The source surface of an air mass defines its main characteristics. A warm air mass is created by prolonged contact with a hot surface, and vice versa, a cold air mass is created by prolonged contact with a cold surface. Heat transfer processes that heat or cool the air take place slowly. It can take a week or more for the air to be heated by 10°C by the atmosphere, and for these changes to occur, a large air mass must stagnate over an area. The parts of the Earth`s surface where air can stagnate and gradually acquire the properties of the underlying surface are called source regions. In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover several hundred or thousands of square kilometers, adapting to the characteristics of the surface beneath them. They are classified by latitude and by continental or maritime source region. Colder air masses are called polar or arctic, while warmer air masses are considered tropical. Continental and upper air masses are dry, while maritime and monsoon air masses are humid. Weather fronts separate air masses with different density properties (temperature or humidity). As soon as an air mass moves away from its region of origin, the vegetation and underlying waters can quickly change character. Classification schemes deal with both the properties of an air mass and its modification.

Cold fronts and hidden fronts generally move from west to east, while warm fronts move towards the poles. Due to the higher air density, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm waters can slow down the movement of fronts. [10] When a front becomes stationary and the density contrast disappears across the frontal boundary, the front can degenerate into a line separating areas of different wind speed, called the scissor line. [11] This is more common on the high seas. Continental polar air masses (cP) are cold and dry air masses due to their continental source region. Continental polar air masses affecting North America form over Canada`s interior. Continental tropical air masses (cT) are a type of tropical air generated by the subtropical ridge over large tracts of land and usually originate from low-latitude deserts such as the Sahara in North Africa, which is the main source of these air masses. Other smaller sources that produce cT air masses include the Arabian Peninsula, the dry/semi-arid central part of Australia, and the deserts of the southwestern United States. Continental tropical air masses are extremely hot and dry. [6] Arctic, Antarctic, and polar air masses are cold. The qualities of Arctic air develop above ice and snowy ground.

Arctic air is cryogenic and colder than polar air masses. Arctic air can be shallow in summer and change rapidly as it moves equatorially. [7] Polar air masses evolve over higher latitudes above land or ocean, are very stable and generally flatter than Arctic air. Polar air above the (maritime) ocean loses its stability as it gains moisture over warmer ocean waters. [8] The Bergeron classification is the most widely used form of air mass classification, although others have produced refined versions of this scheme for different regions of the world. [1] The air mass classification consists of three letters. The first letter describes its moisture properties – “c” stands for continental air masses (dry) and “m” stands for maritime (wet) air masses. Its region of origin follows: “T” means tropical, “P” means polar, “A” means Arctic or Antarctic, “M” means monsoon, “E” means equatorial and “S” stands for adiabatic drying and warming of the air formed by significant downward movements in the atmosphere. For example, an air mass formed over the desert in the southwestern United States in summer may be called a “cT.” An air mass formed over northern Siberia in winter can be called “cA”. [2] The main source areas are the high-pressure belts in the subtropics (creating tropical air masses) and around the poles (source of polar air masses). Start your free trial today and get unlimited access to America`s largest dictionary with: Large areas of essentially uniform surface conditions on land or water, usually of large-scale subsidence and lateral divergence, where air masses develop their first properties. Another convention that uses these symbols is to specify the change or transformation from one type to another.

For example, an Arctic air mass blowing over the Gulf of Alaska may be displayed as “cA-mPk”.