An air mass is a large body of air that has similar temperature and moisture properties throughout. The best source regions for air masses are large flat areas where air can be stagnant long enough to take on the characteristics of the surface below. Maritime tropical air masses (mT), for example, develop over the subtropical oceans and transport heat and moisture northward into the U.S.. In contrast, continental polar air masses (cP), which originate over the northern plains of Canada, transport colder and drier air southward.

Once an air mass moves out of its source region, it is modified as it encounters surface conditions different than those found in the source region. For example, as a polar air mass moves southward, it encounters warmer land masses and consequently, is heated by the ground below. Air masses typically clash in the middle latitudes, producing some very interesting weather.

Source of an air mass

The temperature of an air mass will depend largely on its point of origin, and its subsequent journey over the land or sea. This might lead to warming or cooling by the prolonged contact with a warm or cool surface. The processes that warm or cool the air mass take place only slowly, for example it may take a week or more for an air mass to warm up by 10 °C right through the troposphere. For this to take place, an air mass must lie virtually in a stagnant state over the influencing region. Hence, those parts of the Earth’s surface where air masses can stagnate and gradually attain the properties of the underlying surface are called source regions.

The main source regions are the high pressure belts in the subtropics, which produce tropical air masses, and around the poles, that are the source of polar air masses.

Air-mass modification

It is in the source regions that the air mass acquires distinctive properties that are the characteristics of the underlying surface. The air mass may be cool or warm, or dry or moist. The stability of the air within the mass can also be deducted. Tropical air is unstable because it is heated from below, while polar air is stable because it is cooled from below.

As an air mass moves away from its source region towards the British Isles, the air is further modified due to variations in the type or nature of the surface over which it passes. Two processes act independently, or together, to modify an air mass.

An air mass that has a maritime track, i.e. a track predominantly over the sea, will increase its moisture content, particularly in its lower layers. This happens through evaporation of water from the sea surface. An air mass with a long land or continental track will remain dry.

A cold air mass flowing away from its source region over a warmer surface will be warmed from below making the air more unstable in the lowest layers. A warm air mass moving over a cooler surface is cooled from below and becomes stable in the lowest layers

links:

http://ww2010.atmos.uiuc.edu

Air Massws :Wikipedia

Air masses and fronts