Rain. Whatever lowers the temperature of the air below the point of saturation, or the dew-point, may be regarded as a cause of rain. Various causes may conspire to bring about this change of temperature, but by far the most important of these originate in winds and other movements of the atmosphere. The more prominent principles of the connection of the winds to the rainfall are these: (1) When the winds have traversed a considerable extent of ocean before reaching land the rainfall is large; (2) when the winds, on arriving at the land, advance into higher latitudes or into colder regions the rainfall is largely increased, for the simple reason that the air is now more rapidly brought below the point of saturation; (3) if the winds, even though they arrive directly from the ocean, have not traversed a considerable breadth of it, the rainfall is not large—indeed, in the case of the sea-board of Lower California the mean annual amount, as at San Diego, is only 10 inches; (4) if the winds, even though they have crossed a great extent of ocean, yet on arriving at the land at once advance into lower latitudes or into warmer regions, the rainfall is small; (5) if a range of mountains lie across the onward course of the winds, the rainfall is largely increased on the side facing the winds, but reduced over the region on the other side of the range; the reason being that, as the air on the windward side of the ridge is suddenly raised to a greater height in crossing the ridge, the temperature is still more reduced by mere expansion, and a more copious precipitation is the consequence. On the lee side, as the air descends to lower levels, it gradually gets drier, and hence the rainfall of necessity diminishes with every stage of the descent to lower levels.
Attention may be here drawn to the diminished velocity of the wind over land as compared with the open sea, as has been fully shown by the observations of the Challenger expedition. From these it has been proved that an envelope of stiller air, or air of less velocity as compared with that of the ocean, broods over the land, and by its presence forces the wind blowing across the land to a greater height, thus augmenting the rainfall. This dragging effect of the land on the wind, and the important consequences resulting from it, explain how it is that during north-easterly storms of rain the foreshores of the Firth of Forth, Moray Firth, and Pentland Firth, which look to the north-east, receive a much heavier rainfall than other parts of Scotland in these circumstances. On the Ayrshire coast the annual rainfall at Ayr is 38 inches, but at Girvan it rises to 51 inches. Both stations are close to the coast, the only difference being that the hills to the eastward approach much nearer the coast at Girvan.
For short periods the heaviest rainfalls occur with thunderstorms, and with tornadoes, waterspouts, and other forms of the whirlwind, for the reason that not only is their rapid expansion due to the rapid ascent of the air, but also to great rarefaction produced by the extreme velocity of the gyrations of the air round the axis of the whirlwind. One of the heaviest rainfalls yet recorded in the British Islands was 2.24 inches in 40 minutes at Lednathie, Forfarshire, during a severe thunderstorm on 18th June 1887. At Camberwell, London, 3.12 inches fell in 2 hours 17 minutes on 1st August 1846. Of heavy falls during one day the following may be mentioned: Ben Nevis Observatory, 7.29 inches, 3d October 1890; Seathwaite, 6.78 inches, 8th May 1884; Tongue, 6.00 inches, 7th September 1870; Newport, Wales, 5.33 inches, 14th July 1875; and Camusinas, Argyllshire, 5.60 inches, 24th January 1868.
In the United States, where severe thunder- storms and tornadoes more frequently occur, the daily rainfalls repeatedly exceed these amounts. Thus, during the years 1885-89 the following heavy rainfalls of one day were recorded: Brownsville, Texas, 12.94 inches in September 1886; Pensacola, 10.70 inches, June 1887; Key West, 7.80 inches, September 1889; Chattanooga, 7.61 inches, March 1886; Shreveport, 7.54 inches, January 1885; and a day's rainfall of from 5 to 7 inches is repeatedly recorded in the United States. These amounts are greatly exceeded in lower latitudes. Thus, on the Khasi Hills, India, 30 inches on each of five successive days have been recorded; at Bombay, 24 inches in one night; and at Gibraltar, 33 inches in 26 hours.
As regards the ocean little is yet known from observation. This, however, is clear, that in the equatorial belt of calms between the regions swept by the two trades the rainfall of the ocean reaches the maximum; and the parts of the Atlantic and Pacific which are longest within the belt of calms as it shifts northward and southward with season have the heaviest ocean rainfall. But, though the cloud-screen is unquestionably dense and the rainfall frequent and heavy, the observations of the Challenger and the Novara show that the statements generally made as to these are very greatly exaggerated. Over the open sea in the regions of the trades the rainfall is everywhere small, owing to the circumstance that these winds are an immediate outflow from anti-cyclonic regions, their dryness being further increased since their course is directed in regions that become constantly warmer.
