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For airmass in astronomy, see airmass.

In meteorology, an 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.

Source regions of global air masses

Air masses are classified according to their temperature and moisture content. The temperature characteristics of an air mass are defined by the terms Arctic, Polar and Tropical, with Arctic being the coldest and tropical being the warmest. The moisture content of an air mass are defined by the terms Maritime and Continental. Maritime air is a moist air mass, whereas Continental air is relatively dry. The temperature and moisture terms are then combined to describe the air mass. For example, a maritime tropical air mass would be warm and moist, and a continental Arctic would be cold and dry.

The air mass terms refer to the fact that air masses acquire the properties of the terrain over which they move. 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 contact occurs. This region is called a weather front; visible and significant weather changes will occur there. Air masses do not mix, rather they 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, they move much quicker. Though these smaller events do involve masses of air, the term air mass is reserved for weather systems that span large areas.

In Europe an Equatorial air mass is often defined as an airmass developing in equatorial lows and distinguished from a tropical air mass, which develops in subtropical heights. Others define monsoon and superior air masses.

• Weather front