« ForrigeFortsett »
arrives from the sea, the rainfall in almost any month is from four to six times greater than at Gros Cailloux on the northwest coast, where neither mountain nor forest exists, and where the air arrives partly deprived of its moisture."
406. The south-west monsoon discharges from 60 to 80 inches of rain over the parts of Hindostan not bounded by high mountains to the west, before reaching the Himalayas, after which it discharges the greater part of its remaining moisture, 120 to 140 inches, on the outer Himalaya range, at elevations of 4000 to 8000 feet. Thus four times less rain falls annually on the Himalayas, as compared with the Khasia Hills, because (1) they present a less abrupt face to the south, and (2) are separated from the ocean by a sandy burning district, raising the temperature of the air above the dew-point, or by hilly ground, which drains the winds of much of their moisture as they pass.
407. The following are a few of the more interesting annual rainfalls in the tropics :-Singapore, 97 inches ; Canton, 78 inches ; St Benoit, Isle of Bourbon, 163 inches ; Sierra Leone, 87 inches ; Caraccas, 155 inches ; Pernambuco, 106 inches ; Rio Janeiro, 59 inches ; Georgetown, 100 inches; Barbadoes, 72 inches ; St Domingo, 107 inches ; Bahamas, 52 inches ; Vera Cruz, 183 inches; and Ceara, 60 inches; Doldrums of the Atlantic, 225 inches; Maranhao, 280 inches. In many places in the interior of continents, and places situated a little to the leeward of mountain-ranges, considered with reference to the prevailing winds within the tropics, the rainfall is small, being not greater than what occurs in temperate countries ; thus, as already stated, the fall at Poonah is only 24 inches.
408. Rainfall in Europe.—The periodicity of the rainfall disappears as we recede from the tropics, and the times of the year during which it falls are different, the greatest quantity falling within the tropics in summer, whereas in temperate regions the greatest quantity falls generally in winter. In respect of the rainfall, Europe may be divided into two distinct regions—Western Europe, extending in a modified degree into the interior of the continent; and the countries bordering on the Mediterranean.
409. A vast ocean on the one hand, a great continent on the other, and a predominance of westerly winds, are the determining circumstances in the distribution of the rainfall over Western Europe. Since south-west winds prevail from the south of Europe northwards, it follows that the western parts, especially where mountain-ranges stretch north and south, are rainy districts. Hence the wettest regions are Norway, Ireland, the west of Great Britain and of France, Spain, and Portugal. At the Stye, in the lake district of England, being, so far as known, the wettest spot in Great Britain, 38.9 inches fell in January 1831: at Drishaig, near Ben Cruachan, 33.2 inches ; and at Portree, 32.4 inches in December 1863 ; and in the same month from 23 to 30 inches at many places in the Scottish Highlands.
410. In the west of Great Britain and Ireland, in the immediate neighbourhood of high hills, the average rainfall is from 80 to 150 inches, and in some years it is higher : thus, at Seathwaite, in Cumberland, it was 1834 inches in 1861; and at the Stye, 2241 inches in 1866. At Bergen, in Norway, the rainfall is 89 inches; in the Peninsula, at Coimbra, 118 inches; at Oviedo, 74 inches; and at St Jago, 73 inches. In France, it is 51 inches at Nantes, and 49 at Bayonne. At places at some distance from hills, and in more inland situations, the annual fall is much diminished. Thus in the west of Great Britain, away from hills, it is from 30 to 45 inches; while in the east of the island it is only from 20 to 28 inches. In France the average is 30 inches; and in the plains of Germany and Russia, 20 inches; while in some parts of Sweden and Russia, it is as low as 15 inches. But in the interior of Europe, in mountainous districts, it rises much above these amounts ; thus at the Brocken it is 59 inches, and at St Maria, in the Alps, 104.35 inches.
411. An important distinction between the mode of distribution of the rainfall in the west of Europe and at more inland places, is that the greater part of the annual quantity of the west falls in winter, whilst in the interior the amount in summer is rather greater than in winter. This peculiarity is shown by the east and west sides of Great Britain. It is probably owing to the clouds being much lower in winter, by which they are arrested and drained of their moisture by the less elevated hills, thus leaving little to be deposited eastward; but in summer, the clouds, being high, pass above and discharge their moisture in the interior. * For every 10 inches of rain which fall at the following places in winter, there fall in summer respectively, 8.1 inches in the west of Great Britain, 11 inches in the east of Great Britain and in west of France, 15 inches in the east of France, 20 inches in Germany, and 27 inches in the north and east of Russia.
