air mass

A large body of air with only small horizontal variations of temperature, pressure, and moisture.

In meteorology, an extensive body of the atmosphere which is relatively homogeneous horizontally. An air mass may be followed on the weather map as an entity in its day-to-day movement in the general circulation of the atmosphere. The expressions air mass analysis and frontal analysis are applied to the analysis of weather maps in terms of the prevailing air masses and of the zones of transition and interaction (fronts) which separate them.

The relative horizontal homogeneity of an air mass stands in contrast to sharper horizontal changes in a frontal zone. The horizontal extent of important air masses is reckoned in millions of square miles. In the vertical dimension an air mass extends at most to the top of the troposphere, and frequently is restricted to the lower half or less of the troposphere. See also Front; Meteorology; Weather map.

The occurrence of air masses as they appear on the daily weather maps depends upon the existence of air-mass source regions, areas of the Earth's surface which are sufficiently uniform that the overlying atmosphere acquires similar characteristics throughout the region. See also Atmospheric general circulation.

The thermodynamic properties of air mass determine not only the general character of the weather in the extensive area that it covers, but also to some extent the severity of the weather activity in the frontal zone of interaction between air masses. Those properties which determine the primary weather characteristics of an air mass are defined by the vertical distribution of water vapor and heat (temperature). On the vertical distribution of water vapor depend the presence or absence of condensation forms and, if present, the elevation and thickness of fog or cloud layers. On the vertical distribution of temperature depend the relative warmth or coldness of the air mass and, more importantly, the vertical gradient of temperature, known as the lapse rate. The lapse rate determines the stability or instability of the air mass for thermal convection and consequently, the stratiform or convective cellular structure of the cloud forms and precipitation. The most unstable moist air mass is characterized by severe turbulence and heavy showers or thundershowers. In the most stable air mass there is observed an actual increase (inversion) of temperature with increase of height at low elevations.

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, which confer uniform properties to the overlying air. They become less uniform with movement into different areas, and with the effects of wind shear. Air masses are classified first by the source region: over land, continental air mass, c, over sea, maritime air mass, m; and secondly 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. Some include equatorial, E, and monsoon, M, in classifications. See also secondary air masses.

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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.

Classification

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.

Movement and fronts

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.

Other air masses

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.

See also

• Weather front

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