Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/41

Rh during the whole year. The depression round the north pole is divided into two distinct centres, at each of which there is a diminution of pressure greatly lower than the average north polar depression. These two centres lie in the north of the Atlantic and Pacific Oceans respectively. The distribution of pressure in the different months of the year differs widely from the annual average, particularly in January and July, the two extreme months. In January the highest pressures are over the continents of the nor thern hemisphere, and the larger the continental mass the greater the pressure, and the lowest pressures are over the northern portions of the Atlantic and Pacific, Suuth America and South Africa, and the Antarctic Ocean. In the centre of Asia the mean pressure of the atmosphere in this month is fully 30 -400 inches, whereas in the North Atlantic, round Iceland, it is only 2 9 340 inches, or upwards of an inch lower than in Central Asia. The area of high barometer is continued westwards through Central and Southern Europe, the North Atlantic between 5 and 45 N. lat., North America, except the north and north-west, and the Pacific for some distance on either side of 15 N. lat. It is thus an exaggerated form of the high belt of annual mean pressure, spreading, however, over a much greater breadth in North America, and a still greater breadth in Asia. In July, on the other hand, the mean pressure of Central Asia is only 29 468 inches, or nearly an inch lower than during January ; or, putting this striking result in other words, about a thirtieth of the pressure of the atmo sphere is removed from this region during the hottest months of the year as compared with the winter season. The lowest pressures of the northern hemisphere are now distributed over the continents, and the larger the con tinental mass the greater is the depression. At the same time, the highest are over the ocean between 50 N. and 50 S. lat., particularly over the North Atlantic and the North Pacific between 25 and 40 N. lat., and in the southern hemisphere over the belt of high mean annual pressure, which in this month reaches its maximum height. Pressure is high in South Africa and in Australia, just as in the winter of the northern hemisphere pressures are high over the continents. Over the ocean, if we except the higher latitudes, atmospheric pressure is more regular throughout the year than over the land. In the ocean to westwards of each of the continents there occurs at all seasons an area of high pressure, from O lO inch to 30 inch higher than what prevails on the coast westward of which it lies. The distance of these spaces of high pressure is generally about 30 of longitude ; and their longitudinal axes lie, roughly speaking, about the zones of the tropics. The maximum is reached during the winter months, and these areas of high pressure are most prominently marked west of those continents which have the greatest breadth in 30 lat.; and the steepest barometric gradients are on their eastern sides. It is scarcely possible to over-estimate the importance of these regions of high and low mean pressures, from their intimate bearing on atmospheric physics, but more par ticularly from their vital connection with prevailing winds and the general circulation of the atmosphere. This rela tion will be apprehended when it is considered that winds are simply the flowing away of the air from regions where there is a surplus (regions of high pressure) to where there is a deficiency of air (regions of low pressure). Every where over the globe this transference takes place in strict accordance with Buys-Ballot s &quot;Law of the Winds,&quot; which may be thus expressed : The wind neither blows round the space of lowest pressure in circles returning on them selves, nor does it blow directly toward that space ; but it takes a direction intermediate, approaching, however, more nearly to the direction and course of circular curves than of radii to a centre. More exactly, the angle is not a right angle, but from 45 to 80. Keeping this relation between wind and the distribution of pressure in mind, the isobaric lines give the proximate causes of the prevailing winds over the globe, and through these the prominent features of climates. As regards the ocean, the prevailing winds indicate the direction of the drift-currents and other sur face-currents, and thereby the anomalous distribution of the temperature of the sea as seen in the Chili, Guinea, and other ocean currents, and the peculiarly marked climates of the coasts past which these currents flow, are explained; for observations have now proved that the prevailing winds and surface-currents of all oceans are all but absolutely coincident. As regards the annual march of pressure through the months of the year, curves representing it for the different regions of the earth differ from each other in every con ceivable way. It is only when the results are set down in their proper places on charts of the globe that the subject can be well understood. When thus dealt with, many of the results are characterised by great beauty and simplicity. Thus, of all influences which determine the barometric fluctuation through the months, the most impor tant are the temperature, and through the temperature the humidity. Comparing, then, the average pressure in January with that in July, which two months give the greatest possible contrasts of temperature, the following is the broad result : The January exceeds the July pressure over the whole of Asia except Karntchatka and the extreme north-east, the greatest excess being near the centre of the continent ; over Europe to south and east of a line drawn from the White Sea south-westward to the Naze, thence southward to the mouth of the Weser, then to Tours, Bordeaux, and after passing through the north of Spain, out to sea at Coruna ; over North America, except the north-east and north-west. On the other hand, the July exceeds the January pressure generally over the whole of the southern hemisphere, over the northern part of the North Atlantic and regions immediately adjoining (the excess amounting in Iceland to - 397 inch), and over the northern part of the North Pacific and surrounding regions. Thus the pres sure which is so largely removed from the Old and New Continents of the northern hemisphere in July is trans ferred, partly to the southern hemisphere, and partly to the northern portions of the Atlantic and Pacific Oceans. Atmospheric pressure is more uniformly distributed over the globe in April and October than in any of the other months. In May and November, being the months immediately following, occur the great annual rise and fall of temperature ; and since these rapid changes take place at very different rates, according to the relative distribution of land and water in each region, a comparison of the geographical distribution of May with that for the year brings out in strong relief the more prominent causes which influence climate, and some of the more striking results of these causes. This comparison shows a diminu tion of pressure in May over tropical and sub-tropical regions, including nearly the whole of Asia, the southern half of Europe, and the United States. An excess prevails over North America to the north of the Lakes, over Arctic America, Greenland, the British Isles, and to the north of a line passing through the English Channel in a north easterly direction to the Arctic Sea. The excess in the southern hemisphere includes the southern half of south America and of Africa, the whole of Australia, and adjacent parts of the ocean. The influence of the land of the southern hemisphere, which in this month is colder than the surrounding seas, brings about an excess of pressure ; on the other hand, the influence of land over those regions

