SUPPLEMENTAL INFORMATION

To complement the Daily Summary for Thursday, 28 August 2008

AIR MASSES, THEIR SOURCE REGIONS and AIR MASS MODIFICATION


Various weather elements, such as temperature and moisture, can be used to identify air masses. An "air mass" is a large body of air (hundreds of miles in extent) in which these weather elements are relatively uniform in their horizontal extent at each particular altitude. Consequently, the density is essentially uniform in the horizontal. While horizontal uniformity is an ideal condition, variations do exist, but they are usually gradual with no abrupt discontinuities within the same air mass.

AIR MASS SOURCE REGIONS

In a general sense, the temperature of the air mass is determined by the latitude of the source region, while its moisture content is determined by the type of surface over which the air mass developed. When air stagnates over the snow-covered polar latitude continents in winter, the near surface air rapidly loses heat through radiation during the long winter nights. Although the entire air column can cool, the greatest cooling occurs near the surface. In the tropics and subtropics, the intense sunlight heats the surface during the day. The air immediately above the surface is heated upon contact.

AIR MASS CLASSIFICATION

Air masses are usually classified using the following scheme devised during the early 20th century that contains a one or two letter code:

AIR MASS TYPE

SOURCE REGIONS

THERMAL & MOISTURE
PROPERTIES AT SOURCE

Maritime Polar (mP)

Polar and subpolar oceans poleward of approximately 50 degrees

Prominent in all seasons.
Cool, moist (damp).

Continental Polar (cP)

Continents in vicinity of Arctic and Antarctic Circles or Antarctica

Prominent in winter.
Winter: Very cold and dry
Summer: cool and dry.

Maritime Tropical (mT)

Tropical, Trade wind belt and major subtropical ocean basins.

Prominent in all seasons.
Moist and warm.

Continental Tropical (cT)

Tropical continents, especially tropical and subtropical deserts, chiefly Sahara and Australian deserts

Prominent in summer.
Hot and very dry.

Arctic (A)

Arctic or Antarctic
especially the Polar anticyclone

Found all seasons, prominent in winter.
Bitterly cold and very dry in winter.

Equatorial (E)

Equatorial belt

Prominent in all seasons.
Hot and high humidity.

AIR MASS MODIFICATION

When the air mass travels away from the source region, the air mass will remain for a time as a recognizable entity, retaining thermal and moisture properties. However with time and new underlying surface conditions, the air mass does undergo a slow process called "airmass modification". The amount of modification depends upon the route that the air mass traveled, the nature of the underlying surface and the time spent enroute. Usually, polar air masses moving over warmer surfaces are modified more rapidly than tropical air moving over cold ground. Heating of the polar air mass from below enhances vertical air motions that mix the air mass, and increase the likelihood that clouds with a puffy appearance (called cumulus type clouds) will form. Conversely, cooling of the air by contact with a colder surface, suppresses vertical motions restricting cloud formation.


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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2008, The American Meteorological Society.