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.
The travelling cyclones and anticyclones make the weather highly variable, because there is an element of uncertainty in their time of arrival as well as frequency. Various wave cyclones and vortices are superimposed on the general circulation and pressure systems of the mid-latitude zone.
Precipitation, an important climatic element, is also controlled by various atmospheric disturbances visiting the temperate lands. The convergent wind-system of the cyclones is helpful in the lifting and cooling of extensive air masses which results in precipitation.
Extra tropical cyclones that travel from the nearby oceans to the continents make the atmosphere humid. It is these cyclones which account for a major part of rainfall in different parts of our globe. On the contrary, the anticyclones with their divergent wind system are producers of dry weather.
In maintaining the latitudinal heat balance and in the transfer of heat across different latitudinal zones the atmospheric disturbances contribute a lot. Besides, in the transference of humidity from one part of the earth to another, these disturbances have no less important part to play.
An understanding of the atmospheric disturbances of various magnitudes enables the weather scientist to make prediction as regards their time and place of occurrence. In addition to this, storm warnings can be given well in advance, so that protective measures are taken to save life and property.
Moreover, environmental protections as well as weather modification are directly related to our capacity to understand as completely as possible the natural environmental processes involved in these weather phenomena and to make use of this knowledge in a positive and constructive manner.
The atmospheric disturbances which involve a closed circulation about a low pressure centre, anticlockwise in the northern hemisphere and clockwise in the southern hemisphere, are called cyclones.
They fall into the following two broad categories: (a) Extra tropical cyclones (also called wave cyclones), and (b) Tropical cyclones. Extra tropical cyclones are the weather disturbances of temperate zone and high latitude regions.
In common usage, extra tropical cyclones stand for the wave cyclones that originate at the polar front. But the extra tropical regions have various types of such weather disturbances, besides the wave cyclones, that are non-frontal in origin.
In the present chapter only the extra tropical cyclones that originate at the polar front have been described. Tropical cyclones have been discussed in the next chapter in detail.
However, it may be pointed out that there are other types of cyclonic storms also that are non-frontal in origin and that occasionally visit the temperate regions.
A brief description of these non-frontal atmospheric disturbances of the temperate regions would not be out of place here.