Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/160

Rh 150 METEOROLOGY [RAINFALL. Rainfall. Whatever tends to lower the temperature of the air below the dew-point is a cause of rain. It is there fore to the winds we must chiefly look for an explanation of the rainfall, and the broad principles of the connexion may be stated to be these five : (1) when the winds have previously traversed a considerable extent of ocean, the rainfall is moderately large ; (2) if the winds advance at the same time into colder regions, the rainfall is largely increased, because the temperature is sooner reduced below the point of saturation ; (3) if the winds, though arriving from the ocean, have not traversed a considerable extent of it, the rainfall is not large; (4) if the winds, even though having traversed a large extent of ocean, yet on arriving at the land proceed into lower latitudes or regions markedly warmer, the rainfall is small or nil; (5) if a range of mountains lies across the onward path of the winds, the rainfall is largely increased on the side facing the winds, and reduced over the regions on the other side of the range. The reason here is that, the air on the windward side of the ridge being suddenly raised to a greater height in crossing the range, the temperature is further reduced by mere expansion, and a more copious precipita tion is the result whereas on the leeward side as the air descends to lower levels it becomes gradually drier, and accordingly the rainfall rapidly diminishes with the descent. We have drawn attention to the diminished velocity of the wind over land as compared with the open sea (p. 125). From this it follows that an envelope of stiller air or air of less velocity than that of the prevailing wind broods over the land, and by its presence forces the prevailing wind to a greater height, thus tending to increase the rainfall. If the foreshore rises within a few miles to a height of 200 or 300 feet, the result is very striking when the wind from the sea blows straight upon it. Thus at Spittal, near Berwick, on September 1877, a N.E. wind blew straight ashore at an estimated velocity of 25 miles an hour. To eastward the sky was singularly clear down to the horizon, but to westward all the country beyond a mile from the shore was enveloped in what appeared a dense mist or fog. About 15 to eastward of the zenith of an observer on the shore, the thinnest rack of cloudlets was seen emerging without intermission from the deep stainless blue of the sky, which as they drifted landward increased so rapidly in volume and density that the zenith was three-fourths covered with clouds. A similar phenomenon was seen in September 1879 on board the Orkney steamer at the magnificent cliff of Hoy Island, Orkney. A heavy storm had just cleared away, and a strong &quot;W.N.W. wind was blowing right against the cliff. The sky was absolutely cloud less all round, except the upper 300 feet of Hoy Hill, 1570 feet high, which was enveloped in a thick mist that stretched away to wind ward, some distance to westward of the steamer s course, which was about 2 miles from land. The western termination of the cloud was the thinnest rack of cloud, which emerged unceasingly from the blue sky at a distance not less than 4 miles to wind ward of the cliff. The constituent parts of the cloud itself were in rapid motion eastward, but, owing to the fresh accessions it was constantly receiving, the cloud itself appeared stationary. Thus the wind was forced upward into the atmosphere for some consider able distance to windward of the ridge lying across its path. It is this dragging effect of the land on the wind, and the con sequences which result from it, that explain how it is that during storms of wind and rain from the north-east the rainfall over the foreshores of the Firth of Forth, the Moray Firth, and the Pentland Firth looking to the north-east is so much in excess as compared with the rest of Scotland. The same principle explains the heavy rainfall in plains at some distance from the range of hills lying across the wind s path and on the side of the rain-bringing winds. For short intervals of time the heaviest rainfalls occur with tornadoes, waterspouts, and some other forms of the whirlwind, the reason being that not only is there rapid expansion due to the rapid ascent of the air, but also great rarefaction is produced by the extreme velocity of the aerial gyrations round the axis of the tornado. On August 1, 1846, 3-12 inches of rain fell at Camberwell, London, in two hours and seventeen minutes. Of heavy falls may be mentioned 4 60 inches in London, April 13, 1878; 6-00 inches at Tongue, September 7, 1870; 5 36 inches in Monmouthshire, July 14, 1875; 6 62 inches at Seathwaite, Cumberland, November 27, 1848; and 7 12 inches at Drishaig, Argyllshire, December 7 to 8, 1863. But it is in lower latitudes that the heaviest single showers have been recorded. The following are among the most remarkable: at Joyeuse, France, 31 17 inches in twenty- two hours; at Genoa, 30 00 inches in twenty-four hours; at Gibraltar, 3 3 00 inches in twenty-six hours; on the hills above Bombay, 2 4 00 inches in one night ; and on the Khasi Hills, India, 30 00 inches on each of five successive days. As regards the ocean, there are no available data from which an estimate could be formed as to the amount of the rainfall, since the rainfall statistics of the ocean must be regarded as giving hardly anything more than the comparative frequency of the fall. It is, however, certain that the equatorial belt of calms in the Atlantic and Pacific between the trades is the region where the ocean rainfall reaches the maximum, and the parts of these oceans are the rainiest which are the longest within the belt of calms as it shifts its position northward and south ward with season. While the cloud-screen is undoubtedly dense, and the rainfall frequent and heavy, the careful observations of the &quot; Challenger &quot; and &quot; Novara &quot; show that the statements generally made as to these points are greatly exaggerated. In the regions of the trades the rainfall is everywhere small over the open sea, seeing that the trade-winds are essentially an outflow from anticyclonic regions, and their original dryness is to a large extent maintained because their course is directed into regions which become con stantly warmer. Thus at Ascension, lat. 8 45 S., which is throughout the whole year within the S.E. trades, the mean rainfall for the two years 1854-55 was only 8 85 inches. At St Helena, which lies constantly within the same trades, five years give a mean rainfall of 5*36 inches on the coast; but in the same island at a height of 1763 feet the annual amount rises to 2 3 98 inches. Maiden Island and some other islands in the Pacific, about long. 150 W., and for some distance on each side of the equator, have been pointed to by Scott as practically almost rain less, as is shown by their containing extensive guano deposits. These islands are situated somewhat similarly to Ascension with respect to the zone of calms. In Mauritius the annual rainfall on a mean of four years was 30 inches at Gros Cailloux, but at Cluny, only 16 miles distant, for the same four years it was 146 inches; in regard to which Meldrum remarks that at Cluny, which is in the vicinity of mountains and forests, in the south east of the island, and thus directly exposed to the trade- wind as it arrives from the sea, the rainfall in almost any month is from four to six times greater than at Gros Cailloux on the north-west coast, where neither mountain nor forest exists, and where the S.E. trade arrives con siderably drained of its moisture. From what has been said it is evident that the heaviest rains will be brought by the winds which have traversed the greatest extent of ocean within the tropics, and which accordingly of all ocean winds have the highest temperature and humidity. These conditions are most completely fulfilled during the summer months of the northern hemisphere by the winds which, commencing from near lat. 30 S., blow home on southern Asia as the well-known S.W. monsoon of these regions. Accordingly it is by the winds of this monsoon that a larger rainfall is distributed over a larger portion of the earth s surface than occurs anywhere else in any season ; and this large rainfall is in many regions still farther greatly increased by the mountain ranges which lie across the path of the rain- bringing winds. It is on these winds that the rainfall of India chiefly depends. Along the whole of the west coast from the