In all places within the tropics where the trade-winds are blowing regularly an d stead ily, rain is of rare occurrence, the reason being, that as these winds come from higher latitudes, their temperature is increasing, and hence they are in the condition of taking up moisture rather than of parting with it; and the return trade-winds, which blow above them in an opposite direction, having discharged the greater part of their moisture in the region of the calms, are also dry and cloudless. Where, however, these winds are forced up mountain-ranges in their course: as on the e. of Hindustan. they bring rain. which falls chiefly during night. when the earth's surface is coolest. The region of calms is a broad intertropical belt about 5° in breadth, characterized by calms, and toward which the northern and southern trades (see TRADE-WINDS) blow. This, the region of calms, is at the same time the region of constant rains. Here the sun almost invariably rises in a clear sky; but about mid-day clouds begin to gather; and in a short time the whole face of the sky is covered with dense black clouds, which pour down prodigious quanti ties of rain. Toward evening, the clouds disappear, the sun sets in a clear sky, and the nights are serene and tine. The reason of this daily succession of phenomena in the belt of calms is, that there the air, being heated to a high degree by the vertical rays of the sun. ascends, drawing with it the whole mass of vapor which the trade-winds have brought with them, and which has been largely added to by the rapid evaporation from the belt of calms; this vapor is condensed as it is raised toward the line of junction of the lower and upper trade-winds, and the discharge is in some cases so copious, that fresh water has been collected from the surface of the sea. As evening sets in, the surface of the earth and the superincumbent air are cooled, the ascending currents cease, the cooled air descends, and the dew-point is consequently lowered, clouds are disipated, and the sky continues clear till the heat of the following day hrings round a recurrence of the same phenomena. Since the belt of calms, which determines the rainy season within the tropics, moves northward or southward with the sun's declination, carrying the trade-winds with it on each side, it follows that there will be only one rainy and one dry season in the year at its extreme northern and southern limits; but at all intermediate places there will be two rainy and two dry seasons, at the equator these will be equally distant from each other.
This state of things is only of strict application to the Pacific ocean, whose Vast expanse of water, presenting a uniformly radiating and absorbing surface, is sufficient to allow the law to take full effect. But over the great& part of the earth's surface disturb ing influences draw the trade-winds more or less out of their normal course, and some times produce a total reversal, as in the case of the monsoons (q.v.). These winds determine entirely the rainfall of India, and but for them the eastern districts of Hin dustan would be constantly deluged with rain, and the western parts constantly dry and arid. As it is, each part of s. Asia has its dry and wet season, summer being the wet season of the western parts and interior as far as the Himalaya, and winter the wet season of the eastern, and especially south-eastern parts.
The heaviest annual rainfall on the globe is 527 in., at Cherra Punji, on the Khasia hills, 494 iu. of which falls from April to September during the s.w. monsoon. This astdn ishing amount is due to the abruptness of the mountains which face the bay of Bengal, from which they are separated by 200 m. of low swamps and marshes. The winds not only arrive among the hills heavily charged with the vapor they have absorbed from the wide expanse of the Indian ocean, but being near the point of saturation, their tempera ture not being raised in passing.over these swamps, they are, so to speak, ready to burst in torrents over the abrupt cliffs which divert them from their horizontal course into the higher regions of the atmosphere. At 20 m. inland, the annual fall is reduced to 200in.; 30 m, further s., it is only 100 in.; n., at Gowhatty in Assam, it is only 69 inches. In the n.w. of the bay of Bengal, at Cutlack, it is only 52 in.; while in the n.e., in Arracan, owing to flies. w. direction of the winds, it is from 200 to 240 inches. At Madras the annual fall is 48 in. ; at Seringapatam, only 24 in. ; at Bombay, 71 in.; at Uttra-Mullay, 263 in.,
and at INIaliabalish war, 254 in., both. on the western Ghauts; and at Poona inland, 27 inches. The s.w. monsoon discharges from 50 to 90 in. of rain over the parts of Hindustan not bounded by high mountains to the w., before reaching the Himalayas, after which it dis charges the greater part of its moisture, 120 to 140 in., on the outer Himalayan range, at elevations of 4000 to 8000 feet. Thus, four times more rain falls annually on the Khashr hills than on the Himalaya, to the less abrupt face these latter mountains present to the s., to the sandy burning plain, which raise the winds considerably above the dew point, and to the larger tract traversed by the winds, over which their moisture continues to be discharged as they pass.
