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

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:--

At 7,000 feet it amounts to 8-9 degrees in Aug. and Sept., and 17

degrees in Dec. At 11,000 feet it amounts to 12 degrees in Aug. and Sept., and 30 degrees in Dec. At 15,000 feet it amounts to 15 degrees in Aug. and Sept., and 40 degrees in Dec. At London it amounts to 20

degrees in Aug. and Sept., and 10 degrees 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* [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 degrees or 15 degrees above the mean of the day; which excessive heat (usually transient) the maximum thermometer registers, and consequently gives too high a mean.] by Dr. Chapman, is nearly the same as that of March and October: January, the coldest month, is more than 13.4 degrees colder than the mean of the year;

but the hottest month is only 8.3 degrees 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 degrees hotter, and January 14

degrees colder than the mean of the year.

This distribution of the seasons has a most important effect upon

vegetation, to which sufficient attention has not been paid by

cultivators of alpine Indian plants; in the first place, though

English winters are cold enough for such, the summers are too hot and dry; and, in the second place, the great accession of temperature,

causing the buds to burst in spring, occurs in the Himalaya in March, when the temperature at 7000 feet rises 8 degrees above that of

February, raising the radiating thermometer always above the freezing point, whence the young leaves are never injured by night frost: in England the corresponding rise is only 3 degrees, and there is no

such accession of temperature till May, which is 8 degrees warmer

than April; hence, the young foliage of many Himalayan plants is cut off by night frosts in English gardens early in the season, of which Abies Webbiana is a conspicuous example.

The greatest heat of the day occurs at Dorjiling about noon, owing to the prevalent cloud, especially during the rainy months, when the sun shines only in the mornings, if at all, and the clouds accumulate as the day advances. According to hourly observations of my own, it

occurred in July at noon, in August at 1 p.m., and in September (the most rainy month) there was only four-tenths of a degree difference between the means of noon, 1 p.m., and 2 p.m., but I must refer to

the abstracts at the end of this chapter for evidence of this, and of the wonderful uniformity of temperature during the rainy months.

In the drier season again, after September, the greatest heat occurs between 2 and 3 p.m.; in Calcutta the hottest hour is about 2.45

p.m., throughout the year; and in England also about 3 p.m.

The hour whose temperature coincides with the mean of the day

necessarily varies with the distribution of cloud and sunshine; it is usually about 7 a.m. and 7 p.m.; whereas in Calcutta the same

coincidence occurs at a little before 10 a.m., and in England at

about 8 a.m.

Next to the temperature of the air, observations on that of the earth are perhaps of the greatest value; both from their application to

horticulture, and from the approximation they afford to the mean

temperature of the week or month in which they are taken. These form the subject of a separate chapter.

Nocturnal and solar radiation, the one causing the formation of dew and hoar-frost when the air in the shade is above freezing, end

killing plants by the rapid abstraction of heat from all their

surfaces which are exposed to the clear sky, and the other scorching the skin and tender plants during the day, are now familiar

phenomena, and particularly engaged my attention during my whole

Indian journey. Two phenomena particularly obstruct radiation in

Sikkim--the clouds and fog from the end of May till October, and the haze from February till May. Two months alone are usually clear; one before and one after the rains, when the air, though still humid, is transparent. The haze has never been fully explained, though a

well-known phenomenon. On the plains of India, at the foot of the

hills, it begins generally in the forenoon of the cold season, with the rise of the west wind; and, in February especially, obscures the sun's disc by noon; frequently it lasts throughout the twenty-four

hours, and is usually accompanied by great dryness of the atmosphere.

It gradually diminishes in ascending, and have never experienced it at 10,000 feet; at 7000, however, it very often, in April, obscures the snowy ranges 30 miles off, which are bright and defined at

sunrise, and either pale away, or become of a lurid yellow-red,

according to the density of this haze, till they disappear at 10 a.m.

I believe it always accompanies a south-west wind (which is a

deflected current of the north-west) and dry atmosphere in Sikkim.

The observations for solar radiation were taken with a black-bulb

thermometer, and also with actinometers, but the value of the data

afforded by the latter not being fixed or comparative, I shall give the results in a separate section. (See Appendix K.) From a multitude of desultory observations, I conclude that at 7,400 feet, 125.7

degrees, or + 67 degrees above the temperature of the air, is the

average maximum effect of the sun's rays on a black-bulb thermometer*

[From the mean of very many observations, I find that 10 degrees is the average difference at the level of the sea, in India, between two similar thermometers, with spherical bulbs (half-inch diam.), the one of black, and the other of plain glass, and both being equally

exposed to the sun's rays.] throughout the year, amounting rarely to + 70 degrees and + 80 degrees in the summer months, but more

frequently in the winter or spring. These results, though greatly

above what are obtained at Calcutta, are not much, if at all, above what may be observed on the plains of India. This effect is much

increased with the elevation. At 10,000 feet in December, at 9 a.m., I saw the mercury mount to 132 degrees with a difl: of + 94 degrees, whilst the temperature of shaded snow hard by was 22 degrees; at

13,100 feet, in January, at 9 a.m., it has stood at 98 degrees, diff.

degrees. This is but a small selection from many instances of the

extraordinary power of solar radiation in the coldest months, at

great elevations.

Nocturnal and terrestrial radiation are even more difficult phenomena for the traveller to estimate than solar radiation, the danger of

exposing instruments at night being always great in wild countries.

I most frequently used a thermometer graduated on the glass, and

placed in the focus of a parabolic reflector, and a similar one laid upon white cotton,* [Snow radiates the most powerfully of any

substance I have tried; in one instance, at 13,000 feet, in January, the thermometer on snow fell to 0.2 degree, which was 10.8 degrees

below the temperature at the time, the grass showing 6.7 degrees; and on another occasion to l.2 degrees, when the air at the time (before sunrise) was 21.2 degrees; the difference therefore being 20 degrees.

I have frequently made this observation, and always with a similar

result; it may account for the great injury plants sustain from a

thin covering of ice on their foliage, even when the temperature is but little below the freezing-point.] and found no material

difference in the mean of many observations of each, though often 1

degree to 2 degrees in individual ones. Avoiding radiation from

surrounding objects is very difficult, especially in wooded

countries. I have also tried the radiating power of grass and the

earth; the temperature of the latter is generally less, and that of the former greater, than the thermometer exposed on cotton or in the reflector, but much depends on the surface of the herbage