Throughout the greater part of the year the prevailing wind is from the south-east, and comes laden with moisture from the Bay of Bengal: it rises at sunrise, and its vapours are early condensed on the forests of Sinchul; billowy clouds rapidly succeed small patches of vapour, which rolling over to the north side of the mountain, are carried north-west, over a broad intervening valley, to Dorjiling. There they bank on the east side of the spur, and this being partially clear of wood, the accumulation is slow, and always first upon the clumps of trees. Very generally by 9 a.m., the whole eastern sky, from the top of Dorjiling ridge, is enveloped in a dense fog, while the whole western exposure enjoys sunshine for an hour or two later. At 7 or 8 a.m., very small patches are seen to collect on Tonglo, which gradually dilate and coalesce, but do not shroud the mountain for some hours, generally not before 11 a.m. or noon. Before that time, however, masses of mist have been rolling over Dorjiling ridge to the westward, and gradually filling up the valleys, so that by noon, or 1 p.m., every object is in cloud. Towards sunset it falls calm, when the mist rises, first from Sinchul, or if a south-east wind sets in, from Tonglo first.
The temperature is more uuiform at Mr. Hodgson's bungalow, which is on the top of the Dorjiling ridge, than on either of its flanks; this is very much because a good deal of wood is left upon it, whose cool foliage attracts and condenses the mists. Its mean temperature is lower by nearly 22 degrees than that of Mr. Muller's and Dr. Campbell's houses, both situated on the slopes, 400 feet below. This I ascertained by numerous comparative observations of the temperature of the air, and by burying thermometers in the earth it is chiefly to be accounted for by the more frequent sunshine at the lower stations, the power of the sun often raising the thermometer in shade to 80 degrees, at Mr. Muller's; whereas during the summer I spent at Mr. Hodgson's it never rose much above 70 degrees, attaining that height very seldom and for a very short period only. The nights, again, are uniformly and equally cloudy at both stations, so that there is no corresponding cold of nocturnal radiation to reduce the temperature.
The mean decrease of temperature due to elevation, I have stated (Appendix I.) to be about 1 degree for every 300 feet of ascent; according to which law Mr. Hodgson's should not be more than 1.5 degrees° colder than Mr. Muller's. These facts prove how difficult it is to choose unexceptionable sites for meteorological observatories in mountainous countries; discrepancies of so great an amount being due to local causes, which, as in this case, are perhaps transient; for should the top of the spur be wholly cleared of timber, its temperature would be materially raised; at the expense, probably, of a deficiency of water at certain seasons. Great inequalities of temperature are also produced by ascending currents of heated air from the Great Rungeet valley, which affect certain parts of the station only; and these raise the thermometer 10 degrees (even when the sun is clouded) above what it indicates at other places of equal elevation.
The mean temperature of Dorjiling (elev. 7,430 feet) is very nearly 50 degrees, or 2 degrees higher than that of London, and 26 degrees below that of Calcutta (78 degrees,* [Prinsep, in As. Soc. Journ., Jan. 1832, p. 30.] or 78.5 degrees in the latest published tables* [Daniell's Met. Essays, vol. ii. p. 341.]); which, allowing 1 degree of diminution of temperature for every degree of latitude leaves 1 degree due to every 300 feet of ascent above Calcutta to the height of Dorjiling, agreeably to my own observations. This diminution is not the same for greater heights, as I shall have occasion to show in a separate chapter of this Appendix, on the decrement of heat with elevation.
A remarkable uniformity of temperature prevails throughout the year at Dorjiling, there being only 22 degrees difference between the mean temperatures of the hottest and coldest months; whilst in London, with a lower mean temperature, the equivalent difference is 27 degrees. At 11,000 feet this difference is equal to that of London. In more elevated regions, it is still greater, the climate becoming excessive at 15,000 feet, where the difference amounts to 30 degrees at least.* [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.] The accompanying table is the result of an attempt to approximate to the mean temperatures and ranges of the thermometer at various elevations.
Altitude 11,000 feet 15,000 feet 19,000 feet
Mean shade 40.9 29.8 19.8
Mean warmest month 50.0 40.0 32.0
Mean coldest month 24.0 11.0 0.0
Mean daily range
of temperature 20.0 27.0 35.0
Rain-fall in inches 40.0 20.0 10.0
1 degree equals 320 feet 350 feet 400 feet
Supposing the same formula to apply (which I exceedingly doubt) to heights above 19,000 feet, 2 degrees 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 degrees on the south flank of Kinchinjunga, and of 22 degrees 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 degree=320 feet at 6000 to 10,000 feet, 1 degree=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 degree 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 degrees 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 degrees 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 degrees 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 degrees; at Delhi, which has a continental one, to 24.6 degrees; and in London to 17.5 degrees. At 11,000 feet it amounts to about 20 degrees, and at 15,000 feet to 27 degrees. These values vary widely in the different months, being much less in the summer or rainy months. The following is probably a fair approximation:—