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Chapter 8: Air Masses, Fronts Mid-Latitude Cyclones
 Air Masses

large body of air whose properties of temperature and
humidity are fairly similar in any horizontal direction at
any given latitude
 Source Regions
 Regions
where air masses originate
 Air Mass Classification/Characteristics
 Table
8.1
Air Masses of North America
 Continental Polar (cP)
 Continental Arctic (cA)
Air masses originate over ice and snow covered regions
 Clear night allow for strong radiational cooling

 Maritime Polar (mP) Maritime Arctic (mA)
Source region is over the ocean of higher latitudes
 North Pacific and north Atlantic
 Responsible for west coast low clouds and fog
 This air mass also contributes to rainfall that occurs
during the winter months

Air Masses of North America
 Maritime Tropical (mT)
May be associated with warm and unstable air
 This air mass may combine with maritime polar air to
give us more rain from a storm
 Pineapple Express type storms
 Atmospheric Rivers: stretches of maritime tropical air
from the west Pacific

 Continental Tropical (cT)
May bring hot and dry weather to the mountains and
deserts
 May be associated with dust devils, wind storms in the
deserts

Fronts
 The leading edge of an air mass may serve as
a frontal boundary.
 A frontal boundary can either be a
continental polar air mass moving south or
maybe a maritime tropical airmass moving to
the north
 The airmass can either be moist or dry,
depending on its source region
Fronts cont…
 Front: is the transition zone between two airmasses
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of different densities.
Also the humidity can differ with the airmasses
Cold air is often heavier and drier than moist, less
dense air
Warm air is often lighter and more moist than
heavy, more dense air
Ocean air masses are more moist than land air
masses, which are typically drier.
Fronts
 Stationary Front: has essentially no movement
 Cold Front: Cold, dry, stable air is replacing warm,
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moist, unstable tropical air
Warm Front: air from the south (Gulf of Mexico)
moves northward and moves over colder air to the
north.
Occluded Front: a cold front catches up to a warm
front.
See Table 8.3 page 220
Also see Figure 8.20
Dry from the North
Warm Unstable Air
Middle Latitude Cyclones
 Low pressure system forms where cold air is moving
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south and warm air is moving north
Fronts represent ‘battle grounds’ where air masses
meet.
Middle latitude cyclones represent this ‘battle
ground’.
Middle latitude cyclones are associated with rain,
wind and snow.
Middle latitude cyclones move with the upper level
winds at 500mb.