1888.] Pedler and Crombie — Tornado which occurred at Dacca, Sfc. 185 



VI. — On recent Tornadoes in Bengal loith special reference to the Tornado 

 at Bacea on April 7th, 1888. In two Parts. Part I. A Description 

 of the Meteorological Conditions in Bengal which accompanied the 

 Formation of the Dacca Tornado.— By Alex. Pedler, Offg. Meteorologi- 

 cal Reporter to the Government of Be^igal. Part II. A fall Description 

 of the actual Phenomena of the Dacca Tornado. — By A. Ceombie,M. D., 

 Civil Surgeon of Dacca. 



(With Plates XXIV.— XXIX.) 

 [Received and Read May 3rd, 1888.] 

 Part I. 

 Meteorological Conditions accompanying the Dacca Tornado, 

 By Alex. Pedler. 

 Amongst the very varied meteorological phenomena which are 

 commonly met with in India or in Bengal, tornadoes are fortunately of 

 rare occurrence. So rare are they in most countries that few people, 

 except those living in the United States, ever have an opportunity of 

 witnessing one. On account of their rarity in India and of the rather 

 loose manner in which such names as cyclones, whirlwinds, dust-storms, 

 and nor '-westers are sometimes applied to classes of storms which have 

 no right to them, it may be well to state that whirlwinds, waterspouts, 

 tornadoes, dust-storms, and even nor'-westers, are all closely connected 

 phenomena, differing from each other in such particulars as dimensions 

 and intensity, or the degree in which the moisture present is condensed 

 and becomes visible, though more or less closely connected in the causes 

 which give them birth. Such storms as these are, however, widely 

 different from true cyclones both in the manner of their formation 

 and in their phenomena. Thus, the largest tornadoes are vastly smaller 

 than the smallest cyclones, so that there is no difficulty in distinguish- 

 ing between the two classes. The cyclones with which we are 

 familiar in India, and particularly in the Bay of Bengal, are formed 

 over sea areas when the conditions of pressure are very uniform, 

 when the air motion is small over the area where the storm forms, and 

 when the air is of high temperature and nearly saturated with moisture. 

 The formation of a cyclone is apparently only possible wheii the energy 

 of the storm can be supplied by the rajDid inrush of moisture-laden 

 winds in large volumes, and the actual formation of it appears to 

 follow on or to be connected with excessively heavy and torrential 

 rain over a small area. Again, cyclones take time to generate, and they 

 are frequently in existence for days before they attain their maximum 

 strength, and the diameter of the area of hurricane winds is rarely less 

 than 100 miles. Cyclones are also as a rule slow moving storms in 



