182 J. Blioi-rrThe South-West Monsoon Storms [No. 2, 



form, and rapidly decreases during the disintegration of the cyclone, 

 ceasing with the disappearance of the cyclonic vortex. It is thus a 

 phenomenon parallel in character and duration with the cyclonic motion 

 or disturbance. 



It is also equally certain that when aqueous vapour is condensed 

 into rain, practically the whole of the solar thermal energy utilized to 

 perform the work of evaporation is given out by the mass of vapour 

 during condensation, and is transferred to the adjacent mass of air. 

 Major Cunningham's Hydraulic Experiments at Eoorkee appear to 

 establish that the sun's heat under the most favourable conditions, that 

 is, dry weather and high air temperature, does not evaporate more than 

 one-tenth of an inch per diem from the surface of slowly moving water. 

 The inverse process of condensation, in consequence of certain features 

 of air motion dependent on rainfall, usually proceeds much more 

 rapidly, and frequently restores the aqueous vapour in the form of 

 rain to the earth's surface at the rate of one to two inches per hour. 

 Prolonged rainfall at the rate of 10 to 30 inches ;per diem for periods 

 varying from 24 to 72 hours are by no means uncommon during the 

 passage of the larger cyclones of the Bay of Bengal across the Bengal 

 or Madras coasts. It is probable, judging from the expressions used by 

 sailors to describe the rainfall during cyclones in the Bay, that it is more 

 intense and prolonged than on land. 



If we therefore compare the rates at which evaporation and condensa- 

 tion can occur, it is certain that the energy released during the act of con- 

 densation is transferred to the atmosphere with very great rapidity during 

 heavy rainfall and probably at a rate occasionally amounting to 100, 200, 

 or even 400 times that at which it was absorbed during the process 

 of evaporation. The effect of a continuous fall of 20 or 30 inches of 

 rain over any portion of the earth's surface would, on the assumption 

 of Major Cunningham's results, be equivalent to that of a sun 250 times 

 as powerful as our present luminary acting directly on the mass of 

 the atmosphere above the area of rainfall, instead of indirectly by means 

 of convection currents due to the heating of the earth's land surface. 

 The action is also usually continuous, and is not interrupted, as in the 

 case of the direct solar action, by the succession of night and day. 

 There is therefore the strongest probability that so powerful a disturbing 

 action can produce very large and rapidly accumulating effects on the 

 mass of the earth's atmosphere affected and influenced by it in a com- 

 paratively short space of time. 



There hence appears to be no doubt that the energy transferred to 

 the atmosphere during heavy rainfall is very large, and that the source 

 of the energy thus indicated is adequate from every point of view to 

 account for the production of the largest and most intense cyclonic cir- 



