422 HUMIDITY OF DIFFERENT ELEVATIONS. Appendix G. 



G. 



ON THE RELATIVE HUMIDITY, AND ABSOLUTE AMOUNT OF YAPOUIt 

 CONTAINED IN THE ATMOSPHERE AT DIFFERENT ELEYATIONS 

 IN THE SIKKIM HIMALAYA. 



My observations for temperature arid wet-bulb being for the most 

 part desultory, taken at different dates, and under very different 

 conditions of exposure, &c, it is obvious that those at one station are 

 hardly, if at all, comparative with those of another, and I have there- 

 fore selected only such as were taken at the same date and hour 

 with others taken at the Calcutta Observatory, or as can easily be 

 reduced ; which thus afford a standard (however defective in many 

 respects) for a comparison. I need hardly remind my reader that 

 the vapour-charged wind of Sikkim is the southerly one, which blows 

 over Calcutta ; that in its passage northwards to Sikkim in the 

 summer months, it traverses the heated plains at the foot of the 

 Himalaya, and ascending that range, it discharges the greater part 

 of its moisture (120 to 140 inches annually) over the outer Hima- 

 layan ranges, at elevations of 4000 to 8000 feet. The cooling effect 

 of the uniform covering of forest on the Sikkim ranges is particularly 

 favourable to this deposition, but the slope of the mountains being 

 gradual, the ascending currents are not arrested and cooled so sud- 

 denly as in the Khasia mountains, where the discharge is conse- 

 quently much greater. The heating of the atmosphere, too, over the 

 dry plains at the foot of the outer range, increases farther its capa- 

 city for the retention of vapour, and also tends to render the rain-fall 

 less sudden and violent than on the Khasia, where the south wind 

 blows over the cool expanse of the Jheels. It will be seen from the 

 following observations, that in Sikkim the relative humidity of the 

 atmosphere remains pretty constantly very high in the summer 

 months, and at all elevations, except in the rearward valleys ; and 

 even there a humid atmosphere prevails up to 14,000 feet, every- 

 where within the influence of the snowy mountains. The uniformly 

 high temperature which prevails throughout the summer, even at 

 elevations of 17,000 and 18,000 feet, is no doubt proximately due to 



