ABSTRACT
Three factors determine the nature and degree of uniformity of air-mass characteristics: (1) the nature of the source area where the air mass obtains its original qualities; (2) the direction of movement and changes that occur as an air mass moves over long distances; and (3) the age of the air mass. Air masses are classified on the basis of two primary factors. The first is temperature, giving arctic, polar and tropical air, and the second is the surface type in their region of origin, giving maritime and continental categories.
