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parts of the lower atmosphere, or troposphere, whose horizontal dimensions are comparable to large parts of continents and oceans. Each air mass has a definite homogeneity of properties and moves as an entity in one of the currents of the general atmospheric circulation. In addition, a given air mass is separated from neighboring masses by border zones known as fronts. The division of the troposphere into air masses is constantly changing: in the complex system of air currents, air masses move from one region of the earth to another, while changing their properties, disappearing as separate entities, and reforming.
The properties of an air mass are determined primarily by the geographical conditions of the region in which the air mass formed (source region of the mass). Such a source region can be a broad area with a sufficiently homogeneous underlying surface and resulting homogeneous influences on the air—for example, the ocean expanses in tropical latitudes, arctic ice, taiga areas, and large deserts. When an air mass remains in such a region for a long period of time (as in a persistent anticyclone) or when there is prolonged shifting over such a surface, the air acquires the properties of a homogeneous air mass: variations in space (horizontal lapse rates) of temperature, moisture, and certain other meteorological elements become slight, and cloud cover and precipitation acquire features typical of the given air mass. In connection with particular features of atmospheric movements (such as the presence of confluence of flow lines), the blurred boundaries between air masses turn into clear fronts, or narrow zones where horizontal gradations of meteorological elements are much greater than within the air masses. Every air mass carries a definite type of weather, which it transports when it shifts, thereby creating the most important nonperiodic weather changes. When air masses move to a new region far from their source region, their properties change under the influence of altered geographic circumstances (primarily geographic latitude and the nature of the underlying surface). Air mass transformation occurs, which is expressed in changes in the weather regimes associated with such circumstances.
The most general division of air masses is into cold, warm, and local types. A cold air mass is a mass moving into a warmer atmosphere—that is, usually into lower latitudes and toward a warmer underlying surface; its arrival brings cooling into the area. A warm mass is a mass moving into a cooler region—that is, usually into higher latitudes and over a colder surface; its arrival brings warming. A local air mass is one that does not change its geographical location over an extended period. The weather systems vary greatly in masses of these three types. Thus, a cold air mass moving onto a warmer surface and being warmed from below acquires unstable stratification; convection, together with the corresponding clouds and torrential precipitation, develops in it. The winds take on a turbulent, gusty character, visibility improves, and so on. A warm air mass, on the other hand, is characterized by stable stratification, which gives the clouds a specific stratified form, with corresponding light precipitation; it may also cause fog. Local air masses can have stable or unstable stratification, depending on the season of the year.
Air masses also differ in relation to the geographical location of their source. Four zonal types are differentiated according to this trait: arctic air (in the southern hemisphere, antarctic air), whose masses form in the earth’s highest latitudes; polar air (temperate air), whose masses form outside of the tropical latitudes but excluding the highest latitudes; tropical air, whose source regions are located in tropical or subtropical latitudes; and equatorial air, which comes from the lowest, equatorial latitudes. Each type is further divided into oceanic and continental air. There are also more detailed geographic classifications for specific regions, such as the territory of the USSR. Statistical characteristics of air masses have also been established for various regions of the earth. The concept of air masses is one of the fundamental concepts in contemporary climatology and synoptic meteorology.
REFERENCESKhromov, S. P. Osnovy sinopticheskoi meteorologii. Leningrad, 1948.
Alisov, B. P. Klimaticheskie oblasti zarubezhnykh stran. Moscow, 1950.
S. P. KHROMOV