Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/156

Rh 146 METEOROLOGY [PREVAILING WINDS. highly heated by the summer sun, and, besides, the rainfall over southern parts is excessive. Consequently atmospheric pressure is very low, being fully 40 inch lower in the Punjab than it is in the south of Ceylon. From the interior pressure rises continuously on advancing to the eastward, southward, westward, and northward, and from all these directions the prevailing winds of summer flow inwards upon the interior, and these bring rain or parching drought according to the vapour they bring from the ocean they have traversed, and according as they advance into warmer or colder regions. The prevailing summer winds of Asia, being an inflow inwards upon the interior, have, generally speaking, exactly the reverse direction of that prevailing in winter. The winds of Europe are mainly determined by the extraordinarily high pressure of the Atlantic in its relations to the low-pressure systems of Central Asia and Central Africa at this time. The winds in the Spanish Peninsula are north-west ; in the north of Africa they are northerly, and again north-westerly in Syria. The winds of the British Islands and western Europe have less southing and more northing than the prevailing winds of winter, and to the east of long. 40 E. they become decidedly north-west. It is to the Atlantic origin of these winds that the summer climates of these large and important regions owe the com paratively large rainfall of this season, it being at this time that the rainfall reaches the annual maximum. The bear ing of the low-pressure areas and mountain systems of the north of Italy and Scandinavia on the climates of these countries will be afterwards referred to. The centre of lowest pressure in North America is over the central States about Utah, from which it rises all round, least to northward and most in south-easterly and north-westerly directions. In California N.W. winds necessarily blow in wards upon this central low-pressure area; and, as these winds pass successively over regions the temperature of which con stantly increases, the summer climate is rainless. On the other hand, southerly and south-easterly winds from the Gulf of Mexico blow up the western side of the basin of the Missis sippi inwards upon the low-pressure area of the centre, de positing in their course, in a rainfall more or less abundant, the moisture they have brought from the Gulf. To the north of lat 50, and to westward of Hudson s Bay, the prevail ing winds become easterly and north-easterly, distributing over Manitoba, Saskatchewan, and neighbouring regions, as they continue their westerly course towards the low- pressure area, the rainfall they have transported thither from the wide expanse of Hudson s Bay. An attentive examination of the arrows of fig. 1 7 shows that the prevail ing winds over all the States to the east of the Mississippi river are rather to be regarded as an outflow from the region of very high pressure over the Atlantic to south eastward. Thus in Florida the winds are S.E., in the southern States S., and in the lake region, in the New Eng land States, and on the Atlantic seaboard S.W. Since the origin of these winds is thus essentially oceanic, and since in their course northwards no mountain range crosses their path, the whole of this extensive region enjoys a large but by no means excessive rainfall, which, taken in connexion with the temperature, renders the summer climate of these States one of the best to be met with anywhere on the globe for the successful prosecution of agricultural industries. The remarkable protrusion of high pressures from the southern hemisphere, where they are massed at this time of the year, northwards into the Atlantic is, as has already been referred to, one of the outstanding features of the meteorology of the summer months of the northern hemi sphere. In the central area of this large region the climate is remarkable for its prevailing calms, light winds, occasional squalls, and clear skies. From this comparatively calm space the wind blows outwards in all directions towards and in upon the surrounding regions of low pressure. These winds, owing to the high temperature, clear skies, and strong sunshine of the region from which they issue, carry with them a great amount of vapour near the surface, by which to a large extent the north of South America, the east of North America, the greater part of Europe, and a large por tion of Africa are watered. The prevailing winds over this region are further interesting, not merely from the striking illustration they give of the intimate relation of the winds to the distribution of the pressure, but as being of no small importance in determining the best routes to be taken over this great highway of commerce, and the more so inasmuch as the currents of the ocean are coincident with these pre vailing winds. In the Antarctic regions, or rather to the south of lat. 45 S., the normal atmospheric pressure is low at all seasons, there being a gradual diminution of pressure to 2 9 20 inches about lat. 60 S. Pressure is probably even still lower nearer to the south pole, as seems to be indi cated by the observations made by Sir James C. Ross, and in the &quot; Challenger &quot; and other expeditions. Over this zone the prevailing winds are W.N.W. and N.W. This is the region of the &quot; brave west winds,&quot; the &quot; roaring forties &quot; of sailors, which play such an important part in navigation,, and which determine that the outward voyage to Australia be round the Cape of Good Hope and thence eastward, and the homeward voyage eastward round Cape Horn, the globe being thus circumnavigated by the double voyage. That the general drift of these winds is inwards upon the south pole is strongly attested by the existence of the enormously thick wall of ice which engirdles these regions, from which are constantly breaking away the innumerable icebergs that cover the Southern Ocean, none of which is ever seen of a calculated thickness less than 1400 feet. The snow and rainfall which must take place in the south polar regions for the formation of icebergs of such a thickness must be peculiarly heavy, but not heavier than might be expected from the strength and degree of satura tion of the &quot; roaring forties &quot; which unceasingly precipitate their moisture over these regions. To sum up : so far as the prevailing winds are con cerned, it has been shown that where pressure is high, that is to say, where there exists a surplus of air, out of such a region winds blow in all directions ; and, on the other hand, where pressure is low, or where there is a deficiency of air, towards such a region winds blow from all directions in an in-moving spiral course. This outflow of air-currents from a region of high pressure upon a region of low pres sure is reducible to a single principle, viz., the principle of gravitation. Given as observed facts the differences of pressure, it is easy to state with a close approximation to accuracy what are the prevailing winds, before calculating the averages from the wind observations. Indeed so pre dominating is the influence of gravitation where differences of pressure, however produced, exist that it may practically be regarded as the sole force immediately concerned in causing the movements of the atmosphere. If there be any other force or forces that set the winds in motion independently of the force called into play by differences of mass or pressure, their influence must be altogether insig nificant as compared with gravitation. It has been abundantly proved that the wind does not blow directly from the region of high towards that of low pressure, but that, in the northern hemisphere, the region of lowest pressure is to the left of the direction towards which the wind blows, and in the southern hemisphere to the right of it. This direction of the prevailing wind with reference to the pressure is in strict accordance with Buys