Air Mass Designations

The characteristics of an air mass are acquired in the source region, which is the surface area over which the air mass originates. The ideal source region has a uniform surface (all land or all water), a uniform temperature, and is an area in which air stagnates to form high-pressure systems. The properties (temperature and moisture content) that an air mass acquires in its source region are dependent upon a number of factors. These include the time of year (winter or summer), the nature of the underlying surface (whether land, water, or ice covered), and the length of time it remains over its source region.

Arctic (A) Air. A permanent high-pressure area exists in the vicinity of the North Pole. In this region, a gentle flow of air over the polar ice fields allows an Arctic air mass to form. This air mass is characteristically dry aloft and very cold and stable in the lower altitudes.

Antarctic (A) Air. Antarctica is a great source region for intensely cold air masses that have continental characteristics. Before the Antarctic air reaches other land areas, it becomes modified and is properly called maritime polar. The temperatures are colder than in the Arctic regions. Results of Operation Deepfreeze have revealed the coldest surface temperatures in the world to be in the Antarctic.

Continental Polar (cP) Air.—The continental polar source regions consist of all land areas dominated by the Canadian and Siberian high-pressure cells. In the winter, these regions are covered by snow and ice. Because of the intense cold and the absence of water bodies, very little moisture is taken into the air in these regions. Note that the word polar, when applied to air mass designations, does not mean air at the poles (this area is covered by the words arctic and antarctic). Polar air is generally found in latitudes between 40 and 60 degrees and is generally warmer than arctic air. The air over northern and central Asia are exceptions to this.

Maritime Polar (mP) Air. The maritime polar source regions consist of the open unfrozen polar sea areas in the vicinity of 60 degrees latitude, north and south. Such areas are sources of moisture for polar air masses; consequently, air masses forming over these regions are moist, but the moisture is sharply limited by the cold temperature.

Continental Tropical (cT) Air. The continental tropical source regions can be any significant land areas lying in the tropical regions; generally these tropical regions are located between 25 degrees north latitude and 25 degrees south latitude. The large land areas located in these latitudes are usually desert regions (such as the Sahara or Kalahari Deserts of Africa, the Arabian Desert, and the interior of Australia). The air over these land areas is hot and dry.

Maritime Tropical (mT) Air. The maritime tropical source regions are the large zones of open tropical sea along the belt of the subtropical anticyclones. High-pressure cells stagnate in these areas most of the year. The air is warm because of the low latitude and can hold considerable moisture.

Equatorial (E) Air. The equatorial source region is the area from about 10 degrees north latitude to 10 degrees south latitude. It is essentially an oceanic belt that is extremely warm and that has a high moisture content. Convergence of the trade winds from both hemispheres and the intense insolation over this region causes lifting of the unstable, moist air to high levels. The weather associated with these conditions is characterized by thunderstorms throughout the year.

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