Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/762

738 738 INDIA [PHYSICAL METEOROLOGY. Meteor- The great peninsula of India, with its lofty mountain ology. ranges behind and its extensive seaboard exposed to the first violence of the winds of two oceans, forms an excep tionally valuable and interesting field for the study of meteorological phenomena. But only within the last few years have trustworthy statistics been obtained for some of its most important registration stations. Meteorological. Geography. After the general description of the country which has been given at the beginning of this article, it is only necessary here to sketch very briefly the meteorological geography of the peninsula. The follow ing sentences are condensed from an interesting account in the first Report on the Meteorology of India (for 1875), by Mr H. F. Blanford. From the gorge of the Indus to that of the Dihong (Brahmaputra), a distance of 1400 miles, Hima- the Himalayas form an unbroken watershed, the northern k*y as - flank of which is drained by the upper valleys of these two rivers; while the Sutlej (Satlaj), starting from the southern foot of the Kailas Peak, breaks through the watershed, dividing it into two very unequal portions, that to the north-west being the smaller. The average elevation of the Himalaya crest may be taken at not less than 19,000 feet, and therefore equal to the height of the lower half of the atmosphere ; and indeed few of the passes are under 16,000 or 17,000 feet. Across this mountain barrier there appears to be a constant flow of air, more active in the day-time than at night, northwards to the arid plateau of Tibet. There is no reason to believe that any transfer of air takes place across the Himalayas in a southerly direction, unless, indeed, in those most elevated regions of the atmo sphere which lie beyond the range of observation ; but a noc turnal flow of cooled air, from the southern slopes, is felt as a strong wind where the rivers debouch on the plains, more especially in the early morning hours ; and this probably contributes in some degree to lower the mean temperature of that belt of the plains which fringes the mountain zone. Indus At the foot of the great mountain barrier, and separating plain. jt f roni the more ancient land which now forms the high lands of the peninsula, a broad plain, for the most part alluvial, stretches from sea to sea. On the west, in the dry region, this is occupied partly by the alluvial deposits of the Indus and its tributaries and the saline swamps of Kahhch (Cutch), partly by the rolling sands and rocky surface of the desert of Jaisalmlr and Bikanir, and the more fertile tracts to the eastward watered by the Lunai. Over the greater part of this region rain is of rare occur rence ; and not infrequently more than a year passes with out a drop falling on the parched surface. On its eastern margin, however, in the neighbourhood of the Aravalli hills, and again on the northern Punjab, rain is more fre quent, occurring both in the south-west monsoon, and also at the opposite season in the cold weather. As far south as Sfrsa and Multan (Mooltan), the average rainfall does not much exceed 7 inches. Gangetic The alluvial plain of the Punjab passes into that of the plain. Gangetic valley without visible interruption. Up or down this plain, at opposite seasons, sweep the monsoon winds, in a direction at right angles to that of their nominal course ; and thus vapour which has been brought by winds from the Bay of Bengal is discharged as snow and rain on the peaks and hillsides of the Western Himalayas. Nearly the whole surface is under cultivation, and it ranks among the most productive as well as the most densely populated regions^of the world. The rainfall diminishes from 100 inches in the south-east corner of the Gangetic delta to less than 30 inches at Agra and Delhi, and there is an average difference of from 15 to 25 inches between the northern and southern borders of the plain. Eastward from the Bengal delta, two alluvial plains Eas stretch up between the hills which connect the Himalayan Ben system with that of the Burmese peninsula. The first, or the valley of Assam and the Brahmaputra, is long and narrow, bordered on the north by the Himalayas, on the south by the lower plateau of the Garo, Khasi, and Naga hills. The other, short and broad, and in great part occu pied by swamps and jhils, separates the Garo, Khasf, and Naga hills from those of Tipperah and the Lushai country. The climate of these plains is damp and equable, and the rainfall is prolonged and generally heavy, especially on the southern slopes of the hills. A meteorological peculiarity of some interest has been noticed, more especially at. the stations of Sibsagar and Silchar, viz., the great range of the diurnal variation of barometric pressure during the afternoon hours, which is the more striking, since at Riirki (Roorkee), Lahore, and other stations near the foot of the Western Himalayas, this range is less than in the open plains. The highlands of the peninsula, which are cut off from c the encircling ranges by the broad Indo-Gangetic plain, are tat divided into two unequal parts, by an almost continuous lan chain of hills running across the country from west by south to east by north, just south of the Tropic of Cancer. This chain may be regarded as a single geographical feature, forming one of the principal watersheds of the peninsula, the waters to the north draining chiefly into the Narbada and the Ganges, those to the south into the Tapti, the Goddvari, the Mahanadi, and some smaller streams. In a meteorological point of view it is of considerable importance. Together with the two parallel valleys of the Narbada (Nerbudda) and Tapti (Taptee), which drain the flanks of its western half, it gives, at opposite seasons of the year, a decided easterly and westerly direction to the winds of this part of India, and condenses a tolerably copious rainfall during the south-west monsoon. Separated from this chain by the valley of the Narbada on the west, and that of the Son on the east, the plateau of Malwa and Baghelkhand occupies the space intervening between these valleys and the Gangetic plain. On the western edge of the plateau are the Aravalli hills, which run from near Ahmadabad up to the neighbourhood of Delhi, and include one hill, Mount Abu, over 5000 feet in. height. This range exerts an important influence on the direction of the wind, and also on the rainfall. At Ajmir (Ajmere), an old meteorological station at the eastern foot of the range, the wind is predominantly south-west, and there and at Mount Abu the south-west monsoon rains are a regularly recurrent phenomenon, which can hardly be said of the region of scanty and uncertain rainfall that extends from the western foot of the range and merges in the Bikanir desert. The peninsula south of the Satpura range consists chiefly So of the triangular plateau of the Deccan, terminating pi abruptly on the west in the Sahyadri range (Western Ghats), and shelving to the east (Eastern Ghats). This plateau is swept by the south-west monsoon, but not until it has surmounted the western barrier of the Ghats ; and hence the rainfall is, as a rule, light at Poona and places similarly situated under the lee of the range, and but moderate over the more easterly parts of the plateau. The rains, however, are prolonged some three or four weeks later than in tracts to the north of the Satpuras, since they are also brought by the easterly winds which blow from the Bay of Bengal in October and the early part of November, when the recurved southerly wind ceases to blow up the Gangetic valley, and sets towards the south-east coast. This was formerly thought to be a north-east monsoon, and is still so spoken of by certain writers ; but the rainy wind is really a diversion of the south-west monsoon.