Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/163

Rh RAINFALL.] METEOKOLOGY 153 May in Scotland north of the Grampians, June in Orkney, Shetland, Iceland, the west of Ireland, and the north-west of France, and July over the whole of the south of Europe. The driest month occurs nowhere in Europe in any of the five months from August to December. Rainfall of North America. West of the Rocky Mountains the rainfall is very unequally distributed, the annual amounts varying from 86 inches at Astoria, near the mouth of the Columbia river, to 8 inches at San Diego on the coast, and 3 inches at the head of the Gulf of California. Over the whole of the region between the Cascade and Rocky Mountains the rainfall at all seasons is extremely small, this being indeed that feature in the climate to which the formation of the canons of that region is chiefly to be referred. On the other hand, in the United States and Canada to east of long. 100 W. the distinguishing feature of the rainfall is the comparative equableness of its distribution, an annual rainfall exceeding 50 inches occurring only over restricted districts, and a rainfall as low as 20 inches being scarcely met with anywhere. The regions where the rainfall exceeds 50 inches are Florida, the lower basin of the Mississippi, and the Atlantic seaboards of Nova Scotia and New foundland. In January the annual maximum rainfall occurs over the whole of the west coast from Sitka to lower California; but in the interior between long. 120 and 95 W. the amount is everywhere small, and over a considerable part in the south-west of this region no rain falls. The region of largest rainfall extends from Louisiana to West Virginia, where the mean varies from 4 to 6 inches. Over nearly the whole of the Dominion of Canada, by much the greater part of the winter precipitation is in the form of snow, which has been carefully measured and recorded by the Meteorological Ser vice. The average snowfall for January exceeds 30 inches at St John s, Newfoundland, in Anticosti, Prince Edward Island, and in many other regions. In July the rainfall is everywhere small in the west, a large part of this extensive region being absolutely rainless. The remarkable dry ness of the climate at this season is due to the N. W. winds that set in towards the low pressure of the interior, which thus blow towards warmer regions. The rainfall to the east of the Rocky Mountains is distributed by the winds which are connected with the low-pressure region of the interior and with the high- pressure region of the Atlantic. The result is two regions of larger rainfall, the one in the south-east of the States and the other to the west of the lakes. The summer winds of the south-eastern coasts are southerly, and as they are anticyclonic in their origin and have in their course traversed some extent of ocean, they arrive well- but not super-saturated, and pour down a rainfall in July of 6 inches and upwards along the coasts and for some distance inland from Louisi ana to Chesapeake Bay. Further, since in July these winds attain their maximum force and persistency, the rainfall at the same time reaches the maximum along the whole coast from Boston to some distance west of New Orleans. Since the summer winds blow- in the line of the Alleghany mountains and not across them, the rainfall diminishes in ascending their slopes. The comparative equableness of the rainfall over the eastern States is the necessary result of the winds passing into higher latitudes, and, therefore, cooler regions. A broad region where the rainfall is less than on each side of it, extends from Michigan to the south-west as far as Canadian River. To the west of the lakes the rainfall rises above 4 inches, and, since over this region the winds become somewhat easterly as they flow towards the low-pressure area, it is probable that the larger rainfall of this prairie region has its origin in no small degree in the evaporation of the lakes. On ascending the higher reaches of the Mississippi, the amount diminishes, but scarcely falls lower than 2 inches, being thus analogous to the summer rains of the Upper Ganges. On crossing the water-parting into the basin which drains into Hudson Bay, we encounter E. and N.E. winds laden with vapour licked up in their passage over Hud son s Bay, which they distribute in a generous rainfall of probably 3 to 5 inches over the rising colonies of Manitoba and Saskat chewan. An important point in the climate of the States is that over nearly the whole of the extensive region stretching between Alleghanies and Rocky Mountains, except the south coast already referred to, the annual maximum rainfall does not occur in summer but in spring, the month of largest rainfall in the great majority of cases being May. In the basin of Hudson s Bay July is the month of largest rainfall. Rainfall of Central and South America. The following are, in inches, the larger and more interesting annual rainfalls round the coasts: V era Cruz, 182; Belize, 75; Maracaibo, 163; Caracas, 155; Georgetown, 95; Paramaribo, 142; Cayenne, 140; Para, 71; Pernambuco, 109; Buenos Ayres, 34; Bahia Blanca, 19; Puerto Montt, 102; Valdivia, 109; Valparaiso, 100; Serena, 93; Lima, 9 ; and a large part of Peru, nil. A remarkable feature of the rainfall of South America is the large amounts that fall in the basins of the Orinoco and Amazon; the fall is 91 inches in the upper basin of the Madeira, and 112 inches at Yquitos (lat. 