Physical GeographyLecture 26

Air Masses and Fronts

Air Masses

Definition:◦ Large (diameter > 1600

km) body of air

◦ Similar characteristics throughout

◦ Recognizable entity; travel as one

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Figure 7-2

◦ Modifies area over which it moves

◦Is changed by area over which it moves

Classification based on Source Region

MOISTURE CONTENT c—continental, dry m—maritime, humid

Temperature P—polar source region T—tropical source region A—arctic source region E—equatorial source region

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North American Air Masses

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Figure 7-1

Southwestern Deserts – cT More moderate conditions in NW vs NE – mP vs. cP

Effects on North American Weather

Lake Effect Snows

Properties and Sources

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Fronts

“Front” of an air mass Midlatitude clashes Types of Fronts:

◦ Cold front: cold air advancing◦ Warm front: warm air advancing◦ Stationary front: no advance of air masses◦ Occluded front: cold front overtakes warm front

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Cold Front◦ Fast moving, Steep frontal interface, warm air rises

at front ◦ Tall clouds, severe weather, “Squall line”

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Warm Front◦ Gentle slope of warm air rising above cool air◦ Slow moving◦ Sequence of clouds precedes front◦ Steady precipitation

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Stalled warm and cold fronts

Can produce prolonged rain

Stationary Front

Cold front overtakes warm front

Occluded Front