Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/870

830 HIMALAYA winds : their reezing All the rivers in Ladak are frozen over every winter f rivers, down to 8000 feet, a phenomenon that has been nowhere seen in the Himalaya, the rivers that are accessible during winter being at too small an elevation to be ever frozen, and the main streams probably everywhere too rapid. )iurnal The diurnal range of temperature is directly dependent n<i on the condition of the surface and the state of the sky. f On all well-wooded ranges it is very small, particularly in emper- ^ ae cloudy and rainy mouths, when it will not exceed 5 or ,ture. 6. In the dry months of April or May, before the summer rains begin, and in the autumn when clear skies again prevail, the variation is greatest. In Tibet it is greatest in summer and least in winter, which is the most cloudy season. The annual variation follows a similar law, 20 being about the minimum mean monthly range on the wooded outer mountains, while in Tibet it amounts to as much as 50. Jom- The difference of temperature between ranges densely larison c ] a( j with forest and the Indian plains may be twice as . . much in April and May as in December or January; and m d the differences between the temperatures of a well-wooded ivestem hill-top and an open valley below have been observed to parts of vary as much as from 9 in the coldest to 24 in the hottest " hour. Precisely the converse may be found to hold good in comparing greater altitudes in Tibet with lesser in the Himalaya, where the humidity is greater. General It is generally true here as elsewhere (1) that the decre- relation men k O f temperature with altitude is most rapid in summer ; neratui-e (^) ^at ^ ie annua ^ range diminishes with elevation ; and to alti- (3) that the diurnal range also diminishes with elevation. tude. But the greater quantity of cloud and the more dense covering of forest between 9000 and 10,500 feet render comparisons between this zone and the more open and less cloudy regions above and below anomalous., for reasons tiiat will be apparent after the above remarks. Rainfall. The chief rainfall on the Himalaya, like that of India periodi- g ener ally, occurs in the suinmer months between May and caused"^ October, the remainder of the year being comparatively dry. The physical causes which give rise to the south-west monsoon, under which name is known the season of summer rain as well as the south-westerly winds which accompany it, have been sufficiently explained in the article on ASIA, to which the reader is referred for further details. C4eneral The fall of rain over the great plain of northern India distribu- g ra( j ua l]y diminishes in quantity, and begins later, as we pass from east to west. At the same time the rain is heavier as we approach the Himalaya, and the greatest falls are measured on its outer ranges ; but the quantity is again much diminished as we pass onward across the chain, and on arriving at the border of Tibet, behind the great line of snowy peaks, the rain falls in such small quantities as to be hardly susceptible of measurement. Moun- An important agent in distributing the rain which falls over the Himalaya, if indeed it be not the essential one, is found in the diurnal currents of wind which are established from the plains towards the mountains, and vice versa, blowing up the valleys towards the main watershed from about 8 A.M. to 8 P.M., and in the opposite direction during the corresponding hours of the night. These diurnal winds, which are a very striking phenomenon, and blow in the afternoon with great force along the Himalayan and Turkish watersheds and the neighbouring parts of the plain of Tibet, are common to all mountains, and are no doubt due to the disturbances of the planes of equal atmospheric pressure in a direction transverse to the mountains, caused by the unequal diurnal expansion and contraction of the columns of air respectively over the plains and mountains, the former being obviously longer than the latter, so that the planes of equal pressure are lifted by expansion higher over the plains during the day, and depressed by contraction during the night, thereby setting up currents to and from the highest region of the mountains, as these alternating actions take place. The condensation of vapour from the ascending currents Then thus set up across the Himalaya from the Indian plain, effect and their gradual exhaustion by repeated precipitations J elati as they cross successive ranges, is manifested to the " traveller on the face of the mountains themselves, by the changes observed in the vegetation on passing to leeward of any important range, and more especially the line of great snow peaks. In the rainy months in the higher parts of the chain the southern face of each ridge is clothed by the clay upwind with a crest of cloud, which hangs upon it, the northern face being often left entirely free, thus showing how large a portion of the vapour is arrested, and showing also that it is only up the deeper gorges that a small supply of moisture finds its way to the Tibetan table-land. The yearly rainfall, which amounts to between 60 and Quar 70 inches in the delta of the Ganges, is reduced to about of ra 40 inches where that river issues from the mountains on mea " the 78th meridian, and is no more than 30 inches near the foot of the mountains at the issue of the Indus. At Darje- ling, at about 7000 feet on the east, on the outer border of the mountains, it amounts to about 120 inches ; at Naini- tal, at about the same elevation and similarly situated in 79| E. long., it is about 90 inches; and further west it is still more reduced in quantity. In passing from the exterior to the interior of the chain the quantity greatly diminishes. In Kumaon, while in three months 49 inches was registered at Naini-tal on the outer range, only 19 inches fell at Almora, at 5600 feet, 30 miles further in. At Joshimath, 50 miles onward and close under great snowy peaks, at 6500 feet, 22 inches were measured in the same period; while at Niti, 11,500 feet behind the snowy peaks, the fall did not exceed 5| inches. In a week of rainy weather, when 1| inches fell at Joshimath, half an inch was measured at Niti ; while beyond the Tibetan watershed during the same period the rain fell in a very faint drizzle which could not be measured at all. Similar facts might be quoted from the eastern mountains. In the eastern Himalaya the ordinary winter limit of snow Sno is 6000 feet, and it is rare for even 3 inches to remain as fall, many days on the ground on a southern exposure at 7000 feet. In Kumaon, on the west, it is pretty certain that snow will fall every year at about 6500 feet, and at 5000 feet it will not fail one year out of ten ; the lowest level to which sporadic snowfalls are known to descend is about 2500 feet, of which two or three instances have occurred in the present century. At Peshawar, at an elevation of a little above 1000 feet, snow occasionally falls, but very rarely. At L6", in western Tibet, the results of two years observations indicate that the depth of snow that falls is less than 2 feet, the heaviest continued fall measuring 8 inches and lasting three days, at an elevation of 12,500 feet Depths of 3 feet have been measured on passes between 17,000 feet and 19,000 feet above the sea. On the Indian watershed the falls are much heavier, and even in Septem ber the passes are sometimes quite blocked by the falls which commonly occur after the equinox, and they are not usually open again till the middle of June. It is now satisfactorily established that the snow line, or The the level to which snow recedes in the course of the year, sno<n ranges from 15,000 to 16,000 feet on the southern ex- linc- posures of the Himalaya that carry perpetual snow, along all that part of the chain that lies between Sikim and the Indus. It is probably not till December that the snow begins to descend for the winter, though after September light falls occur which cover the higher mountains down

to 12,000 feet, but these soon disappear. On the Indian