Himalayan Journals, vol 2, J. D. Hooker [small books to read txt] 📗

154

Considering how desultory the observations in Sikkim are, and how

much affected by local circumstances, the above results must be

considered highly satisfactory: they prove that the relative humidity of the atmospheric column remains pretty constant throughout all

elevations, except when these are in a Tibetan climate; and when

above 18,000 feet, elevations which I attained in fine weather only.

Up to 12,000 feet this constant humidity is very marked; the

observations made at greater elevations were almost invariably to the north, or leeward of the great snowy peaks, and consequently in a

drier climate; and there it will be seen that these proportions are occasionally inverted; and in Tibet itself a degree of relative

dryness is encountered, such as is never equalled on the plains of

Eastern Bengal or the Gangetic delta. Whether an isolated peak rising near Calcutta, to the elevation of 19,OOQ feet, would present similar results to the above, is not proven by these observations, but as the relative humidity is the same at all elevations on the outermost

ranges of Sikkim, which attain 10,000 feet, and as these rise from

the plains like steep islands out of the ocean, it may be presumed

that the effects of elevation would be the same in both cases.

The first effect of this humid wind is to clothe Sikkim with forests, that make it moister still; and however difficult it is to separate cause from effect in such cases as those of the reciprocal action of humidity on vegetation, and vegetation on humidity, it is necessary for the observer to consider the one as the effect of the other.

There is no doubt that but for the humidity of the region, the Sikkim Himalaya would not present the uniform clothing of forest that it

does; and, on the other hand, that but for this vegetation, the

relative humidity would not be so great.* [Balloon ascents and

observations on small mountainous islands, therefore, offer the best means of solving such questions: of these, the results of ballooning, under Mr. Welsh's intrepid and skilful pioneering (see Phil. Trans.

for 1853), have proved most satisfactory; though, from the time for observation being short, and from the interference of belts of

vapour, some anomalies have not been eliminated. Islands again are

still more exposed to local influences, which may be easily

eliminated in a long series of observations. I think that were two

islands, as different in their physical characters as St. Helena and Ascension, selected for comparative observations, at various

elevations, the laws that regulate the distribution of humidity in

the upper regions might be deduced without difficulty. They are

advantageous sites, from differing remarkably in their humidity.

Owing partly to the indestructible nature of its component rock (a

glassy basalt), the lower parts of Ascension have never yielded to

the corroding effects of the moist sea air which surrounds it; which has decomposed the upper part into a deep bed of clay. Hence

Ascension does not support a native tree, or even shrub, two feet

high. St. Helena, on the other hand, which can hardly be considered more favourably situated for humidity, was clothed with a redundant vegetation when discovered, and trees and tree-ferns (types of

humidity) still spread over its loftiest summits. Here the humidity, vegetation, and mineral and mechanical composition reciprocate their influences.]

The great amount of relative humidity registered at 6000 to 8000

feet, arises from most of the observations having been made on the

outer range, where the atmosphere is surcharged. The majority of

those at 10,000 to 12,000 feet, which also give a disproportionate

amount of humidity, were registered at the Zemu and Thlonok rivers, where the narrowness of the valleys, the proximity of great snowy

peaks, and the rank luxuriance of the vegetation, all favour a

humid atmosphere.

I would have added the relative rain-fall to the above, but this is so very local a phenomenon, and my observations were so repeatedly

deranged by having to camp in forests, and by local obstacles of all kinds, that I have suppressed them; their general results I have

given in Appendix F.

I here add a few observations, taken on the plains at the foot of the Sikkim Himalaya during the spring months.

_Comparison between Temperature and Humidity of the Sikkim Terai

and Calcutta, in March and April, 1849._

Elev.

No. above TEMP. D.P. TENSION SAT.

of sea.

Obs. Locality Feet C. T. C. T. C. T. C. T.

