Part 3. Distribution and Movement of Air

Chapter 9

Air Masses and Fronts

IntroductionAir 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

North American air masses and source regions

Continental Polar (cP) and Continental Arctic (cA) Air Masses • Canada and Asia origin for North America• Cold and dry• Inherently stable

Continental Polar AirArctic Air

cP Air Migration and Modification

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

Maritime Tropical (mT) Air Masses• Low latitude ocean origin (Gulf of Mexico)• Warm and moist• Inherently unstable

Weather map symbols that show the four types of fronts between air masses

• Cold fronts– Cold air displaces warm air– Steep uplift of the warm air causes

cumulonimbus clouds and precipitation

Frontal development

Vertical lifting of warm along a cold front

A cold front depicted on a satellite picture (a) andradar composite (b)

• 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

Profile of a warm front

• 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

Occlusion sequence (next three slides)

Some occlusions form when the surface low elongates and moves away from the junction of the cold and warm fronts

Some occlusions form when the intersectionof the cold and warm fronts slides alongthe warm front

• 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