Download Unit Air masses and fronts

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Chapter 19
Air Masses and Fronts
Introduction
Air masses are large volumes of air that contain uniform temperature and humidity
characteristics
Different air masses have different source regions
Air mass properties can modify as the air mass travel over continents and oceans
Air mass properties will modify as the air mass moves north or south
Fronts are the boundaries between air masses
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Continental Polar (cP) and Continental Arctic (cA) Air Masses
• Canada and Asia origin for North America
• Cold and dry
• Inherently stable
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Maritime Polar (mP) Air Masses
• Upper latitude ocean origin
• Cool and moist
Continental Tropical (cT) Air Masses
• Desert southwest of U.S. and northern Mexico origin
• Hot and very dry
• Inherently unstable
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Maritime Tropical (mT) Air Masses
• Low latitude ocean origin (Gulf of Mexico)
• Warm and moist
• Inherently unstable
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• Cold fronts
• Cold air displaces warm air
• Steep uplift of the warm air causes cumulonimbus clouds and precipitation
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• Warm fronts
• Warm air overruns and displaces colder air
• Lifting along a warm front usually produces stratus clouds and often light
precipitation
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• Stationary fronts
• Neither air mass on either side of the front can make the front move very
much
• The warmer air can move aloft over the colder air at a stationary front
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• Occluded fronts form when a cold front overtakes a warm front. The front at
the surface divides two cold air masses, while the warm air is aloft over the
front.
• Cold-type occlusion has a colder air mass pushing out a cooler air mass
• Common in the eastern half of North America
• Warm-type occlusion has a cooler air mass pushing out a colder air mass
• Common along the western edge of North America
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• Drylines are fronts with little temperature change but a strong humidity contrast
• Often form when cT air moves into mT air
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