AIR MASSES The fact that air may have the same temperature and humidity over a wide geographical area has given rise to the concept of the discrete air mass, with properties different from adjacent air masses and separated from them by `fronts'. When an air mass remains stationary for some time, it acquires a temperature and humidity dependent on the surface on which it rests. These characteristics will be retained for some time when the air mass moves into a new region. Air masses are classified, therefore, according to their origin, and we may have, for example: polar maritime, polar continental, tropical maritime, and tropical con-. tinental types, as well as air masses of indeterminate origin (Belasco, 1952). The different air masses interest us, not only because they bring different kinds of weather but also because they might conceivably bring an air spora characteristic of their place of origin.