SOME FEATURES OF PRE-MONSOON THUNDERSTROMS OVER NORTH EAST INDIA

Thunderstorms are localized phenomena which have been defined as storms produced by a cumulonimbus cloud always accompanied by lighting and thunder. They are usually of short duration, seldom over two hours. Generally they are accompanied by wind gusts, heavy rain and sometimes hail. Thunderstorms are serious weather hazard to aviation, A large percentage of aircraft accidents occur due to it. So, the aviators dread thunderstorm and tried to keep it away. Therefore, this severe weather phenomenon has always been considered as an important subject to be investigated by
the weather scientist.

Atmospheric instability, lifting of potentially unstable air and sufficient supply of warm moist air are the main factors favouring the formation of a thunderstorm (Glode, 1977). There are marked seasonal and latitudinal variations in the height, horizontal extent and duration of thunderstorms. Generally, they range from 4 km to 20 km in height above mean sea level. The maximum height is observed in tropical region during warm season. Thunderstorms may cause destruction of properties rendering thousands of
people homeless, over rooting trees and damaging standing crops in the fields.


In Northeast India, thunderstorms (Nor’westers) are very common phenomenon during the period February to May. The areal distribution of thunderstorm activity in India during the pre-monsoon months (February to May) is depicted in Figure 4, f and 4.2 (Rao,1983). The figures show that during pre-monsoon months the areas with marked thunderstorm activity are North eastern part of the country including Northeastern states, Bengal and Orissa, some parts of central India and extreme south of the subcontinent.

Thunderstorms have diurnal variation. In Northeast India, generally they occur during afternoon hours. During night and early morning hours also the region experiences considerable thunderstorms events. Pre-monsoon thunderstorms are sometimes accompanied by hail usually in and near sub-montane regions.


Analyses of IMD Tephidiagrams show that there are no days during the Nor’wester season in the region without latent instability being present in the atmosphere in some form. There is usually an inversion between 1 to 2 km(at Guwahati airport), and if the necessary trigger action becomes available, the energy that can be realized on most days is considerable. If the whole of the energy was realized, the velocity of the squall will exceed 75 mph on many occasions. (IMD 1944)

On most occasions the energy has only been partly used up and on a few occasions the realizable energy has shown an increase after thunderstorms, which suggest maintenance of intensification of state due to air movements. In vertical sections θs (saturation potential temperature) decrease from about 330K to about 292K at 5km, and thereafter increases, reaching the surface value at about 13km. Therefore convection maybe expected to extend up to about 13 km and convective clouds to reach that level. The vertical sections as well as winds on isentropic surface indicate in a general way both upward and downward currents and after a thunderstorm θs at the surface has nearly the same value as the minimum θs in the upper air suggesting a descent of air from about 5 km. It is also noticed from an examinations of all available thermograms that the value of the wet bulb is, after a squall, of the order of 294 °K, irrespective of locality.

From past records of both surface and upper air data it has been observed that the occurrence of pre-monsoon thunderstorm in Northeast India is most favored on those days when certain latent instability is present in the atmosphere.

The main feature in the pressure distribution over north India during the Nor'wester season is the trough of low pressure extending in a W-E direction across the plains.

There is no cooling process operating on the westerlies reaching the Brahmaputra valley at the level of the plateau. Therefore, the Tibetan plateau will continue to remain colder than the air over the Brahmaputra valley at the corresponding level.