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Such disturbances fall into the following types which have been briefly described below: tropical depressions, thermal lows, lee depressions, and polar air depressions.

The atmospheric disturbances play a vital role in the complex processes of exchange of heat between different latitude zones. True, the maximum control of these disturbances on the day-to-day weather is seen in the middle latitude zone.

As a result of the observations of atmospheric conditions at the surface and aloft, the following types of fronts are identified: (1) cold front, (2) warm front, (3) occluded front, and (4) stationary front.

Fronts do not form everywhere. Their development is confined to certain well-defined zones. Fronts usually develop in those areas of the world where air masses have strong temperature contrasts.

It was Tor Bergeron who for the first time used the term ‘frontogenesis’ for the creation of new fronts. Later on, the term was extended to include the process of regeneration of the old and decaying fronts.

Fronts may differ in their types, location, or areal extent, but the following characteristics are more or less common to all of them.

The air masses differ in their temperature and in their moisture content.

All the physical properties of air masses that govern the weather and climate of Europe are determined by its shape and geographical location. Europe, which forms the western part of the great Eurasian continent, is broader in the south and narrow in the north.

From the point of view of air masses, the largest continent of Asia is a land of extremes. Since the landmass of this continent extends from near about the North Pole to the Equator, the hottest as well as the coldest air masses are found here.

The continent of North America is triangular in shape with it base towards the north in the higher latitudes, and apex towards the south in the low latitudes.

All the physical properties of air masses which determine the weather characteristics produced by them are acquired from their source regions. When the air masses move out from their source regions, the temperature, humidity and the nature of the underlying surface bring about certain changes in them.

According to Byers, an air mass may undergo changes because of any one or more than one of the following mechanisms:-

Thermodynamic processes include such effects as heating from below which decreases the vertical stability. Other thermodynamic effects include evaporation of water into the air mass from below or into the intermediate layers by precipitation from the overlying moist air layers.

Air masses normally migrate from their source regions to other regions which have different surface properties. As the air masses move out from their source regions, they not only modify the weather of the areas they occupy, but they are also modified to a certain extent by the surface over which they are moving.

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.

Areas where air masses form are known as source regions. The nature of the source region largely determines the temperature and humidity characteristics of an air mass.