The climate of Tibet is in every part extremely arid, because it is surrounded almost entirely by ranges of mountains so elevated that the rarefied air which passes over them can contain only a very small proportion of aqueous vapour. Along the Indus, indeed, no mountain chains are interposed to obstruct the passage of moist air, but the lower course of that river lies entirely in a comparatively dry climate, so that the winds which blow over the plains of Sind and the lower mountains of Eastern Affghanistan cannot convey any excess of moisture to lower Tibet. In the few Tibetan valleys which, like that of the Sutlej, are traversed by rivers debouching on the plains of India in a rainy climate, the quantity of moist air which they can receive being limited to that which proceeds directly up the valley, the upward current, even when saturated with moisture at the commencement, being gradually rarefied by the increasing elevation of the river-bed, and meeting with descending currents of cold air in its course, it very early deposits its moisture, first in the form of light showers, afterwards of fog and mist, and in its further progress is just as dry as the air in the more interior parts of the country.
RAIN-FALL.
It will probably be long before lengthened registers of meteorological phenomena will be obtained from all the different stages between India and the central parts of Tibet, so as satisfactorily to establish the gradual transition of climate. Till such shall be the case, the best evidence from which to deduce the fact of the alteration of climate, is afforded by the gradual change in the vegetation of the country as one advances towards the interior. Direct observation will probably at some future period fix the point in the outer Himalaya, at which the quantity of rain—always greater, cæteris paribus, among mountains than in level countries—is a maximum. I believe that in the Western Himalaya the greatest quantity of rain will be found to fall on mountains elevated from seven to nine thousand feet. Ranges of mountains which attain an elevation of from ten to eleven thousand feet have already (in the Western Himalaya) a very sensible effect in diminishing the quantity of moisture, as indicated by the vegetation; and when the mountain chains became sufficiently elevated to be capped by perpetual snow, they condense a very great proportion of the moisture of the air-currents which pass over them.
To a traveller who penetrates directly to the Tibetan interior from the plains of India, the change of climate is perceptible to the senses; most markedly so of course if his journey occurs during the Indian rainy season. Even during the rains, however, the irregularities which everywhere occur in the fall of rain prevent the gradations of climate from being ascertained during a journey with the precision which a lengthened series of observations would permit; but the phenomena of vegetable life, which are dependent on the average seasons, are not affected by accidental irregularities, and therefore form an unerring guide.
CLOUDS.
Though the climate of the whole of Western Tibet may, in general, be characterized as extremely dry, it is by no means cloudless. The winter months in particular are often very cloudy, and a good deal of snow falls. During the summer the sky is either bright and clear, or overcast with very light clouds. These clouds, usually cirrhi, are in general elevated and extremely thin. The cirrhus, when it remains for any length of time, changes or increases into a uniform hazy stratum, which covers the whole sky; more rarely, and perhaps only by an optical deception, it is seen under the form of stratus. Cumuli are very uncommon. After several dull days the clouds generally accumulate, descend lower in the atmosphere, and rest on the mountain; as a few drops of rain fall in the valleys, the clouds disappear, and the highest peaks are seen to have received a slight sprinkling of snow, which is soon melted by the rays of the sun. It is only very rarely that the quantity of rain exceeds a few drops, or merits the appellation of a shower. The few occasions on which I have observed any fall of rain, at all deserving of being called by that name, have mostly been in early spring or in the latter part of autumn.
TEMPERATURE.
When the sky is clear, the sun, in all parts of Tibet, even at great elevations, but especially in the valleys at and below ten and eleven thousand feet, is extremely powerful. The shade temperature depends, of course, in a great measure on the elevation above the level of the sea, but also on the situation, exposure, and many other accidental circumstances. In the lower part of the Indus valley, at elevations of seven and eight thousand feet, it is said to be frequently very high[36], the clear dry atmosphere allowing the full influence of the sun to be exerted on the bare, often black rocks. Even as far up as 11,000 feet, in narrow valleys, the heat is often great in the middle of the day, but the more open plains are generally very temperate in the shade, and the nights and mornings are always cool.
On the tops of the lower passes, and in the alpine valleys, the temperature of the nights and mornings is, in clear weather, very much depressed by radiation, so that the mornings, except when the sky is overcast, are intensely frosty, at elevations of 15,000 and 16,000 feet, or far below the level of perpetual snow. This is the case even in the month of August, which is the hottest of the year. The shade temperature at these high elevations rarely rises very high, even when the heat of the sun is oppressive, as it is moderated by the action of the violent winds which so generally prevail.
The periods of cloudy sky, which now and then alternate with the bright sunshine, which is the prevailing weather, are in the alpine regions extremely cold. The stratum of cloud, at first high in the atmosphere, gradually lowers itself, and the traveller is enveloped in a frozen mist, followed most commonly during the night by a fall of snow. The quantity of snow which falls is very small, seldom, so far as I have seen, more than an inch or two in depth, and it speedily disappears as soon as the clouds have been dissipated and the sky resumes its usual serenity.