WINDS.
The whole of Western Tibet is subject to extremely violent winds, the course and direction of which could only be satisfactorily studied by a resident. From the great depth of the valleys, the wind in general follows their course, blowing at one time up them, at other times down. In unsettled weather the direction is extremely variable, often changing repeatedly in the course of the day, but in clear settled weather the direction of the wind is, during the day at least, more frequently up the valleys than in the contrary direction. I have not observed any constancy in the course of the wind on the passes, on which it would be principally important to be acquainted with it, but it probably varies in direction according to the period of the day, so that a traveller, whose time does not permit him to delay to register the changes as they occur, is not likely to be able to discover any general law.
The Tibetan wind, in the ordinary state of the atmosphere, commences after the sun has nearly attained the meridian, the mornings being in general quite calm. It increases in violence during the afternoon, sometimes till after sunset, ceasing to blow after dark, or at all events before midnight. This wind seems to be pretty constant over the whole country, from the upper Sutlej as far west as Rondu; and as a very similar wind blows in the valleys of Affghanistan, which have an identical summer climate in respect of moisture, it must, I presume, be caused by the influence of the sun, in heating the barren rocky plains and hills.
During periods of cloud, and throughout the winter, the wind is much less regular in its direction, as well as in the periods during which it blows. It frequently changes its direction very abruptly. About the equinoxes, or at the commencement and end of winter, at which times there seems to be generally a good deal of unsettled weather, it blows for some days with extreme violence. In March, 1848, at Iskardo, for several nights the wind almost amounted to a hurricane; its direction was from the south, or directly across the mountains. This was very commonly the case at Iskardo, in unsettled weather, during the winter, but never when the days were bright and cloudless.
SNOW-FALL.
The amount of snow-fall varies much, diminishing as we advance into the interior of the country, but being always much greater on the mountains than in the valleys at their feet. In the outer Himalaya, the amount at equal distances from the plains diminishes as we advance westward, but in the Kouenlun, where the source of moisture lies to the westward, the snow-fall diminishes rapidly from west to east. The same is the case in the valley of the Indus, where the amount of winter's snow, except in the most westerly parts, is quite insignificant.
It is probably owing to the absence of cumular clouds, and to the general uniform expansion of the condensed vapours over the whole sky, that the outward manifestations of electricity—thunderstorms—are of very rare occurrence in Tibet. I find only one instance of a thunderstorm recorded as having been observed while I was in a Tibetan climate. This was at Le, in September, 1848, at which time there was a good deal of cloudy weather for several days. From the extreme dryness of the air, electricity is evolved with great facility by friction: all articles of woollen clothing, blankets, and even the hair, emit sparks when rubbed in the dark. I have even observed this to be the case at the elevation of 15,500 feet, in cloudy weather, when snow was falling.
GLACIERS.
In every part of the Himalaya, and of Western Tibet, wherever the mountains attain a sufficient elevation to be covered with perpetual snow, glaciers are to be found. The occurrence of glacial ice is a sufficient indication of the existence of snow of more than one year's duration, and (setting aside trifling cases of masses of ice in deep and sunless ravines, which, indeed, are not an exception, as they have no motion,) it may be laid down as a general law, that every glacier has its origin in perpetual snow.