Lapse rate change due to advection at lower levels and occurrence of pre-monsoon thunderstorms

Abstract

Using the method of vertical wind shear vector difference, the change in lapse rate brought about by advection alone (upto a height of 5000-7000 ft) was estimated for Calcutta from the upper wind data of 0200 GMT ascent for every day, spread over a period of one month of the usual peak activity of nor'westers which (violent thunderstorms). This change applied to the morning tephigram is found to provide a moderately successful tool for prediction of local afternoon thunderstorms. The percentage of success for the period studied is nearly 80 per cent.