4 Rummai 293 82.2 70.6 61.7 60.5 .553 .532 .517 .717

4 Belakoba 368 92.8 85.5 62.6 63.0 .570 .578 .382 .485

3 Rangamally 275 84.2 75.0 68.7 62.5 .695 .568 .605 .665

3 Bhojepore 404 90.1 81.2 54.1 44.3 .429 .308 .313 .295

4 Thakyagunj 284 84.9 77.1 61.3 60.8 .547 .537 .466 .588

3 Bhatgong 225 87.4 74.9 64.7 54.6 .611 .436 .480 .512

2 Sahigunj 231 80.2 68.0 66.2 53.1 .642 .414 .635 .409

8 Titalya 362 85.5 80.0 55.4 56.1 .448 .459 .376 .459

31 Means 305 85.9 79.0 61.8 56.9 .562 .479 .472 .516

May, 1850 ) 131 89.7 K78.6 76.7 K71.4 .904 K.759 .665 K.793 Kishengunj)

Vapour in a cubic foot--Kishengunj 8.20 Terai 5.08

Calcutta 9.52 Calcutta 5.90

Mean difference of temperature between Terai and Calcutta, from

31 observations in March, as above,

excluding minima Terai--6.9

Mean difference from 26 observations in March,

including minima Terai--9.7

Mean difference of temperature at Siligore on May 1, 1850-- 10.9

Mean difference of temperature at Kishengunj on May 1, 1850-- 11.1

From the above, it appears that during the spring months, and before the rains commence, the belt of sandy and grassy land along the

Himalaya, though only 3.5 degrees north of Calcutta, is at least 6

degrees or 7 degrees colder, and always more humid relatively, though there is absolutely less moisture suspended in the air. After the

rains commence; I believe that this is in a great measure inverted, the plains becoming excessively heated, and the temperature being

higher than at Calcutta. This indeed follows from the well known fact that the summer heat increases greatly in advancing north-west from the Bay of Bengal to the trans-Sutledge regions; it is admirably

expressed in the maps of Dove's great work "On the Distribution of

Heat on the Surface of the Globe."

APPENDIX H.

ON THE TEMPERATURE OF THE SOIL AT VARIOUS ELEVATIONS.

These observations were taken by burying a brass tube two feet six

inches to three feet deep, in exposed soil, and sinking in it, by a string or tied to a slip of wood, a thermometer whose bulb was well padded with wool: this, after a few hours' rest, indicates the

temperature of the soil. Such a tube and thermometer I usually caused to be sunk wherever I halted, if even for one night, except during

the height of the rains, which are so heavy that they communicate to the earth a temperature considerably above that of the air.

The results proved that the temperature of the soil at Dorjiling

varies with that of the month, from 46 degrees to 62.2 degrees, but is hardly affected by the diurnal variation, except in extreme cases.

In summer, throughout the rains, May to October, the temperature is that of the month, which is imparted by the rain to the depth of

eleven feet during heavy continued falls (of six to twelve inches a day), on which occasions I have seen the buried thermometer

indicating a temperature above the mean of the month. Again, in the winter months, December and January, it stands 5 degrees above the

monthly mean; in November and February 4 degrees to 5 degrees; in

March a few degrees below the mean temperature of the month, and in October above it; April and May being sunny, it stands above their

mean; June to September a little below the mean temperature of

each respectively.

The temperature of the soil is affected by:--1. The exposure of the surface; 2. The nature of the soil; 3. Its permeability by rain, and the presence of underground springs; 4. The sun's declination;

The elevation above the sea, and consequently the heating power of the sun's rays: and 6, The amount of cloud and sunshine.

The appended observations, though taken at sixty-seven places, are

far from being sufficient to supply data for the exact estimation of the effects of the sun on the soil at any elevation or locality;

they, however, indicate with tolerable certainty the main features of this phenomenon, and these are in entire conformity with more ample series obtained elsewhere. The result, which at first sight appears the most anomalous, is, that the mean temperature of the soil, at two or three feet depth, is almost throughout the year in India above

that of the surrounding atmosphere. This has been also ascertained to be the case in England by several observers, and the carefully

conducted observations of Mr. Robert Thompson at the Horticultural

Society's Gardens at Chiswick, show that the temperature of the soil at that place is, on the mean of six years, at the depth of one foot, 1 degree above that of the air, and at two feet 1.5 degrees. During the winter months the soil is considerably (l degree to 3 degrees)

warmer than the air, and during summer the soil is a fraction of a

degree cooler than the air.

In India, the sun's declination being greater, these effects are much exaggerated, the soil on the plains being in winter sometimes 9

degrees hotter than the air; and at considerable elevations in the

Himalaya very much more than that; in summer also, the temperature of the soil seldom falls below that of the air, except where copious

rain-falls communicate a low temperature, or where forests interfere with the sun's rays.

At considerable elevations these effects are so greatly increased,

that it is extremely probable that at certain localities the mean

temperature of the soil may be even 10 degrees warmer than that of

the air; thus, at Jongri, elevation 13,194 feet, the soil in January was 34.5 degrees, or 19.2 degrees above the mean temperature of the month,