An air mass in an immense body of air, usually 160C kilometers or more across, and perhaps several kilometers thick, which is characterized by homogeneous physical properties (in particular temperature and moisture content) at any given altitude.
Trewartha defines air mass as “an immense body of air that moves over the earth’s land- sea surface as a recognizable entity, with temperature and humidity characteristics which are relatively uniform in a horizontal direction at different levels.”
Since it may extend through 20 degrees or more of latitude and cover hundreds of thousands to millions of square kilometers, the horizontal homogeneity of an air mass is never complete. Small differences in the physical properties (especially temperature and humidity) from one point to another at the same level are bound to occur.
But the internal differences of an air mass are small in comparison with the much more rapid rates of change that are experienced across the boundaries between different air masses.
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Since an air mass traverses an area in several days, the region occupied by it will generally have the same weather conditions except some day-to-day variations here and there. But the weather conditions in an adjacent air mass are altogether different. Along the margins of the air masses the weather changes are rather very sharp.
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When air remains in contact with a large and uniform surface for a couple of days, its temperature and moisture attain equilibrium with the surface. If the underlying surface is warm, the overlying air will be warmed and the entire mass of air lying above it will be heated gradually by conduction.
If the surface is cold, the air above it will naturally be cooled and the heat will be removed from the lower part of the air. Similarly, the moist surface will impart its moisture to the air above it, whereas the overlying air will lose moisture to the underlying dry surface.
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Under any circumstances, the air always tends to reach equilibrium with the surface. The time taken to reach a state of equilibrium may vary from two or three days to a week or two. It shows that once equilibrium is reached, it will change only slowly with time.
Such an extensive portion of the atmosphere that has acquired some sort of equilibrium with the surface by remaining into its contact over a large area is designated as an air mass. Such a large uniform surface is called an air mass source region.
Air-masses retain their identity even after they have moved far from their source region. But the original physical properties are conserved only in the upper parts of such air masses.
This is so because the process of conduction is rather slow, convection is almost absent in stagnant and stable air and the process of radiation is not effective in the free air except at cloud surfaces.
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The following two characteristics of an air mass control the weather associated with it: (a) the vertical distribution of temperature in an air mass, and (b) the moisture content of the air. The vertical distribution of temperature is indicative of the stability of an air mass besides its warmness or coldness.
The vertical distribution is closely related to the ascent or descent of air currents within the body of the air mass itself. The presence or absence of condensation forms is determined by the moisture content in the air.
According to Trewartha, air masses are an important and inseparable part of the planetary wind system. Therefore every air mass is related with one or the other permanent wind belt. Tropical maritime and tropical continental air masses are found in the trade wind belts.
However, sometimes tropical as well as polar air masses co-exist in the belt of westerlies. These air masses may be either continental or maritime. In fact, when the air masses move out of their source regions, they may be taken to be the streams of tropical or polar air on a gigantic scale.
Actually it is through these extensive air streams that heat is transferred from the tropical to the Polar Regions. Thus, the air masses play an important role in removing latitudinal imbalances in heat.
An air mass with temperature lower than that of the underlying surface is designated as a cold air mass. On the other hand, a mobile air mass which is warmer than the underlying surface is labeled as a warm air mass. The same air mass may be called warm or cold with reference to the temperature of the surface over which it moves.
For example, if a warm air mass moves from a cold land surface to warm oceanic surface, it will be known as a cold air mass. Similarly, what is a cold air mass during the day becomes a warm air mass during the night because of the rapid cooling of land by nocturnal radiation.
But the most important feature of air masses is that they are independent in respect of their temperature and humidity characteristics. Air masses with different densities, even when they come into contact with each other, do not merge together but retain their identity.
The cold air masses are marked by instability and atmospheric turbulence. Because of the heating of the air lying close to the surface, convectional currents are set up which make the air mass unstable. On the contrary, the warm air masses are cooled from below because of which vertical movements in the atmosphere are non-existent. Therefore such warm air masses have marked stability in them.