[411] This is contrary to the conclusions of all meteorologists who have studied the climate of the Alps, and is entirely due to the local disturbances which I have so often dwelt upon, and principally to the unequal distribution of moisture in the loftier rearward regions, and the aridity of Tibet. Professor James Forbes states (Ed. Phil. Trans., v. xiv. p. 489):—1. That the decrement of temperature with altitude is most rapid in summer: this (as I shall hereafter show) is not the case in the Himalaya, chiefly because the warm south moist wind then prevails. 2. That the annual range of temperature diminishes with the elevation: this, too, is not the case in Sikkim, because of the barer surface and more cloudless skies of the rearward loftier regions. 3. That the diurnal range of temperature diminishes with the height: that this is not the cane follows from the same cause. 4. That radiation is least in winter: this is negatived by the influence of the summer rains.
| Altitude | Mean Shade | Mean Warmest Month | Mean Coldest Month | Mean Daily Range of Temperature | Rain-fall in inches | |
| 11,000 feet 15,000 feet 19,000 feet | 40·9 29·8 19·8 | 50·0 40·0 32·0 | 24·0 11·0 0·0 | 20·0 27·0 35·0 | 40·0 20·0 10·0 | 1°=320 feet 1°=350 feet 1°=400 feet |
Supposing the same formula to apply (which I exceedingly doubt) to heights above 19,000 feet, 2° would be the mean annual temperature of the summit of Kinchinjunga, altitude 28,178 feet, the loftiest known spot on the globe: this is a degree or two higher than the temperature of the poles of greatest cold on the earth’s surface, and about the temperature of Spitzbergen and Melville island.
The upper limit of phenogamic vegetation coincides with a mean temperature of 30° on the south flank of Kinchinjunga, and of 22° in Tibet; in both cases annuals and perennial-rooted herbaceous plants are to be found at elevations corresponding to these mean temperatures, and often at higher elevations in sheltered localities. I have assumed the decrease of temperature for a corresponding amount of elevation to be gradually less in ascending (1°=320 feet at 6000 to 10,000 feet, 1°=400 feet at 14,000 to 18,000 feet). My observations appear to prove this, but I do not regard them as conclusive; supposing them to 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 elevated above 16,000 feet, and consequent radiation of heat; also to the greater amount of sunshine there; and to the less dense mists which obstruct the sun’s rays at all elevations. In corroboration of this I may mention that the decrease of temperature with elevation is much less in summer than in winter, 1° of Fahr. being equivalent to only 250 feet in January between 7000 and 13,000 feet, and to upwards of 400 feet in July. Again, at Dorjiling (7,430 feet) the temperature 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 the winter nights especially, are disproportionately cold at great heights, the thermometer falling upwards of 40° below the Dorjiling temperature at an elevation only 6000 feet higher.
The diurnal distribution of temperature is equally and similarly affected by the presence of vapour at different altitudes. The lower and outer ranges of 6000 to 10,000 feet, first receive the diurnal charge of vapour-loaded southerly winds; those beyond them get more of the sun’s rays, and the rearward ones more still. Though the summer days of the northern localities are warmer than their elevation would indicate, the nights are not proportionally cold; for the light mist of 14,000 feet, which replaces the dense fog of 7000 feet, effectually obstructs nocturnal radiation, though it is less an obstacle to solar radiation. Clear nights, be it observed, are as rare at Momay (15,300 feet) as at Dorjiling, the nights if windy being rainy; or, if calm, cold currents descend from the mountains, condensing the moist vapours of the valleys, whose narrow floors are at sunrise bathed in mist at all elevations in Sikkim. The rise and dispersion of these dense mists, and their collection and recondensation 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 Calcutta, which has almost an insular climate, it amounts to 17°; at Delhi, which has a continental one, to 24·6°; and in London to 17·5°. At 11,000 feet it amounts to about 20°, and at 15,000 feet to 27°. These values vary widely in the different months, being much less in the summer or rainy months. The following is probably a fair approximation:—
At 7,000 feet it amounts to 8–9° in Aug. and Sept., and 17° in Dec.
At 11,000 feet it amounts to 12° in Aug. and Sept., and 30° in Dec.
At 15,000 feet it amounts to 15° in Aug. and Sept., and 40° in Dec.
At London it amounts to 20° in Aug. and Sept., and 10° in Dec.
The distribution of temperature throughout the day and year varies less at Dorjiling than in most mountainous countries, owing to the prevailing moisture, the effect of which is analogous to that of a circumambient ocean to an island: the difference being, that in the case of the island the bulk of water maintains an uniform temperature; in that of Dorjiling the quantity of vapour acts directly by interfering with terrestrial and solar-radiation, and indirectly by nurturing a luxuriant vegetation. The result in the latter case is a climate remarkable for its equability, and similar in many features to that of New Zealand, South-west Chili, Fuegia, and the damp west coasts of Scotland and Ireland, and other countries exposed to moist sea winds.
The mean temperature of the year at Dorjiling, as taken by maxima and minima thermometers[[412]] by Dr. Chapman, is nearly the same as that of March and October: January, the coldest month, is more than 13·4° colder than the mean of the year; but the hottest month is only 8·3° warmer than the same mean: at Calcutta the months vary less from the mean; at Delhi more; and in London the distribution is wholly different; there being no rains to modify the summer heat, July is 13° hotter, and January 14° colder than the mean of the year.
[412] The mean of several of the months, thus deduced, often varies a good deal from the truth, owing to the unequal diurnal distribution of heat; a very few minutes’ sunshine raises the temperature l0° or 15° above the mean of the day; which excessive heat (usually transient) the maximum thermometer registers, and consequently gives too high a mean.