Appendix F. SOLAR RADIATION". 409 



dew and hoar-frost when the air in the shade is above freezing, and 

 killing plants by the rapid abstraction of heat from all their surfaces 

 which are exposed to the clear sky, and the other scorching the skin 

 and tender plants during the day, are now familiar phenomena, and 

 particularly engaged my attention during my whole Indian journey. 

 Two phenomena particularly obstruct radiation in Sikkim — the 

 clouds and fog from the end of May till October, and the haze from 

 February till May. Two months alone are usually clear ; one before 

 and one after the rains, when the air, though still humid, is trans- 

 parent. The haze has never been fully explained, though a well- 

 known phenomenon. On the plains of India, at the foot of the hills, 

 it begins generally in the forenoon of the cold season, with the rise of 

 the west wind; and, in February especially, obscures the sun's disc 

 by noon ; frequently it lasts throughout the twenty-four hours, and 

 is usually accompanied by great dryness of the atmosphere. It 

 gradually diminishes in ascending, and I have never experienced it at 

 10,000 feet ; at 7000, however, it very often, in April, obscures the 

 snowy ranges 30 miles off, which are bright and denned at sunrise, 

 and either pale away, or become of a lurid yellow-red, according to 

 the density of this haze, till they disappear at 10 a.m. I believe it 

 always accompanies a south-west wind (which is a deflected current 

 of the north-west) and dry atmosphere in Sikkim. 



The observations for solar radiation were taken with a black-bulb 

 thermometer, and also with actinometers, but the value of the data 

 afforded by the latter not being fixed or comparative, I shall give the 

 results in a separate section. (See Appendix K.) From a multitude 

 of desultory observations, I conclude that at 7,400 feet, 125° 7, 

 or + 67° above the temperature of the air, is the average maximum 

 effect of the sun's rays on a black-bulb thermometer * throughout the 

 year, amounting rarely to + 70° and + 80° in the summer months, 

 but more frequently in the winter or spring. These results, though 

 greatly above what are obtained at Calcutta, are not much, if at all, 

 above what may be observed on the plains of India. This effect is 



* From the mean of very many observations, I fiud that 10° is the average 

 difference at the level of the sea,, in India, between two similar thermometers, 

 with spherical bulbs ^-inch diam.), the one of black, and the other of plain glass, 

 and both being equally exposed to the sun's rays. 



