content/visualizations/es2002/es2002page01.cfm?chapter_no=20

•Occluded front: when a cold front “catches up” to a warm front

•Stationary front: when a front does not move (it’s simply an air mass boundary)

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What are the Front Types?To help protect your privacy, PowerPoint prevented this external picture from being automatically downloaded. To download and display this picture, click Options in the Message Bar, and then click Enable external content.

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Thunderstorms• Much smaller (a few kilometers across) than low pressure systems

(mid-latitude lows) and hurricanes

Thunderstorms•Cumulonimbus clouds can bring:

• Rain, hail, damaging winds, lightning, thunder, & tornadoes

• May occur anytime; most

numerous in the afternoon

Thunderstorm “Triggers”

1. A source of lift (cold front, etc.)

2. Moisture

3. Heat

Thunderstorms can occur in Wisconsin any month of the year!

http://www.youtube.com/watch?v=IatoWCinM9w

http://www.youtube.com/watch?v=exlVSEPEXKc

Thunderstorm Life Cycle Air rises and a cumulus cloud forms.

The rising air is called an updraft.

The updraft prevents precipitation from reaching the ground.

Photos: National Weather Service

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Thunderstorm Life CycleThe precipitation becomes heavy enough to fall through

the updraft and reach the ground.

The falling precipitation

creates a downdraft.

Photos: National Weather Service

Thunderstorm Life CycleThe downdraft weakens the updraft, eventually cutting off

the supply of moist air rising to the cloud.

The cloud begins to

evaporate.

Photos: National Weather Service

Thunderstorms•Severe thunderstorms defined as:

•Contains a tornado

•Winds 57 mph or higher

•Hail 1 in. across or larger