An area of the atmosphere extending for hundreds of kilometres that, horizontally, has generally uniform properties, especially of temperature and humidity, and with similar vertical variations of temperature and pressure throughout. Air masses obtain these attributes from their areas of origin, known as source regions, and become less uniform with movement into different areas, and with the effects of wind shear.

Air masses are classified by the source region: continental air mass, c, or maritime air mass, m; and by the latitude of the source area: the arctic, A, the poles, P, and the tropics, T. These two combine to distinguish most air masses, such as cA— continental arctic, mP— maritime polar, mT—maritime tropical, and so on. Equatorial, E, and monsoon, M, tags also exist. Browell et al. (2003) J. Geophys. Res. 108, D20 identify nine air mass types, quantifying their frequency as a function of altitude, and Chen et al. (2007) J. Geophys. Res. 112, D10S05 classify air mass types along coastal New England. See Peterson et al. (1981) J. Appl. Met. 20, 3 on turbidity and synoptic air masses.

In the case of Bangalore, India, for instance, a continental polar (cP) and continental arctic (cA) mass is predominant in the autumn and winter, which produces dry conditions, while a maritime equatorial (mE) is dominant in the spring and summer, bringing moister conditions.