1904.] 0. Little — The recent excessive heat in Bengal. 11 



That is, there must be not only a large difference of temperature 

 between the air below and the air above, but there must be an excep- 

 tionally large difference. This is described by some as an unstable con- 

 dition of the atmosphere, the use of the word unstable in that connec- 

 tion being fatal to close reasoning unless clearly defined. 



Before proceeding further I will briefly summarise the assumptions 

 which appear to be necessary to account for the high temperature in 

 Bengal in April and May. They are — 



(1) A westerly instead of a north-westerly wind across the west- 

 ern boundary of Bengr.i in the upper region of the atmosphere. 



(2) That the ai^- oa,rried by this current into Bengal was warmer 

 than with the more usual north-west wind. 



(3) That during the hot weather months the air over East Bengal 

 and Assam becomes comparatively still, and that this condition of the 

 air moves slowly westward in advance of the establishment of the mon- 

 soon or rainy season. 



As I am writing for Indian readers and not for professional meteor* 

 ologists or for the information of physicists in general, I venture to 

 make my argument clear by referring to some of the simpler atmos* 

 pheric processes which are constantly going on around us. On a nor- 

 mal hot season day in India, say in Calcutta, in May, when the san is 

 about vertical at midday and there is no cloud to intercept the sun's 

 heat, the maximum shade temperature occurring about 2 p.m. should be 

 about 96°. This is the temperature of the air close to the earth's sur- 

 face, and the increase of temperature from the early morning onwards 

 is largely due to the warming effect of the earth, which first absorbs the 

 radiant heat from the sun and then gives it out slowly to the air in con- 

 tact with it. The temperature of the air would increase more rapidly 

 if it were not for well-known processes the most important of which is 

 convection. The air as it becomes warmer also becomes lighter than the 

 over-lying stratum. Breaking through that over-lying stratum in places, 

 it streams upwards in what are called convection currents, and the 

 colder air settles down from above, to become warmer in turn. 



The resultant effect, in a normal day, of the sun's heat modified by 

 convection currents is to raise the temperature of the air at the ground 

 level to 96° at Calcutta. Now year after year the sun on the same day 

 in May gives out the same amount of heat from the same series of posi- 

 tions, and the ground surface may be taken to be in the same condition 

 as regards absorption of heat, &c. Any considerable variation of tem- 

 perature from the normal, therefore, may, in the assumed absence of 

 local disturbances, be taken to be due to the convection currents being 

 weaker. 



