Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/161

Rh RAINFALL.] METEOROLOGY 151 Gulf of Cambay southward, and on the Western Ghats, the rainfall is excessive. The following are some of the more interesting annual means in inches beginning with Bombay and proceeding southwards : Bombay, 74 ; Matheram, 247; Mahabaleshwar, 252; Ratnagiri, 104; Baura, 255; Goa, 102; Karwar, 115; Honawar, 139; Mangalore, 134; Cannanore, 132; Calicut, 116; and Cochin, 114. In the west of Ceylon the rainfall is also heavy, being at Colombo 87, at Galle 91, and at Ratnapura, at some distance inland among the hills, 149. Since the S.W. monsoon is drained of much of its moisture in crossing these mountains, a greatly diminished rainfall is distributed over the interior and east side of India, and on the eastern slopes of Ceylon. If now we cross to the eastern shores of the Bay of Bengal, we again encounter an excessive rainfall along these coasts and up the slopes of the mountains looking down on them. Thus from south northward the following are among the more characteristic rainfalls in inches : Nancowry, 102; Port Blair, 116; Mergui, 152; Tavoy, 196; Maulmain, 189; Rangoon, 100; Bassein, 98; Sandoway, 212 ; Akyab, 198; and Chittagong 104. On the other hand, at Thyetmio, inland on the Irawadi, the annual rainfall is only 48 inches. We have shown how, in accordance with the peculiar distribution of pressure in India in summer, the monsoon is diverted up the valley of the Ganges as an E.S.E. wind, distributing on its way, even to the head of the valley, in a generous rainfall the moisture it has brought from the Indian Ocean and the Bay of Bengal. The rainfall does not extend farther westward than the basin of the Ganges, and the precipitation is most copious along the lower Himalayas, the largest falls being recorded at heights about 4000 feet, being, as pointed out by Hill, near the level at which the summer monsoon is cooled just below its dew-point. The following are some of the larger rain falls in inches, beginning with the more western: Mus- sooree, 95 ; Naini Tal, 92 ; Khatmandu, 57 ; Darjiling, 121; Kurseong, 154; Buxa, 219; Kuch Behar, 131. The rainfall is very large in the north-east angle of the Bay of Bengal and thence northwards towards Bhutan, or at the angle where the summer monsoon from the bay curves round to a westerly course on its way up the Ganges. Thus at Noakhally, on the coast, it amounts in inches to 109 ; at Tura, on the Brahmaputra, immediately to west .of the Garo Hills, 129; at Silchar and Sylhet to eastward, 117 and 155; whilst at Cherrapunji, on the Khasi Hills, it rises to 493 1 9 inches on a mean of twenty- four years. This last rainfall is the largest known on the globe, the causes of which are the highly saturated state of the monsoon on its arrival at the lower Ganges, the high mountain range of Burmah to eastward of Bengal, which turns the monsoon to the north, and the protrusion westwards of the Khasi and Garo Hills so as to lie in the line of that branch of the monsoon which passes from the lower Ganges into the basin of the Brahmaputra above Goalpara. The consequence is that the highly saturated air of the monsoon in its passage across the Khasi Hills is suddenly raised to a height of about 6000 feet, and being thereby reduced far below the point of saturation the superabundant moisture is precipitated in unequalled deluges of rain. The amount of the annual rainfall at all these places is determined, essentially if not altogether, by the rains of the summer monsoon, the relative intensity of which over India may be taken to be fairly represented by the rainfall of July. The rains which accompany the N.E. monsoon of the winter months may be represented by the rainfall for January. These are heaviest in Ceylon, especially on its east slopes, and in southern India, or where the N.E. monsoon arrives after having traversed a large extent of ocean. The fall for the month exceeds 6 inches over a large portion of the east coast, whilst at Colombo in the west the rainfall is only half that amount, and farther north at Pattalum the January rainfall is only 1*82 inches. In southern India the amount varies from about 1 to 2 inches. Blanford pointed out in 1873 (Phil. Trans., vol. clxiv. p. 618) that, while the surface winds of northern India in win ter are northerly, on the Himalayas, especially the north west portion, southerly winds prevail during the cold months. It is these upper southerly winds which bring the winter rains to the Punjab, Upper India, and the highlands of Assam. It is further to be noted that winter rains also occur in Central India, where the prevailing surface winds are from east and north-east. The mean rainfall of January at Mussooree is 2 &quot;00 inches and at Naini Tal 2 86 inches, and in Assam, at Sibsagar, 1*13 inch. Over a large tract of the east side of southern India from Nellore southward, including Ceylon, the maximum rainfall for the year occurs in the months of October and November. Rainfall of the Malay Archipelago and Australia. Under the direction of the late Dr Bergsma, systematic observations of the rainfall of the Malay archipelago were begun in 1879, the number of stations being 150. The results of the first three years show that the mean annual rainfall over the archipelago varies from about 60 inches in Timor to upwards of 200 inches at some spots among the western slopes of Sumatra. But the most important feature in the rainfall in its relations to climate is not the absolute amount that falls annually, but rather the manner of its distribution through the months of the year. Over the greater number of the islands rain falls copiously every month; but as regards some of the islands the year is divided into dry and wet seasons as marked as are seen in the climates of India. The key to this essential difference among the climates is the distribution of atmospheric pressure during the months of the year from south-eastern Asia to Australia, with the resulting prevailing winds. During the winter months atmospheric pressure is high in south-eastern Asia and low in the interior of Australia, the difference being about three-quarters of an inch. Since between these two regions the fall in the mean pressure is practically uninterrupted, the Malay archipelago lying between them is swept by northerly winds (fig. 14). As these winds have traversed a great breadth of ocean in their course, they arrive in a highly saturated state, and consequently deposit a copious rainfall, particularly on the northern slopes of the higher islands. Hence in these months the rainfall over the islands without exception is large, the mean monthly amount being in many cases more than 30 inches. These winds continue their course to southward towards the low-pressure region in the interior of Australia, and deposit along the north coasts of that continent a monthly rainfall rising generally to from 14 to 20 inches. On advancing into the interior, the mean amount gradually diminishes at the successive telegraphic stations to 3 50 inches at Alice Springs near the tropic of Capricorn. The amount of the rainfall for any particular year, and the distance from the coast to which the rains penetrate inland, depend essentially on the height of the winter pressure of south-eastern Asia as compared with the low mean pressure of central Australia, by which the strength of the northerly monsoon is regulated. On the other hand, during the summer of the northern hemisphere pressure is high in the interior of Australia and low in China, the mean difference being about half an inch. Between the two regions the fall in the mean pressure is continuous and uninterrupted, and as a consequence southerly winds prevail over the intervening archi pelago. These winds, as they advance from the continent into lower latitudes, are absolutely rainless in the north of Australia, and over Timor and the other Malay islands which are separated from Australia only by a comparatively narrow belt of sea. During the three years no rain whatever fell in Timor in July and A igust, and the fall in June, September, and October was small. As, however, the winds pursue their course to northward, they &quot;agerly lick up moisture from the sea, so that by the time they arrive at Amboyna they have become so saturated that the monthly rainfall there rises to nearly 30 inches. Again at some distance to the west of Timor rain falls more or less regularly every month, the amount increas ing in proportion to the extent of ocean traversed by the S. E. winds, which advance towards these islands from the direction of Australia. These marked differences among the climates of the Malay archipelago, which, since they really depend on the geo graphical distribution of land and sea of this part of the globe, must be regarded as permanent differences, have played no inconspicuous part in the singular distribution of animal and vegetable life which characterizes the archipelago.