Air Masses and Fronts

AIR MASSES AND FRONTS
AIR MASSES An air mass is a mass of air with similar properties of temperature and moisture covering a large area of the earth's surface and bounded by fronts. Regions where homogenous air masses tend to be created are known as source regions. Some of the well known source regions are sub-tropical and tropical oceans, low-latitude deserts, and the continental interiors. An air mass is said to be cold when it is colder than the surface over which it rests or is moving. An air mass is said to be warm when it is warmer than the surface over which it rests or is moving.

On the basis of source region, air masses are of three types: tropical air mass, polar air mass, and Arctic or Antarctic air mass. Tropical air mass is an air mass that has its source region within the sub-tropical high pressure areas between 20° and 40° north and south of the equator. Polar air mass is an air mass having its source region between latitudes 40° and 60° north and south of the equator. Arctic or Antarctic air mass is an air mass with its source area between latitude 60° and the poles. On the basis of humidity, tropical and polar air masses are further divided into tropical maritime, tropical continental, polar maritime and polar continental. Maritime air masses originate over an ocean, while continental air masses originate over a con­tinent or landmass.

FRONTS The boundary zones between air masses that have originated in different source areas have differing temperature and humidity characteristics. Where the bound­ary zone intersects the earth's surface, it forms a line of separation, called a front. It is represented by a line in weather charts. A front is usually associated with a trough of low pressure and is very important as a considerable amount of weather is generated along it. Fronts may be nearly vertical as in the case of air masses having little motion relative to one another. They may -be almost horizontal as in cases where one air mass slides over
,another.

Cold Front is a front separating a retreating warm air mass and an advancing cold air mass; the cold air, being heavier, remains near the surface, forcing the warm air to rise over it. The cold front is steeper than the warm front. Another is associated with strong atmospheric disturbances. At a cold front, pressure rises, temperature falls, the wind, veers and there are heavy showers often accompanied by thunder.
Wann Front is a front separating a retreating cold air mass and an advancing warm air mass. Warm air is forced to rise here also. Warm fronts have lower slopes and are commonly attended by stable atmospheric conditions, lack­ing the turbulent air motion of the cold front. If the wal1I\ air is unstable, it results in heavy showers and thunder­storm. The weather associated with warm fronts is seen long before the front reaches the observer. The first indication of its approach is high cirrus cloud, which gradually lowers and thickens into low cirrostratus, fol­lowed by altostratus and then rain-producing nimbostratus at the front itself, frequently a distance of 1000 km over
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all. As the front passes, temperature and humidity rise, pressure stops falling and winds veer.
Occluded Front forms when, during the later stages of the evolution of a cyclonic storm, the more rapidly moving cold front catches up with the warm front and lifts the warm sector above the surface. This is fairly common over Western Europe. The weather associated with an occlusion is similar to that occurring at the original fronts, but does not last as long and eventually dies out as the air in the warm sector riSes and cools.
Stationary Front is formed when neither the warm nor the cold front are able to replace each other. At the stationary fronts rain occurs, sometimes lasting for several days.