184 J. Eliot— r^e South-West Monsoon Storms [No. 2, 



As rainfall does not always appear to produce cyclonic motion, it is 

 clear that, although rainfall may be the source of energy, it is only when 

 the rainfall occurs under special conditions that the accompanying air 

 motion increases and accumulates in the peculiar manner necessary 

 to give rise to a large and intense cyclonic circulation. Experience 

 has shown that the following conditions, which can be proved to have 

 a direct bearing on the formation of cyclones, are always present before 

 and during the generation of cyclones in the Bay of Bengal : — 



1st. The establishment and prevalence of a humid current over the 

 extreme south of the Bay, which brings up large quantities of aqueous 

 vapour into the centre or north of the Bay. 



2nd. The occurrence of approximate uniformity of meteorological 

 conditions, more especially of pressure, over the coasts of the Bay, and 

 frequently over a considerable portion of the Bay. 



3rd. The prevalence of light and variable winds over Bengal and 

 the coasts of the Bay, and over a considerable portion of the Bay extend- 

 ing from the Bengal coast southwards. This condition is practically 

 identical with the previous, as both are due to, and accompany, the same 

 general atmospheric conditions. 



4th. The absence of rainfall, and the prevalence of clear skies with 

 fine weather, over the north and centre of the Bay, and in Bengal. 



The relative importance of these conditions will be evident on very 

 ])rief consideration. The first is evidently necessary to supply the aque- 

 (jus vapour in sufficiently large amounts to give rise to continuous heavy 

 vainfall over such a large area as is covered by a considerable cyclonic 

 disturbance. The Bay of Bengal is not a large enough evaporating area 

 to afford such a supply. Hence cyclonic storms are only formed in the 

 Bay of Bengal when there is a humid current blowing into it from 

 the Indian Ocean. This occurs only during the south-west mon- 

 soon period, when the south-west winds blowing at the entrance of 

 the Bay are the northward continuation beyond the Equator of the south- 

 east Trade Winds of the Southern Tropics. That such is the case is 

 sufficiently proved by the fact that cyclonic storms on the large scale 

 are entirely restricted to that portion of the year when south-west mon- 

 soon winds are blowing over a part or the whole of the Bay, that is, from 

 the beginning of May to the end of December. It is also shown by the 

 fact that, at the commencement and termination of the south-west mon- 

 soon period, any cyclones that are generated, form in the south of the 

 Bay, whilst in the months of July and August, or during the height of 

 the south-west monsoon, they form near the Head of the Bay. In short, 

 the area of cyclonic generation in the Bay of Bengal depends mainly 

 upon the season, and travels northwards or southwards, according as the 

 south-west monsoon is advancing or retreating over the Bay. 



