Thunderstorms have been defined as storms produced by a cumulonimbus cloud and always accompanied by lightning and thunder. They are usually of short duration, seldom over 2 hrs. They are also accompanied by strong wind gusts, heavy rain and, sometimes, hail.
Origin and Structure
Thunderstorms originate from cumulonimbus clouds. The only difference between an ordinary shower cloud and these storms is that they are always associated with thunder and lightning.
According to Trewartha, a thunderstorm is like a thermodynamic machine in which the potential energy of the latent heat of condensation is rapidly converted into the kinetic energy of ascending air currents.
Presence of warm and humid air in the lower layers of the atmosphere is an essential pre-requisite for the development of a thunderstorm. Atmospheric instability and intense convective activity are other important requirements for their origin and growth.
Intense convective activity in warm and moist air makes possible the lightning discharge in the atmosphere. So the thunderstorms have been called by many meteorologists as the violent showers produced by atmospheric instability.
Thus, a thunderstorm represents the weather phenomenon which combines strong wind gusts, thunder, lightning, torrential rains and cumulonimbus clouds etc. all in one. That is why thunderstorms are considered as a weather factory.
The vertical extent of a thunderstorm depends on the intensity of ascending air currents. The height of thunderstorms ranges from 4 to 20 km. The rising air cools at the dry adiabatic lapse rate up to the base of the thunder cloud.
In the convective cells of the thunderstorm, the dry air is sometimes carried to the middle of the clouds whereby the cooling process continues rapidly. Besides, there is a marked seasonal and latitudinal variation in the heights of thunderstorms.
The maximum height is attained during the warm season in the tropical regions. If there is an upper- air inversion, it acts as a barrier to stop convective activity and thus the further growth of a thunderstorm is checked.
The top of the cumulonimbus cloud is anvil-shaped and the forward- bulging crest points towards the direction of the storm movement. There is heavy precipitation from the well-developed thunderstorms.
At times, when the vertically ascending air currents or updrafts are vigorous, hail may fall from the front part of a thunder cloud. Lightning and thunder always accompany torrential rains.
Since intense heating of the earth's surface sets up convectional currents in the atmosphere, it follows that thunderstorms usually develop in summer in the tropical and the middle latitude regions.
That is why these storms have a preference for the latter part of the day when maximum temperatures are recorded. Besides, adequate supply of moisture is also needed, so that the latent heat of condensation may be available to sustain the storm.
Latent heat of condensation and fusion in ascending currents of warm and humid air supplies the requisite amount of energy for the development of thunderstorms.
These atmospheric disturbances consist of an aggregation of cumulonimbus clouds which often extend to the tropopause.
Cumulonimbus clouds may be of an hour's duration and some of them may have a diameter of 50 km or more. Such huge cloud conglomerations have a life span of several hours.
The word 'conglomeration' refers to the fact that a large cloud mass may contain many updraft areas which are rapidly replaced by others.
Virtually a thunderstorm is composed of several convective cells with updrafts and downdrafts joined together. These cells in a thunderstorm are several kilometers in extent and go through a life cycle.