Unlike cyclonic storms, thunderstorms are a meso-scale or localised phenomena, generally occurring between 100 and 1000 km. However, because of its large frequency and associated multiple hazards, cumulative damage is quite substantial. It is estimated that at any given instant more than 2000 thunderstorms are taking place around the world (WMO, 1953). Hailstorms, tornado, lightning, cloud burst, microburst, squall are hazardous phenomena associated with thunderstorms. In aviation, thunderstorms are the most dreaded weather phenomena. It causes pressure perturbation leading to defective altimeter reading, poor visibility due to heavy rain, turbulence, icing, and radio static and communication problem. Many air accidents have been attributed to thunderstorms. Tornadoes are notorious for their ferocity and leave a trail of destruction in the affected areas.
Convection is the building block of a thunderstorm. Convection is initiated due to strong heating of the land mass during the day time, or by mixing of different types of air masses. Presence of atmospheric instability coupled with moisture convergence results in the formation of deep cumulonimbus clouds and thunderstorms. A typical thunderstorm cell may have a size of 1-10 km with life period of less than an hour (Fig. 1). Depending on the atmospheric conditions a thunderstorm undergoes three distinct stages i.e. cumulus, mature and dissipating in its life cycle.
The single cell thunderstorm: The ordinary or single cell thunderstorm is an ephemeral burst of convection. Each burst creates a towering cumulus or ‘cell’ of convection. When the cell gets large (and tall) enough, it is classified as a cumulonimbus or thunderstorm.
The multi cell storm: As the name suggests, it consists of many cells that grow to form a group of cells that move together as one unit. A multi-cell cluster thunderstorm results from a vigorous parent-daughter effect. In this case, an ordinary thunderstorm creates neighbouring storms via the downdraft and gust front. The multi-cell cluster storms last longer than ordinary or single cell storms, and cover a larger area. Each cell in a multi-cell cluster storm usually lasts about 20 minutes, but the cluster of storms itself can persist for several hours. Because of the storm-storm interaction, it is rare to get extreme weather (large hail, severe tornadoes) from a multicell cluster, but heavy rain, strong winds, small to medium sized hail and weak tornadoes are still threats. Sometimes thunderstorms form in a line which can extend laterally for hundreds of miles. These ‘squall lines’ can persist for many hours and produce damaging winds and hail.
The super cell storm: This type of storm is the classic deep cumulonimbus tower (12 km and more height) with the anvil top and on occasion the overshooting updraft tower. These storms are notorious for producing damaging straight-line winds, frequent lightning, flash floods, large hail, and violent tornadoes. This is a stand-alone entity which is so well organised that it actually supports itself and enhances its own growth. In order to form a supercell, three ingredients are necessary: high thermal instability, strong winds in the middle and upper troposphere, and veering of the wind with height in the lowest kilometre.