The followina are a few of the annual rainfalls in the tropics: Singapore, 97 in.; Can ton, 78 in.; St. Benoit (Isle of Bourbon), 163 in. ; Sierra Leone, 126 in. ' • Caracas, 155 in.; Pernambuco, 106 in. • Rio Janeiro, 45 in. • Georgetown, 95 in. ; Barbadoes, 50 in.; St. Domingo, 107 in.; Bahamas, 55 in.; and Vera Cruz, 183 inches, In many places in the interior of continents within the tropics, the rainfall is small—not greater, hi fact, than in temperate countries, such as the eastern parts of England.. At Poona, only 23 in. fall annually.
The periodicity of the rain-fall disappears as we recede from the tropics, and the times of the Year during which it occurs are different—the greater quantity falling in summer at places within the tropics and in the interior of continents, but in winter iu countries bordering on the sea in temperate regions. In respect of the rain-fall, Europe may be divided into two distinct regions: Western Europe and the countries bordering on the Mediterranean. A vast ocean on the one hand, a great continent on the other, and a predominance of w. winds, are the determining circumstances in the distribution of the rain-fall over western Europe. As the s.w. winds, which are the return trades, descend to the earth and blow over the surface of Europe, and as the whole of this con tinent is thus within their influence, it follows that the western parts, especially where mountain-ranges stretch n. and s., are rainy districts; for these mountains, diverting the s.w. winds from their horizontal course, force them up into the higher regions of the atmosphere, where, chilled, they form into clouds, or deposiA in rain the vapor they cam no longer hold in suspension. Ilene° the rainiest regions of Europe are Norway, Ire land, the w. of Great Britain and of France, Spain, and Portugal. At the Stye, in the lake district, 3S.9 in. fell in Jan.. 1851; at Drishaig, 33.2 in., and at Portree, 32.4 in Dec., 1863; and in the same mouth, from 23 to 30 in. at many other places in the Scot tish Highlands. In the w. of Great Britain and Ireland, in the vicinity of hikh hills, the average rain-fall is from 80 to 128 inches. At Bergen, in Norway, it is 70 in.; in the peninsula, at Oporto, it is 54 in. ; at Bilbao, 47 in.; and at St. Jago, 65 iu.; and in France it is 51 in. at Nantes and 49 at Bayonne. At places at some distance front hills, and in more inland districts, the annual fall is much diminished. Thus, in the w. of Great Britain, away from hills, it is from 30 to 45 in., while in the e. it is from 20 to 28 inches. In France it averages 30 in.; and in the plains of Germany and RuSsia, 20 in.; while in some parts of Sweden and Russia it falls as low as 14 inches. In the interior of Europe, in mountainous districts, it rises much above these amounts; thus at Ischl it is 62 inches. An important distinction between the mode of distribution of the rain-fall in the w. of Europe and that of more inland places is that the greater part of the annual amount in the w. falls iu winter; but in the interior in spring or summer. This difference is par ticularly striking on the different sides of Great Britain, and arises from this circum stance, that, as the clouds are umelt lower in winter, they are arrested and drained of their moisture by the less elevated hills, leaving little to be deposited eastward; but in summer being high they pass above and discharge themselves in the interior Thus for every 10 in. of rain which fall at the following places in winter, there fall in summer respectively 81 in. in the w. of Great Britain, 11 in. in the e. of Great Britain a-nd w. of France, 15 in. in the e. of France, 20 in. in Germany; and 27 in. in the n. and e. of Russia.