A mass of air which is largely homogeneous in a horizontal direction. Its physical properties are determined by the nature of the surface over which it forms, and may be subsequently modified when the air mass moves over a different type of surface. Air masses are often separated from each other by frontal surfaces, which are discontinuities.
A large scale portion of the earth's atmosphere whose properties (principally temperature and moisture) are approximately uniform. A system commonly used to classify air according to their source regions. In this system, air masses are identified according to the thermal and moisture properties of their source region. Thermal identifications are tropical (T), polar (P), and less frequently, arctic (A) or antarctic (AA). Moisture characterizations are continental (c), which is relatively dry, and maritime (m), which is relatively moist.