Himalayan Journals, vol 2 - J. D. Hooker (bill gates best books txt) 📗

nights are more frequent, and the cold produced by radiation, at 14,000 feet, is often severe towards the end of the rains in September. Still the amount of clear weather during the night is small; the fog clears off for an hour or two at sunset as the wind falls, but the returning cold north current again chills the air soon afterwards, and rolling masses of vapour

are hence flying overhead, or sweeping the surface of the earth,

throughout the summer nights. In the Tibetan regions, on the other

hand, bright nights and even sharp frosts prevail throughout the

warmest months.

Referring again to the cut, it must be borne in mind that neither of the two meridional ridges runs in a straight line, but that they wind or zigzag as all mountain ranges do; that spurs from each ridge are given off from either flank alternately, and that the origin of a

spur on one side answers to the source of a river (i.e., the head of a valley) on the other. These rivers are feeders of the main

stream, the Teesta, and run at more or less of an angle to the

latter. The spurs from the east flank of one ridge cross, at their

ends, those from the west flank of another; and thus transverse

valleys are formed, presenting many modifications of climate with

regard to exposure, temperature, and humidity.

The roads from the plains of India to the watershed in Tibet always cross these lateral spurs. The main ridge is too winding and rugged, and too lofty for habitation throughout the greater part of its

length, while the river-channel is always very winding, unhealthy for the greater part of the year below 4000 feet, and often narrow,

gorge-like, and rocky. The villages are always placed above the

unhealthy regions, on the lateral spurs, which the traveller

repeatedly crosses throughout every day's march; for these spurs give off lesser ones, and these again others of a third degree, whence the country is cut up into as many spurs, ridges, and ranges, as there

are rills, streams, and rivers amongst the mountains.

Though the direction of the main atmospheric current is to the north, it is in reality seldom felt to be so, except the observer be on the very exposed mountain tops, or watch the motions of the upper strata of atmosphere. Lower currents of air rush up both the main and

lateral valleys, throughout the day; and from the sinuosities in the beds of the rivers, and the generally transverse directions of their feeders, the current often becomes an east or west one. In the branch valleys draining to the north the wind still ascends; it is, in

short, an ascending warm, moist current, whatever course be pursued by the valleys it follows.

The sides of each valley are hence equally supplied with moisture,

though local circumstances render the soil on one or the other flank more or less humid and favourable to a luxuriant vegetation: such

differences are a drier soil on the north side, with a too free

exposure to the sun at low elevations, where its rays, however

transient, rapidly dry the ground, and where the rains, though very heavy, are of shorter duration, and where, owing to the capacity of the heated air for retaining moisture, day fogs are comparatively

rare. In the northern parts of Sikkim, again, some of the lateral

valleys are so placed that the moist wind strikes the side facing the south, and keeps it very humid, whilst the returning cold current

from the neighbouring Tibetan mountains impinges against the side

facing the north, which is hence more bare of vegetation. An infinite number of local peculiarities will suggest themselves to any one

conversant with physical geography, as causing unequal local

distribution of light, heat, and moisture in the different valleys of so irregular a country; namely, the amount of slope, and its power of retaining moisture and soil; the composition and hardness of the

rocks; their dip and strike; the protection of some valleys by lofty snowed ridges; and the free southern exposures of others at

great elevations.

The position and elevation of the perpetual snow* [It appears to me, as I have asserted in the pages of my Journal, that the limit of

perpetual snow is laid down too low in all mountain regions, and that accumulations in hollows, and the descent of glacial ice, mask the

phenomenon more effectually than is generally allowed. In this work I define the limit, as is customary, in general terms only, as being

that where the accumulations are very great, and whence they are

continuous upwards, on gentle slopes. All perpetual snow, however,

becomes ice, and, as such, obeys the laws of glacial motion, moving as a viscous fluid; whence it follows that the lower edge of a

snow-bed placed on a slope is, in one sense, the termination of a

glacier, and indicates a position below that where all the snow that falls melts. I am well aware that it is impossible to define the

limit required with any approach to accuracy. Steep and broken

surfaces, with favourable exposures to the sun or moist winds, are

bare much above places where snow lies throughout the year; but the occurrence of a gentle slope, free of snow, and covered with plants, cannot but indicate a point below that of perpetual snow. Such is the case with the "Jardin" on the Mer de Glace, whose elevation is 9,500

feet, whereas that of perpetual snow is considered by Professor J.

