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.