110 CLIMATE AND VEGETATION OF 



be SO, I attribute it to a combination of various causes, especially 

 to the increased elevation and yet unsnowed condition of the mass 

 of land elev-ated above 16,000 feet, as is shown in the woodcut 

 (p. 85) ; also to the greater amount of sunshine there, and to 

 the lesser density and height of the fogs wiiich obstruct the 

 sun's rays at all elevations. In corroboration of this I may 

 mention that the difference of temperature is much less in sum- 

 mer than in winter, 1° of Fahr. being equivalent to only 2oO 

 feet in January between 7000 and 13,000 feet, and to upwards 

 of 400 feet in July. Again, at Darjiling (7500 feet) tlie tem- 

 perature hardly ever rises above 70° in the summer months, yet 

 it often rises even higher in Tibet at 12,000 to 14,000 feet. 

 On the other hand, the winters, and winter nights especially, 

 are disproportionately cold, the thermometer falling upwards 

 of 40° below the Darjiling temperature at 6000 feet above that 

 elevation. 



The diurnal distribution of temperature is equally and simi- 

 larly affected by the presence of vapour in the tliree different 

 zones. The lower zone is first clouded, because the lower ranges, 

 of 6000 to 10,000 feet, first receive the diurnal charge of vapour- 

 loaded southerly winds ; the middle gets more of the sun's rays, 

 and the upper more still. Though the summer days of the upper 

 zone are warmer than their elevation would indicate, tlie niglits 

 are not proportionally colder ; for the light mist of 14,000 

 feet, which replaces the dense fog of 7000 feet, effectually ob- 

 structs nocturnal radiation, though it is less an obstacle to solar 

 radiation. Clear nights, be it observed, are as rare at 14,000 

 as at 7000 feet, the nights being rainy, if windy ; or, if calm, 

 cold currents descend from the mountains, condensing the 

 moist vapours of the valleys, whose narrow floors are at sun- 

 rise bathed in mist at all elevations in Sikkim. The rise and 

 dispersion of these dense masses, and their collection and recun- 

 densation on the mountains in the morning, is one of the most 

 magnificent phenomena of the Himalaya, when viewed from a 

 proper elevation ; it commences as soon as the sun appears on 

 the horizon. 



The mean daily range of the thermometer at 7000 feet is 13° 

 in cleared spots, but considerably less in wooded, and certainly 

 one-third less in the forest itself. At 11,000 feet it amounts to 

 about 20°, and at 15,000 feet to 27° (London 17-5°). These 

 values vary widely in the different months, being much less in 

 the summer or rainy months. 



At 7,000 feet it amounts to 8°-9° in Aug. and Sept., and 17° in Dec. 

 11,000 ,, 12° ,, ,, 30 



15,000 ,, 15 ,, ,, 40 ,, 



London ,, 20 ,, ,, 10 