The trades, however, deposit a larger rainfall over islands and other land-surfaces which they traverse; the amounts being proportional to the height of the land, but more particularly to the degree in which the high land ranges lie across the paths of the winds. Thus, at Ascension, which is within the south-east trades the whole year round, the annual rainfall is 8.85 inches; and at St Helena on the coast 5.36 inches, but on the high land in the interior of the island it rises to 24 inches. In the western division of the Pacific, for some distance on each side of the equator, the rainfall is very slight, and extensive guano deposits are formed on Malden and other islands scattered over that region. In Mauritius, on the weather shore of the island, the annual rainfall is about 30 inches; but at Cluny, 16 miles inland, it is 146 inches, the latter place being in the neighbourhood of extensive forest-clad mountains.
The heaviest rainfalls of the globe are brought by the winds which have traversed the greatest extent of ocean within the tropics. These conditions are most completely fulfilled from June to September by the winds which, beginning their course from about 30° S. lat., blow home on southern Asia as the south-west monsoon, which accordingly distributes a heavier rainfall over a larger portion of the earth's surface than occurs anywhere else in any season. On these summer winds the rainfall of India chiefly depends, and the 'lie' of the mountain-systems with respect to the winds intensifies the effects. The following in inches are the annual amounts at different points in the west from Surat southward: Surat, 41; Bombay, 74; Mahabaleshwar, 263; Banea, 251; Honawar, 139; Calicut and Cochin, 115. In the west of Ceylon the rainfall is also large, being at Colombo 88, at Galle 90, at Ratnapura, inland among the hills, 150; but in the east of the island, before reaching which the monsoon is deprived of much of its moisture, it is considerably less, being at Mannar 91, at Jaffna 49, and at Trincomalee 62. The rainfall is also very large in Burma, being at Akyab 194, Sandoway 214, Diamond Island 119.
Rangoon 99, Tavoy 197, Mergui 162, and Port Blair 117. In the north-east angle of the Bay of Bengal, and thence north to Bhutan, where the summer monsoon curves to a westerly course up the valley of the Ganges, the rainfall is great, rising at Cherra-punji, in the Khasi Hills district, to 472 inches—or nearly 40 feet—being the largest known rainfall anywhere on the globe. Owing to this diversion in the course of the monsoon, the valley of the Ganges enjoys a generous rainfall. On the other hand, the rainfall is small over the plains of the Punjab, being at Multan 6.79, and at Kurrachee 7.25, increasing over the higher parts of the province, at Lahore 21, Umballa 36, Simla 70, and Dharmasala 122.
The key to the distribution of the rainfall over the East Indian Archipelago and Australia is the distribution of pressure from south-eastern Asia to Australia, with the resulting prevailing winds. During the winter months pressure diminishes rapidly from Asia southwards, and northerly winds prevail. Now, as these winds have travelled a great breadth of ocean, they arrive in a highly saturated state and deposit a heavy rainfall over these islands and the north of Australia. The degree to which these rains penetrate into Australia depends entirely on the strength of the winter monsoon. On the other hand, during the summer months pressure increases from Asia southwards, and southerly winds set in from Australia to Asia, distributing in their course a very large rainfall over the islands of the Eastern Archipelago, rising at several places to 30 inches a month. The small rainfall in such islands as Timor, which are more immediately sheltered, as it were, by Australia during these southerly winds, impresses on the region well-marked dry and wet annual seasons. These marked differences among the climates of this archipelago really depend on the geographical distribution of land and sea in this part of the globe, and must therefore be regarded as permanent differences, and as having played no inconspicuous part in the unique distribution of animal and vegetable life which is so characteristic of the archipelago. Since in the summer of the southern hemisphere the winds blow from the sea inland, and in the winter from the land seaward, it follows that generally the summer is the rainiest season. In the interior, along the Murray River and its affluents, the rainfall everywhere is necessarily small. In the north of New Zealand the winter rainfall is the heaviest, but farther south, where the wet westerly winds prevail with some constancy at all seasons, the rainfall is distributed pretty equally through the year, and is of course largest on the western slopes of these islands. Thus, while in the east it is at Dunedin 34 and at Christchurch 28 inches, in the west it is at Hokitika 120, and at Beaby, inland, 106 inches.
