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 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 movementCold Front: Cold, dry, stable air is replacing
warm, moist, unstable tropical airWarm 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 220Also see Figure 8.20
Dry from the North
Warm Unstable Air
Middle Latitude Cyclones
Low pressure system forms where cold air is moving 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.
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