Part 3' Distribution and Movement of Air

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Part 3. Distribution and Movement of Air

• Chapter 9
• Air Masses and Fronts

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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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North American air masses and source regions
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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

Continental Polar Air
Arctic Air
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cP Air Migration and Modification
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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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Weather map symbols that show the four types of
fronts between air masses
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• Cold fronts
• Cold air displaces warm air
• Steep uplift of the warm air causes cumulonimbus
  clouds and precipitation

Frontal development
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Vertical lifting of warm along a cold front
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A cold front depicted on a satellite picture (a)
and radar composite (b)
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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
• 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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Profile of a warm 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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Occlusion sequence (next three slides)
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Some occlusions form when the surface low
elongates and moves away from the junction of
the cold and warm fronts
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Some occlusions form when the intersection of the
cold and warm fronts slides along the warm front
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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

Continental Tropical Air
Maritime Tropical Air
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