Role of a surface parameter on premonsoon thunderstorm days at DumDum (22.38°N, 88.28°E), Calcutta

121-125Occurrence of premonsoon thunderstorms striking DumDum, Calcutta, over a 12-year period has been studied based on a surface parameter (S) developed from surface observations at 0830 IST only. It has been found that on thunderstorm days the value of S in general lies between 1.00 and 1.60. It has also been found that 59.9% of premonsoon thunderstorms are of northerly type and 35.4% of southerly type. Nearly one-half of northerly developments are associated with wind squall, but in the case of southerly developments the wind squall association is only one-fifth. Southerly thunderstorms with wind squalls have almost a linear relationship between the parameter S and the time of occurrence. Moreover, south-easterly developments with squalls are more moist, have large wind speed of squall and occur early compared to the south-westerly developments with squalls