NOTE: This document is a repeat of that appearing as a Supplementary Information File for 28 January 1999.
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.
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 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 |
| 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 |
Prominent in all seasons. Bitterly cold and very dry in winter. |
| Equatorial (E) | Equatorial belt | Prominent in all seasons. Hot and high humidity. |
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.