3&quot; 40 S., long. 72 57 W.). The reason is that this immense region, where pressure appears to be almost constantly low, is open to the highly saturated winds that blow from the equatorial Atlantic. Quite different is the distribution of the rainfall over the La Plata basin. The annual falls, in inches, are 92 at Joinville, 58 at Corrientes, 44 at Monte Video, 36 at Parana, 24 at Santiago, 22 at San Luis, and only 6 at Mendoza. The fall rapidly rises in ascending the eastern slopes of the Brazil mountains facing the South Atlantic ; thus, while the amount at Rio Janeiro is 45 inches, on the hills to northward it is 116 inches. In January northerly winds prevail on the south coasts of the Gulf of Mexico and the Caribbean Sea, and as they have their origin in the high pressure of the American continent, and in crossing the sea pass into lower latitudes, the January rainfall of these coasts is comparatively small. In July, however, the prevail ing winds are easterly, and as they have traversed a large extent of the equatorial waters of the Atlantic they are highly saturated, and consequently the July rainfall of these coasts is everywhere very large. The following are, in inches, the January and July rainfalls: Caracas, I OO and 14 04 ; Guatemala, 28 and 10*79; Vera Cruz, 5 10 and 35 90. The seasonal distribution of the rainfall in the basin of the Amazon is the reverse of this. In January the position of the belt of calms is about lat. 3 N., and as pressure is relatively low over the basin of the Amazon, especially its southern slopes, the trades and the west portion of the region of calms unitedly spread their highly saturated air over the whole region as far as the Andes, resulting in one of the most widespread heavy rainfalls anywhere to be met with. On the other hand, since in July the belt of calms is about lat. 10 N., the saturated atmosphere of the tropical regions no longer flows up the Amazon, but is carried westward into the Caribbean Sea and Gulf of Mexico. Hence at this season the rainfall of the Amazon valley is small. The following are, in inches, the January and July falls: Para, 6 51 and 3 26; Manaos, 7 33 and 1 82; upper Madeira, 15 90 and 30 ; and Yquitos, 10 24 and 4 26. On the La Plata in January pressure is low, and as winds consequently blow from the ocean in upon the region of low pressure the rainfall is large ; but as pressure is high in the interior in July the rainfall in that month is small. The following are, in inches, the January and July rainfalls : Buenos Ayres, 2 37 and 170; Parana, 4 63 and 1-32; Corrientes, 5 24 and 2 67 ; Joinville, 14 26 and 3 55 ; and San Luis, 2 63 and O OO. Rainfall of Africa. As regards the rainfall, Africa presents the greatest diversity in its climates. The following are the annual amounts in inches at various points on or near the coast : Port Said, 2; Alexandria, 8; Tunis, 12; Algiers 31; Oran, 17; Mogador, 50; mouth of the Senegal, 17; Goree, 21; Sierra Leone, 126; Christiansborg, 23; St Thomas, 40; Gaboon, 106; Loanda, 11; Cape Town, 23; Mossel Bay, 12; Port Elizabeth, 24; Durban, 43; Zanzibar, 58; and mouth of the Zambezi, 61. In the north of tho continent, the rainfall rapidly diminishes inland, and over the great desert of Sahara practically none falls. In the interior of Algiers it diminishes, the amount at Laghouat being 17 inches, and at Biskra 9. In Egypt the rainfall is limited to a narrow strip along the coast ; at Cairo the annual fall scarcely amounts to an inch. The January and July rainfalls are, in inches, as follows : Port Said, 46 and O OO; Alexandria, 1 95 and 20; Algiers, 4 43 and 04; Biskra, 56 and 03; St Louis (Senegal), 28 and 3 00; Goree, O OO and 4 06; Sierra Leone, 69 and 24 20; Christiansborg, 50 and 2 00; Katunga, O ll and 476; Gaboon, 9 35 and 48; Cape Town, 28 and 3 83; Durban, 5 00 and 170; Pretoria, 6 07 and 071; and Zanzibar, 2 - 02 and 2 35. At Zanzibar the heaviest rains occur about the equinoxes, the mean for April being 14 55 inches, and for October 6 80 inches. In the case of this, as the other continents, the explanation of the different amounts is to be had in the seasonal changes of wind. In the north the winter rains arc to a very large extent the accom paniment of the Mediterranean storms of that season, but in summer pressure is diminished in the interior and increased in the Atlantic to the north-west, resulting in strong steady northerly winds, which as they advance into hotter regions are unaccompanied with rain. The heavy summer rains from Senogambia to the Gold Coast are due to the strong monsoonal winds which set in towards the interior, thus drawing over these coasts the highly saturated air of the belt of calms and of the trades immediately to the north and south of it. Since in winter the belt of calms is removed 8 of latitude farther to the south, and the temperature of the interior is greatly reduced, it follows that the winds blowing on these coasts from the sea are drier and less strong, and consequently the rainfall is small. At Sierre Leone the absolutely driest month is February, 31 inch, and the wettest September, 29 15 inches. On the other hand, at Gaboon (lat. 0* 25 N. ) the dry season is from June to August, when the belt of calms is farthest to the north ; and the absolutely rainiest about the equinoxes, the mean of March being 1470 inches and October 19 52 inches. At Loanda (lat. 8* 49 S.) the annual amount is only a tenth of what falls at Gaboon, and it 20