112 CLIMATE AND VEGETATION OF 



January, at 9 a.m., it has stood at 98°, difF. + 68-2°, and at 

 10 A.M. at 114°, difF. + 81-4°, wliilst the radiating thermo- 

 meter on the snow had fallen at sunrise to 0'7°. In December, 

 at 13,500 feet, I have seen it 110°, difF. + 84° ; at 11 a.m., 11,500 

 feet, 122°, difi; + 75°. In November, 9 a.m., at 13,500 feet, 

 112°, difF. 4- 82°. This is but a small selection from many, of 

 the extraordinary power of solar radiation in the coldest months 

 at great elevations. It is accompanied by a great increase of solar 

 light, as I found by means of the black glass photometer. 



Two phenomena particulax'ly obstruct the solar light and heat 

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

 haze from February to May. On the former I have dwelt suffi- 

 ciently at length. Two months alone are usually clear, one 

 before and one after the rains, when the air, thougli still humid, 

 is transparent. The haze has never been fully explained, 

 though a well-known phenomenon. On the plains of India, at 

 the foot of the iiills, 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 24 hours, and is usually accompanied by great 

 dryness of the atmosphere. It gradually diminislies in ascend- 

 ing ; it cannot be said to prevail at 7000 feet, and I have never 

 experienced it at 10,000. At 7000, however, it very often, in 

 April, obscures the snowy ranges 30 miles off, which are bright 

 and defined at sunrise, and either pale away or become of a lurid 

 yellow-red, according to the density of this haze, as they disappear 

 at 10 A.M. I believe it always accompanies a S.W. wind and 

 dry atmosphere in Sikkim. 



Nocturnal radiation. — This is even a more difficult pheno- 

 menon for the traveller to estimate than solar radiation, the 

 dano-er of exposing instruments at night being always great in 

 wild countries. I have used the parabolic reflector and white 

 cotton most frequently, and find no material difference in the 

 means of many observations of each, though often 1° to 2° in 

 individual ones. Avoiding radiation from surrounding objects 

 is very difficult, especially in wooded countries. I have also 

 tried the radiating power of grass and the earth ; the latter 

 generally is lower, the former higher than the thermometer ex- 

 posed on cotton or in the reflector, but much depends on the 

 surface of the herbage and soil. Snow radiates the most power- 

 fully of any substance I have tried ; in one instance, at 13,000 

 feet, in January, the thermometer on snow fell to 0*2°, which 

 was 10"8° below the temperature at the time, grass showing 67°, 

 and on another occasion to 1*2°, when the air at the time (before 

 sunrise) was 21*2°, and the difference 20°. I Jiave frequently 

 made this observation, and always with a similar result ; it may 