412. It will be shown further on that the greater proportion of the rain which falls during storms falls in the front part of the atmospheric depression which accompanies the storm- that is, as respects Great Britain, with easterly and southerly winds. Hence, in those districts where the greater part of the rainfall of the year is made up of the rain which falls during storms, or on the eastern slope of the island, it might be expected that the greater part of the annual rainfall is brought by easterly and not by westerly winds. The Rev. Alexander Beverly, Aberdeen, has examined the rainfall for the different winds at that place for the two years 1864 and 1865. From this examination it appears that as much rain falls with N.E. as falls with S.W. winds; and that while 12.74 inches fell with S.W., W., N.W., and N. winds, as much as 17.85 inches, or nearly a half more, fell with N.E., E., S.E., and S. winds. It will be necessary to make an important distinction here-viz., this large rainfall, which is deposited by easterly winds, is not brought by the polar currents, which the mere direction of the wind might indicate ; but the moisture is originally brought by the southwest wind, which, prevailing first in the higher regions of the atmosphere, as the direction of the cirrus cloud and other high
For a first instalment of an able and thorough discussion of this question by Frederic Gaster, see Symons's 'British Rainfall, 1867,' p. 33.
clouds some time before the breaking out of the storm clearly proves, and then gradually descending, saturates the air with the vapour which it brings from the ocean. Thus, then, the easterly winds (N.E., E., S.E.) which deposit the rain, are part of the same moist current drawn back by and towards the low atmospheric pressure which at the time is advancing from the west.
413. The peculiarity of the rainfall of the basin of the Mediterranean depends on (1) its proximity to the burning sands of Africa, (2) a predominance of northerly winds resulting chiefly from that position, and (3) the Pyrenees and Spanish sierras to the west, on which the south-west winds precipitate their rains before arriving on the north shores of the Mediterranean. In the valley of the Rhone, four times more rain falls in autumn than in summer; and south of the Alps, six times more rain falls with north-east than with south-west winds, being the reverse of what takes place in the west of Great Britain. In Italy the quantity of rain diminishes as we approach the south, because south winds get wetter, and north winds get drier as they proceed on their course. Along the Syrian and North African coasts it rarely rains in summer, but frequently in winter. In the valley of the Rhone, the annual fall ranges from 20 inches on the coast to 63 inches at St Rambert, the average being 30 inches. This is also the average of the valley of the Po; but on ascending the long slopes northward to the Alps, it rises, as at Tolmezzo, to 96 inches.
414. Rainfall of America.-The rainfall in the west of the American continent is distributed similarly to that of Europe -the quantity being chiefly dependent on the physical configuration of the surface over which the westerly winds blow. In North America the yearly amount increases as we proceed northward ; thus at San Francisco it is 22 inches; at Fort Reading, 29 inches ; at Fort Oxford, 72 inches ; at Fort Vancouver, 47 inches; at Astoria, 86 inches; at Steilacoom (Wash. Ter.), 54 inches; and at Sitka, in the north-west of America, 90 inches.
415. But in the United States the manner of the distribution of the rainfall differs greatly from that of Europe. The United States are chiefly dependent for their rain, not on the Pacific Ocean, but on the Gulf of Mexico. The high range of the Rocky Mountains in Central America plays an important part in the rainfall. In the northern parts they drain westerly winds of their moisture as they cross them. Further south they present a barrier to the passage of the easterly winds which blow across the Gulf of Mexico, which are, partly on this account, and partly on account of the heated plains of the States, turned or drawn to the northward, and spread themselves over the States, especially over the low basin of the Mississippi. Thus, then, the greatest part of the moisture will be drawn into the valleys where the heat is greatest, and the least part into the high mountainous regions, where respectively it will be disengaged and fall in rain. If this be the case, then the greatest quantity will fall in the valleys, and the least on the higher grounds--a mode of distribution the opposite of what obtains in Europe. That such is the case the following remarks by Blodget on the rainfall of America, given in the Army Meteorological Register,' will show :-“For much the larger area of the United States, and for all portions east of the Rocky Mountains, the distinguishing feature of the distribution of the rainfall is its symmetry and uniformity in amount over large areas. The quantity has rarely or never any positive relation to the configuration of the surface, which would identify it with Europe and the North Pacific coasts; and in contrast with these it has a diminished quantity at the greater altitudes generally, and the largest amounts in the districts near the sea-level. It also differs from these districts, and from large land areas generally, in having a larger amount in the interior than on the coast, for the same latitude, at least as far north as lat. 42o.” The rainiest districts are Florida, the low flats of the Mississippi, then along the course of its valley, then in Iowa, that remarkable depression at the head of the river; and the least quantities on the Alleghanies, especially their