Himalayan Journals, vol 2 by J. D. Hooker (android pdf ebook reader TXT) 📕

APPENDIX F.

ON THE CLIMATE OF SIKKIM.

The meteorology of Sikkim, as of every part of the Himalayan range, is a subject of growing interest and importance; as it becomes yearly more necessary for the Government to afford increased facilities for a residence in the mountains to Europeans in search of health, or of a salubrious climate for their families, or for themselves on

retirement from the exhausting service of the plains. I was therefore surprised to find no further register of the weather at Dorjiling,

than an insufficient one of the rain-fall, kept by the medical

officer in charge of the station; who, in this, as in all similar

cases,* [The government of India has gone to an immense expense, and entailed a heavy duty upon its stationary medical officers, in

supplying them with sometimes admirable, but more often very

inaccurate, meteorological instruments, and requiring that daily

registers be made, and transmitted to Calcutta. In no case have I

found it to be in the officer's power to carry out this object; he

has never time, seldom the necessary knowledge and experience, and

far too often no inclination. The majority of the observations are in most cases left to personal native or other servants, and the

laborious results I have examined are too frequently worthless.] has neither the time nor the opportunity to give even the minimum of

required attention to the subject of meteorology. This defect has

been in a measure remedied by Dr. Chapman, who kept a twelve-months'

register in 1837, with instruments carefully compared with Calcutta standards by the late James Prinsep, Esq., one of the most

accomplished men in literature and science that India ever saw.

The annual means of temperature, rain-fall, etc., vary greatly in the Himalaya; and apparently slight local causes produce such great

differences of temperature and humidity, that one year's observations taken at one spot, however full and accurate they may be, are

insufficient: this is remarkably the case in Sikkim, where the

rainfall is great, and where the difference between those of two

consecutive years is often greater than the whole annual London fall.

My own meteorological observations necessarily form but a broken

series, but they were made with the best instruments, and with a view to obtaining results that should be comparable inter se, and with those of Calcutta; when away from Dorjiling too, in the interior of Sikkim, I had the advantage of Mr. Muller's services in taking

observations at hours agreed upon previous to my leaving, and these were of the greatest importance, both for calculating elevations, and for ascertaining the differences of temperature, humidity, diurnal

atmospheric tide, and rain-fall; all of which vary with the

elevation, and the distance from the plains of India.

Mr. Hodgson's house proved a most favourable spot for an observatory, being placed on the top of the Dorjiling spur, with its broad

verandah facing the north, in which I protected the instruments from radiation* [This is a most important point, generally wholly

neglected in India, where I have usually seen the thermometer hung in good shade, but exposed to reflected heat from walls, gravel walks, or dry earth. I am accustomed from experience to view all extreme

temperatures with great suspicion, on this and other accounts. It is very seldom that the temperature of the free shaded air rises much

above 100 degrees, except during hot winds, when the lower stratum

only of atmosphere (often loaded with hot particles of sand), sweeps over the surface of a soil scorched by the direct rays of the sun.]

and wind. Broad grass-plots and a gravel walk surrounded the house, and large trees were scattered about; on three sides the ground

sloped away, while to the north the spur gently rose behind.

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