Mechanism of Bengal Tornadoes in the Nor'wester Season

THE first requisite of tornado formation is a violent updraught of moist air. In a paper on the nor'westers in Bengal, which will be published shortly, it is being shown from a large number of weather charts that the nor'wester squalls during the period February to June are due to the katabatic flow of air-wedges advancing along the valleys and undercutting moist southerly winds over the plains of Bengal. In the afternoon when the moist air has attained a certain specified state, conditions become favourable for the formation of squalls near the 'noses' of the valley air-wedges. The strength of the squalls increases with the depth of the valley and of the Bay air, and the temperature differences between the two air masses. The accompanying diagram (Fig. I) is based on the 13 h. I.S.T. chart of April 27, 1930, when a passenger steamer named Condor was caught in a violent squall, believed to have been of tornadic violence, and sank at 17 h. near Nagarbari.