Part 3. Distribution and
Movement of Air
Chapter 9
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
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
Arctic Air
Continental
Polar 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) and
radar 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 intersection
of the cold and warm
fronts slides along
the 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