The quantity of rain which falls during the summer in the outer Himalaya has necessarily a very material influence on the sun's action during the time in which he has most power, and therefore on the mean temperature of the summer months, which at corresponding elevations, notwithstanding the northing of the chain as we advance from east to west, must be higher to the westward. In the interior or Tibetan portion of the Himalaya, this difference is not observed, the climate being the same, or nearly so, from east to west of the region under consideration.
WINTER, THE SEASON OF SNOW.
In the most western part of the Himalaya, in Kashmir and Balti, the winter's fall of snow commences about the beginning of December, and continues on the highest ranges nearly to the beginning of May. The supply of moisture from which the snow is condensed is evidently derived from the Indian seas, and I suppose principally from the south-west, that being the general direction from which I observed snow-storms to arrive at Iskardo. The fall of snow must therefore, equally with that of rain in the rainy season, be greatest in the outermost (snowy) ranges, and very much less in all those in the interior. In the lower parts of Tibet on the Indus the snow-fall during winter is very considerable, though during summer the climate is as dry as elsewhere in Tibet. This difference seems to be explained by the westerly point from which the winter's wind blows, and by the much greater moisture of the atmosphere at that season over Affghanistan and Sind, so that the south-west wind advances loaded with vapour up the valley of the Indus. The increase of elevation in the bed of that river of course causes all the excess of moisture to be deposited without penetrating to any great distance, so that the more eastern parts of the country are not affected by this cause.
The snowy season in the highest mountains is probably in every part of the range very much the same. On the low outer ranges, which do not attain the height of perpetual snow, it is gradually lessened in duration as the elevation diminishes, ceasing entirely, in average years, at about 4000 feet. When the winter is at an end, the influence of a powerful sun and gradually increasing temperature is at once brought to bear on the mass of snow which has fallen; on the inner ranges where the summer is dry, this action proceeds uninterruptedly till the commencement of the next winter, but on the outermost snowy ranges it is modified by the access of the rainy season.
MELTING OF SNOW IN SUMMER.
On the outer ranges of the Himalaya, the crests of which rise to between five and ten thousand feet, the powerful sun soon dissipates all snow. It is in the inner ranges, which rise nearly to the height of perpetual snow, and where the river-beds are from six to eight thousand feet above the level of the sea, that the snow remains for a great length of time. When the valleys are open, the plain on the banks of the stream becomes first of all bare of snow, then the banks which face the south, and lastly the northern slopes. It is not so, however, in the deep narrow valleys and ravines through which the Himalayan rivers generally flow. In these the bottom of the glen is so much sheltered from the sun that a dense mass of snow, the result of accumulation from the avalanches of the winter, remains for a very long time after both slopes are quite bare of snow. These snow-beds have nothing of the nature of a glacier in them, but are simply firm, hard snow. I have, in the month of June, descended along one of them from 13,000 feet (above which height there was perhaps a glacier beneath), to 8500 feet, a distance of seven miles without a break. It was entirely confined to the bottom of the ravine, both banks being throughout all that distance free of snow, and often covered with a most luxuriant herbage.
SNOW-BEDS IN RAVINES.
Similar snow-beds are to be seen in every ravine which is not too wide to be choked up by snow in winter. Their occurrence so universally is probably in a great measure the reason why glaciers were not recognized in our Indian mountains till so recent a period. These beds being so clearly transitory in existence, it was assumed that all masses of snow and ice were equally so. A visit to one of the great glaciers at the end of autumn would of course at once have indicated the dissimilarity.
In many narrow ravines remains of these snow-beds may be seen at surprisingly low elevations throughout the year, their permanence depending much more on the amount of the winter's fall of snow, and of the accumulation in that particular locality, than upon the mean or summer temperature of the place. At Baltal, in the upper part of the Sind valley in Kashmir, the little stream which descends from the Zoji pass was still arched over by a bed of snow several feet thick, in the end of September, at an elevation of not more than 9500 feet. This was not, as might have been expected, in a very shady spot, but fully exposed to the action of the sun; it was, however, in a place where the fall of snow during winter is very great.
The causes which are enumerated by Baron Humboldt as affecting the snow-level are numerous, but several are of only local effect. Two in addition to the latitude seem more important than the others, namely, the amount of fall during winter, and the amount of solar heat during summer. Captain R. Strachey regards the diminished amount of the winter's fall of snow as the main cause of the greater height of the snow-line in the interior of the Himalaya, but I feel disposed to believe that both causes co-operate equally to produce the effect.