I.
ON THE DECREMENT OF TEMPERATURE IN ASCENDING THE SIKKIM HIMALAYA MOUNTAINS AND KHASIA MOUNTAINS
I have selected as many of my observations for temperature of the sir as appeared to be trustworthy, and which, also, were taken contemporaneously with others at Calcutta, and I have compared them with the Calcutta observations, in order to find the ratio of decrement of heat to an increase of elevation. The results of several sets of observations are grouped together, but show so great an amount of discrepancy, that it is evident that a long series of months and the selection of several stations are necessary in a mountain country to arrive at any accurate results. Even at the stations where the most numerous and the most trustworthy observations were recorded, the results of different months differ extremely; and with regard to the other stations, where few observations were taken, each one is affected differently from another at the same level with it, by the presence or proximity of forest, by exposure to the east or west, to ascending or descending currents in the valleys, and to cloud or sunshine. Other and still more important modifying influences are to be traced to the monthly variations in the amount of humidity in the air and the strength of its currents, to radiation, and to the evolution of heat which accompanies condensation raising the temperature of elevated regions during the rainy season. The proximity of large masses of snow has not the influence I should have expected in lowering the temperature of the surrounding atmosphere, partly no doubt because of the more rapid condensation of vapours which it effects, and partly because of the free circulation of the currents around it. The difference between the temperatures of adjacent grassy and naked or rocky spots, on the other hand, is very great indeed, the former soon becoming powerfully heated in lofty regions where the sun’s rays pass through a rarefied atmosphere, and the rocks especially radiating much of the heat thus accumulated, for long after sunset. In various parts of my journals I have alluded to other disturbing causes, which being all more or leas familiar to meteorologists, I need not recapitulate here. Their combined effects raise all the summer temperatures above what they should theoretically be.
In taking Calcutta as a standard of comparison, I have been guided by two circumstances; first, the necessity of selecting a spot where observations were regularly and accurately made; and secondly, the being able to satisfy myself by a comparison of my instruments that the results should be so far strictly comparable.
I have allowed 1° Fahr. for every degree in latitude intervening between Sikkim and Calcutta, as the probable ratio of diminution of temperature. So far as my observations made in east Bengal and in various parts of the Gangetic delta afford a means of solving this question, this is a near approximation to the truth. The spring observations however which I have made at the foot of the Sikkim Himalaya would indicate a much more rapid decrement; the mean temperature of Titalya and other parts of the plains south of the forests, between March and May being certainly 6°–9° lower than Calcutta: this period however is marked by north-west and north-east winds, and by a strong haze which prevents the sun’s rays from impinging on the soil with any effect. During the southerly winds, the same region is probably hotter than Calcutta, there being but scanty vegetation, and the rain-fall being moderate.
In the following observations solitary readings are always rejected.
I.—Summer or Rainy Season observations at Dorjiling.
Observations taken during the rainy season of 1848, at Mr. Hodgson’s (Jillapahar, Dorjiling) alt. 7,430 feet, exposure free to the north east and west, the slopes all round covered with heavy timber; much mist hence hangs over the station. The mean temperatures of the month at Jillapahar are deduced from horary observations, and those of Calcutta from the mean of the daily maximum and minimum.
| Month | No. of Obs. at Jillapahar | Temp. | Temp. Calcutta | Equiv. of 1° Fahr. |
| July August September October | 284 378 407 255 | 61·7 61·7 58·9 55·3 | 86·6 85·7 84·7 83·3 | 364 feet 346 feet 348 feet 316 feet |
| 1,324 | … | Mean | 344 feet |
II.—Winter or dry season observations at Dorjiling.
| 1. | Observations taken at Mr. J.
Muller’s, and chiefly by himself, at “the
Dale”; elev. 6,956 feet; a sheltered spot, with no forest near, and a free west exposure. 103 observations. Months: November, December, January, and February | 1°=313 ft. |
| 2. | Observations at Dr. Campbell’s
(Superintendent’s) house in April; elev. 6,950 feet; similar exposure to the last. 13 observations in April | 1°=308 ft. |
| 3. | Observations by Mr. Muller at
Colinton; elev. 7,179 feet; free exposure to north-west; much forest about the station, and a high ridge to east and south. 38 observations in winter months | 1°=290 ft. |
| 4. | Miscellaneous (11) observations at
Leebong; elev. 6000 feet; in February; free exposure all round | 1°=266 ft. |
| 5. | Miscellaneous observations at
“Smith’s Hotel;” Dorjiling, on a cleared
ridge; exposed all round; elev. 6,863 feet. April and May | 1°=252 ft. ———— |
| Mean of winter
observations Mean of summer observations Mean | 1°=286 ft. 1°=344 ft. ———— 310 ft. |