Forbes, our best authority, to be 8,500 feet. Though limited in area, girdled by glaciers, presenting a very gentle slope to the east, and screened by surrounding mountains from a considerable proportion of the sun's rays, the Jardin is clear, for fully three months of the

year, of all but sporadic falls of snow, that never lie long; and so are similar spots placed higher on the neighbouring slopes; which

facts are quite at variance with the supposition that the perpetual snow-line is below that point in the Mont Blanc Alps. On the Monte

Rosa Alps, again, Dr. Thomson and I gathered plants in flower, above 12,000 feet on the steep face of the Weiss-thor Pass, and at 10,938

feet on the top of St. Theodule; but in the former case the rocks are too steep for any snow to lie, they are exposed to the south-east,

and overhang a gorge 8000 feet deep, up which no doubt warm currents ascend; while at St. Theodule the plants were growing on a slope

which, though gentle, is black and stony, and exposed to warm

ascending currents, as on the Weiss-thor; and I do not consider

either of these as evidences of the limit of perpetual snow being

higher than their position.] vary with those of the individual

ranges, and their exposure to the south wind. The expression that the perpetual snow lies lower and deeper on the southern slopes of the

Himalayan mountains than on the northern, conveys a false impression.

It is better to say that the snow lies deeper and lower on the

southern faces of the individual mountains and spurs that form the

snowy Himalaya. The axis itself of the chain is generally far north of the position of the spurs that catch all the snow, and has

comparatively very little snow on it, most of what there is lying

upon north exposures.

A reference to the woodcut will show that the same circumstances

which affect the distribution of moisture and vegetation, determine the position, amount, and duration of the snow. The principal fall

will occur, as before shown, where the meridional range first attains a sufficiently great elevation, and the air becomes consequently

cooled below 32 degrees; this is at a little above 14,000 feet,

sporadic falls occurring even in summer at that elevation: these,

however, melt immediately, and the copious winter falls also are

dissipated before June. As the depth of rain-fall diminishes in

advancing north to the higher parts of the meridional ranges, so does that of the snow-fall. The permanence of the snow, again, depends

on--1. The depth of the accumulation; 2. The mean temperature of the spot; 3. The melting power of the sun's rays; 4. The prevalence and strength of evaporating winds. Now at 14,000 feet, though the

accumulation is immense, the amount melted by the sun's rays is

trifling, and there are no evaporating winds; but the mean

temperature is so high, and the corroding powers of the rain (which falls abundantly throughout summer) and of the warm and humid

ascending currents are so great, that the snow is not perennial.

At 15,500 feet, again, it becomes perennial, and its permanence at

this low elevation (at P) is much favoured by the accumulation and

detention of fogs over the rank vegetation which prevails from S

nearly to P; and by the lofty mountains beyond it, which shield it

from the returning dry currents from the north. In proceeding north all the circumstances that tend to the dispersion of the snow

increase, whilst the fall diminishes. At P the deposition is enormous and the snow-line low--16,000 feet; whilst at T little falls, and the limit of perpetual snow is 19,000 and 20,000 feet. Hence the anomaly, that the snow-line ascends in advancing north to the coldest

Himalayan regions. The position of the greatest peaks and of the

greatest mass of perpetual snow being generally assumed as indicating a ridge and watershed, travellers, arguing from single mountains

alone, on the meridional ridges, have at one time supported and at

another denied the assertion, that the snow lies longer and deeper on the north than on the south slope of the Himalayan ridge.

The great accumulation of snow at 15,000 feet, in the parallel of P, exercises a decided influence on the vegetation. The alpine

rhododendrons hardly reach 14,000 feet in the broad valleys and

round-headed spurs of the mountains of the Tunkra and Chola passes; whilst the same species ascend to 16,000, and one to 17,000 feet, at T. Beyond the latter point, again, the great aridity of the climate prevents their growth, and in Tibet there are generally none even as low as 12,000 and 14,000 feet. Glaciers, again, descend to 15,000

feet in the tortuous gorges which immediately debouch from the snows of Kinchinjunga, but no plants grow on the debris they carry down,

nor is there any sward of grass or herbage at their base, the

atmosphere immediately around being chilled by enormous accumulations of snow, and the summer sun rarely warming the soil. At T, again, the glaciers do not descend below 16,000 feet, but a greensward of

vegetation creeps up to their bases, dwarf rhododendrons cover the

moraines, and herbs grow on the patches of earth carried down by the latter, which are thawed by the more frequent sunshine, and by the

radiation of heat from the unsnowed flanks of the valleys down which these ice-streams pour.

Looking eastward or westward on the map of India, we perceive that

the phenomenon of perpetual snow is regulated by the same laws.

From the longitude of Upper Assam in 95 degrees E to that of Kashmir in 75 degrees E, the lowest limit of perpetual snow is 15,500 to

16,000 feet, and a shrubby vegetation affects the most humid

localities near it, at 12,000 to 14,000 feet. Receding from the

plains of India and penetrating the mountains, the climate becomes

drier, the snowline rises, and vegetation diminishes, whether the

elevation of the land increases or decreases; plants reaching 17,000

and 18,000 feet, and the snow-line, 20,000 feet. To mention extreme cases; the snow-level of Sikkim in 27 degrees 30 minutes is at 16,000

feet, whereas in latitude 35 degrees 30 minutes Dr. Thomson found the snow line 20,000 feet on the mountains near the Karakoram Pass, and vegetation up to 18,500 feet--features I found to be common also to Sikkim