8 H. F. Blanford— T^ie Theory of tie Winter [No. 1, 



with a rise in the relative and absolute humidity of the air, is simultane- 

 ous with the setting in of the southerly wind ; and this change of wind 

 implies a pre-existing reversal of the barometric gradient, which is the 

 phenomenon to be accounted for. The following considerations may, 

 however, be worthy of attention as tending to throw some light into the 

 prevailing obscurity. 



It has been shewn above, that, at the very moderate elevation of 

 7,000 feet over the outer Himalaya, the barometric gradient is on an 

 average slightly, but distinctly, reversed. At greater elevations, it is 

 most probable that the reversal is more decided, for I have shewn 

 elsewhere* that at Leh (11,500 feet) the pressure in February is at its 

 annual minimum, and the wind-registers of all our hill-stations establish 

 the fact that, throughout the winter months, the prevailing winds are 

 southerly. This preponderance is no doubt, in some measure, perhaps 

 mainly, due to thf fact that the observations are those of 10 A. M. and 

 4 p. M. only ; at which hours the diurnal up-draught of the mountain 

 winds, in an otherwise still atmosphere, is fully active. But I have 

 myself witnessed at Darjeeling, in December, the effects of a strong 

 steady current, sweeping overhead from the south-west, clothing the 

 snowpeaks with cloud-banners. This strong southerly wind is, however, 

 exceptional ; and is that which precedes rain ; and although it is not 

 improbable that, at great elevations, there is a more or less steady flow 

 of air towards Central Asia, to feed the outflow, at low levels, from the 

 anticyclone which, as we know, normally exists in the winter over Nor- 

 thern and Central Asia, there is no reason to question that, up to a 

 considerable elevation over Northern India, the more usual condition is 

 one of comparative stillness or at most of light movement. And, in this 

 state of the atmosphere, even a feeble local action, tending to reduce 

 the density and therefore the pressure, may suffice to set up a centripetal 

 influx of air which may in a short time produce a well- developed baro- 

 metric minimum. How this may be brought about will be shewn 

 presently. 



The southerly surface winds that are invariably the precursors of 

 precipitation, are not merely local ; they prevail also far to the south, 

 indeed over a great part of India ; and they arrive charged with vapour 

 gathered both from the sea and from the warmer land- surface of more 

 southerly regions. 



It seems not improbable, then, that the ulterior conditions which 



give rise to the winter rains, may have their seat in the more elevated 



or middle region of the atmosphere ; and we must look to the formation 



of cloud as the condition which, by disturbing the thermal equilibrium 



* Indian Met. Memoirs, vol. i, p. 224. 



