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thomas mancuso

on 10 May 2011

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Explain how an air mass forms.
List the four main types of air masses.
Describe how air masses affect the weather of North America.
Compare the characteristic weather patterns of cold fronts with those of warm fronts.
Describe how a midlatitude cyclone forms.
Differences in air pressure at different locations on Earth create wind patterns.
air mass:
a large body of air throughout which temperature and moisture content are similar
When air pressure differences are small, air remains relatively stationary.
If air remains stationary or moves slowly over a uniform region, the air takes on characteristic temperature and humidity of that region.
Air masses that form over frozen polar regions are very cold and dry. Air masses that form over tropical oceans are warm and moist.
Air masses are classified according to their source regions.
The source regions for cold air masses are polar areas. The source regions for warm air masses are tropical areas.
Air masses that form over the ocean are called maritime. Air masses that form over land are called continental.
The combination of tropical or polar air and continental or maritime air results in air masses that have distinct characteristics.
Continental Air Masses
There are two types of continental air masses: continental polar (cP) and continental tropical (cT).
Continental polar air masses are cold and dry. Continental tropical air masses are warm and dry.
Maritime Air Masses
When these very moist masses of air travel to a new location, they commonly bring more precipitation and fog.
The two different maritime air masses are maritime polar (mP) and maritime tropical (mT).
Maritime polar air masses are moist and cold. Maritime tropical air masses are moist and warm.
Fronts A cool air mass is dense and does not mix with the less-dense air of a warm air mass.

Thus, a boundary, called a front, forms between air masses. Cold Fronts

cold front the front edge of a moving mass of cold air that pushes beneath a warmer air mass like a wedge

If the warm air is moist, clouds will form. Large cumulus and cumulonimbus clouds typically form along fast-moving cold fronts.

A long line of heavy thunderstorms, called a squall line, may occur in the warm, moist air just ahead of a fast-moving cold front.
Warm Fronts

warm front the front edge of advancing warm air mass that replaces colder air with warmer air

The slope of a warm front is gradual.

Because of this gentle slope, clouds may extend far ahead of the surface location, or base, of the front.

A warm front generally produces precipitation over a large area and may cause violent weather. Stationary and Occluded Fronts

stationary front a front of air masses that moves either very slowly or not at all

occluded front a front that forms when a cold air mass overtakes a warm air mass and lifts the warm air mass of the ground and over another air mass

Sometimes, when air masses meet, the cold moves parallel to the front, and neither air mass is displaced.
midlatitude cyclone:
an area of low pressure that is characterized by rotating wind that moves toward the rising air of the central low-pressure region

Waves are the beginnings of low-pressure storm centers called midlatitude cyclones or wave cyclones. Anticyclones

Unlike the air in the midlatitude cyclone, the air of an anticyclone sinks and flows outward from a center of high pressure.

Because of the Coriolis effect, the circulation of air around an anticyclone is clockwise in the Northern Hemisphere.

Anticyclones bring dry weather, because their sinking air does not promote cloud formation.
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