In meteorology, an airmass or air mass is a large volume of air having fairly uniform characteristics of temperature, atmospheric pressure, and water vapor content. Air masses cover many hundreds or thousands of square miles, and slowly change in accordance with the terrain they are over.
Air masses are classified according to their temperature and moisture content. The terms Arctic, Polar, and Tropical define the temperature of an air mass with arctic being the coldest and tropical being the warmest. Maritime air is a moist air mass, whereas continental air is relatively dry. These terms are combined; i.e. a maritime tropical air mass would be warm and moist. The terms refer to the fact that air masses aquire the properties of the terrian over which they move. Thus, cold arctic air masses are most common in the arctic regions and maritime air masses generally form over water. Air masses do move however, and a maritime air mass that moves over land will slowly lose its moisture and eventually become continental, just as a tropical air mass that moves north will cool and become polar, or even arctic.
Air masses can not be defined by perfect lines or borders, however there is a very small region of interaction of two or more air masses where mixing occurs. This region is called a weather front, and visible weather and significant weather changes will occur there. As air masses move and displace each other, the associated fronts also move, thus causing weather changes for the terrain below. Fronts are always named for the air mass that is advancing. Thus a cold front would occur where a cooler air mass is displacing a warmer one.
Air masses are not to be confused with small scale events like microbursts. Though these smaller events do involve masses of air, the term air mass is reserved for weather systems that span large areas.
See also: weather front.