Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/155

Rh PREVAILING WINDS.] METEOROLOGY 145 eastwards over the Old Continent. Finally, the same low pressure draws over British America and the United States, by the N.W. winds which it induces, the intensely dry cold air-current of the Arctic regions. At Portland, Maine, which is swept by these cold north-westerly winds, the normal temperature in January is 23 6, whereas at Corunna, on the coast of Spain, in nearly the same latitude, where south-westerly winds from the Atlantic prevail, the mean temperature of the month is 49 l, or 25 5 higher. The region of low atmospheric pressure in the north of the Pacific is accompanied by prevailing winds over the region embraced by it and by climatic effects in all respects similar to the above. In Vancouver Island the prevailing winds in January are S.W., at Sitka E.S.E., on Great Bear Lake E.N.E., in Alaska N.E., in Kamchatka N.N.E., and in Japan N.W. In accordance with these winds the winter climate of Vancouver and adjoining regions is mild and humid, and that of the north-east of Asia dry and intensely cold. On the other hand, abnormally high pressure rules over the continent of Asia at this season, and as regards this region of high pressure the arrows represent the winds as blowing outwards from it in all directions. Over the interior of Asia, where the highest normal pressures are, observations show a marked prevalence of calms and light winds, but around this central region the prevailing winds in January are at Calcutta N., at Hong-Kong E.N.E., at Peking N.W., on the Amur W.N.W., S.E. at Nijni- kolynisk and S.S.W. at Ustjansk (in the north of Siberia), and at Bogoslovsk S.W. Hence from this extensive region, where pressure is abnormally high, or where at this season there is a large surplus of air, the prevailing winds flow outwards in all directions towards the lower pressure which surrounds it. Owing to the excessive dry- ness of the air of Central Asia, terrestrial radiation is less obstructed there than anywhere else on the globe, and consequently the temperature falls very low, the mean of January at Werchojansk being 5 5 *8, which is the lowest mean monthly temperature known to occur on the earth s surface. And, since the winds blow outwards from the dry cold climates of the interior, temperatures are low, even on the coasts. Of this China affords good illustrations. Thus the mean January temperature of Peking is 22 0&amp;gt; 7 and of Zi-ka-Wei, near Shanghai, 35 4, whereas at Corfu and Alexandria the normal temperatures for January are respectively 50 9 and 58 0, or 28 2 and 22 6 higher than in corresponding latitudes on the coast of China. The winds of the United States in winter, taken in connexion with the peculiar distribution of pressure already described, are very interesting. There are two regions of high pressure, one in the south-eastern States and the other and larger one in the States around Utah ; and between these there is interposed a trough of lower pressure extend ing from Chicago to the south-west of Texas. On the western side of this depression the winds are north westerly, but to the east of it they become W., W.S.W., and in some places S.W., and again on nearing the Atlantic seaboard they become north-westerly. In connexion with the region of higher pressure in the west, the prevailing winds are seen to flow outward from it. The normal pressure diminishes everywhere to southward of a line drawn from the Canaries to Bermuda, thence westward in nearly the same latitude to Texas, and then to west-north west to San Francisco. The tract of lowest pressure stretches from the basin of the Amazon in the direction of the isthmus of Panama in about latitude 8 N., and thence is continued westward for a considerable distance into the Pacific in nearly the same latitude. It follows from this distribution of the pressure that the north trades in a more or less modified form prevail over South America to the north of the Amazon, and in the Pacific to the north of lat. 8 N., probably as far to westward as long. 150 W. The low-pressure systems which prevail during the summer months in South America and South Africa have each its corresponding system of winds all round. It is, however, in Australia, as being the most compact and isolated continent, that the influence of the summer sun in lowering the pressure is best illustrated. In that continent the lowest pressure occurs in the region situated about midway between the north coast and the tropic of Capricorn, over which the normal pressure does not exceed 29 80 inches. Further, everywhere in Australia pressure diminishes from the coast on advancing upon the inland districts. It follows from this disposition of the pressure that all round the island the prevailing winds in summer blow from the sea towards the interior ; and accordingly it is in these months that the greater part of the rain falls. From the low pressure of the interior southwards to Bass s Straits pressure rises continuously, the increase in the normal over this space being about 200 inch. To north ward it also rises continuously to beyond the north of China, the increase on this side being about f of an inch. In this case the greater part of the increase occurs over the continent, the rate of increase from the north of Australia to the Philippine Islands being only about the rate of increase which obtains southward towards Bass s Straits. It will be shown when the subject of the rainfall is examined that it is the relative excess of these high pressures, the one in the south of Australia and the other in the south-east of Asia, that determines the position of the area of low pressure in Australia in particular years, and with that position the degree and extent to which the whole of the northern portion of Australia is watered by the rainfall. Thus, when pressure is more than usually high in the south-east of Asia, and either low or not ex cessive in the south of Australia, then the low-pressure region is pushed farther southward into the interior, and with it the rainfall spreads inland over a wider area and to a greater depth. Prevailing Winds in July. -In the winter of the southern hemisphere, the geographical distribution of pressure is exactly the reverse in Australia of what obtains during the summer months. Everywhere all round it increases on advancing from the coast into inland districts. The lowest pressure, about 30 00 inches, occurs on the north coast, and the highest over the basin of the Murray river and its affluents, where it rises generally to 30 18 inches. On the south coast it is generally about 30 12 inches, falling, however, at Gabo Island, in the extreme south-east, to 30 050 inches, and to 29 836 in the south of New Zealand. From the Murray river the diminution of pressure is continuous to the north, even to the low pressure of Central Asia. From this arrangement of the pressure, the prevailing winds blow from the interior towards the surrounding ocean all round Australia, with the single exception of the extreme south-west of the continent, where the prevailing winds are south-westerly, being here essenti ally an outflow of the high pressure which overspreads the Indian Ocean to the westward. As these S.W. winds are from the ocean, the rainfall at Perth in July is fully 6 inches, and it is high over south-western districts of West Australia. The prevailing winds round Australia are S.E. on the north coast, S.W. at Brisbane, W.N.W. at Sydney, N. at Melbourne, N.E. at Adelaide. These all represent an outflow from the high-pressure regions of the interior modified by the influence of the earth s rotation, and, in correspondence with the reversal of the distribution of the pressure, are directions the reverse of the prevailing winds of January. In July the central and southern parts of Asia are XVI. 19