air mass

An extensive volume of air whose physical properties are approximately homogeneous, without large differences in temperature and humidity on any horizontal plane considered. Air masses cover areas of several million square kilometers in extent, having horizontal dimensions on the order of a thousand kilometers and thicknesses of between 3 and 5 kilometers. Their homogeneity is a consequence of the prolonged time they remain static over a land region (source area or region) over which an anticyclone persists, such that the air gradually acquires its own characteristics. Any air mass, as it moves across the Earth's surface and moves away from the source region where it originated, sees its properties modified, being subject to progressive denaturing. The concept of an air mass was incorporated into meteorological jargon in the 1920s, thanks to the work of the meteorologists of the Bergen School; in particular, the polar front theory. Air masses are classified according to their temperature (arctic/antarctic, polar, tropical, equatorial) and their moisture content (continental or maritime), each of them being expressed by a two-letter identifying code.