Europe as regards its rainfall may be divided into two regions—the countries bordering on the Mediterranean, and the rest of the continent. A vast ocean on one hand, a great continent on the other, and a prevalence of westerly winds rule the distribution of the rainfall in western and northern Europe. Now, since these westerly winds have traversed a vast extent of ocean before arriving at the land, they distribute a large rainfall, particularly on hilly regions opposing their course. Thus, over a large part of the Highlands of Scotland more than 80 inches fall annually, and over fully one-third of the British Islands the annual amount exceeds 40 inches. On the other hand, in eastern districts on the lee side of the great mountain-ranges, the rainfall does not exceed 30 inches over a large portion of England, and in some of the best agricultural districts of Scotland. In the west of Norway it is 72 inches at Bergen, 46 inches at the Lofoden Isles, and 10 inches at the North Cape; over large portions of Sweden it is 21 inches, and in Russia and Siberia it varies from 20 to 2 inches. Spain presents great extremes, from 70 in the north-west to 13 inches at Saragossa. In the plains of France and Germany it varies from 20 to 35 inches, rising, however, on approaching the Alps to more than 100 inches. In western Europe the greater part of the rain falls in winter, but in the interior of the continent in summer. The summer climates of the extreme south of Europe and in the north of Africa, situated at comparatively low levels, are practically rainless, owing to the northerly winds that set in with considerable force at this time of the year towards the heated plains of the Sahara.
The summer winds in the south-east of the United States are southerly; and, as they have previously traversed a considerable extent of ocean, they arrive well but not super-saturated, and pour down a monthly rainfall of 6 inches or more from Louisiana to Chesapeake Bay. The comparative equalness of the rainfall over the eastern states is the result of no mountain-ridges lying athwart their path and of the winds passing into higher latitudes and therefore cooler regions. Again, the distribution and amount of the summer rainfall in the west and north of the United States is really determined by the low atmospheric pressure which has its centre in the region of the Rocky Mountains. To the west of this low pressure winds are generally north-westerly, and as they thus pass into hotter regions the summer rainfall in these parts of the United States and Canada is either nothing or very small; whereas on the east side winds are southerly, and the rainfall consequently equals, or even exceeds, that of the finest agricultural districts of Great Britain.
As regards the rainfall of the two extreme months, January and July, the following show in inches the amounts for various places: Vera Cruz, 5.10 and 35.90; Para, 6.51 and 3.26; Manaos, 7.33 and 1.82; Buenos Ayres, 2.37 and 1.70; Corrientes, 5.24 and 2.67; and in Africa, Alexandria, 1.95 and 0.20; Algiers, 4.43 and 0.04; Senegal, 0.28 and 3.00; Sierra Leone, 0.69 and 24.20; Capetown, 0.28 and 3.83; Durban, 5.00 and 1.70; and Zanzibar, 2.02 and 2.35. The explanation of these and similar differences is found in the seasonal changes of the wind. The South African colonies present the strongest aspects of climate, so far as the rainfall is concerned, being divided into two totally distinct classes, the climates of the Natal coast and of the inland regions where the rains occur during the summer months, and the climates of the other regions where the rains fall chiefly during the winter months. The driest region of Africa is doubtless the Sahara, and the wettest the region from the Victoria Nyanza northwards, including the gathering-grounds of the Nile.
See METEOROLOGY, RAIN-GAUGE; also BLOOD-RAIN, and, for showers of frogs and fishes, SHOWERS. For maps of annual rainfall of the globe, see Loomis in Silliman's Amer. Jour. Sci., Murray in Jour. Roy. Scot. Geo. Soc., and Hann in Climatological Atlas. For particular countries, Symons, and Buchan for the British Islands; Raulin for France and Algiers; Blanford and Elliot for India; the Dutch for East India Islands; Tod, Russell, Hector Ellery, and Wragge for Australasia; the Signal Service for the United States; Carpmael for Dominion of Canada; the Meteorological Services of the different European countries, &c.