Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/150

Rh 140 [ATMOSPHERIC Pressures greatly under the average cover the northern portions of the Pacific and Atlantic and also the greater part of the Arctic regions. In the north of the Pacific the normal pressure falls to about 29 6 inches between Kamchatka and Alaska. In the north of the Atlantic, however, a still lower mean pressure obtains over a narrow belt stretching from Iceland to the south of Green land, the normal at Stykkisholm in the north-west of Iceland being 29 385 inches, and at Ivigtut in Greenland 29 361 inches. This low average for Ivigtut is the lowest normal known to occur any where and at any season in the northern hemisphere, and it is significant that the place is immediately to the north of that part of the Atlantic where a considerable number of the storms which sweep over Europe have their origin, and where not a few of the storms which cross the Atlantic from America develop intensity. It has been seen that the highest mean pressure occurs near the centre of the largest extent of land ; but as regards the two oceans the lowest pressure is met with in the northern division of the Atlantic, which is the lesser ocean. An inspection of fig. 14 shows, however, that the low-pressure area of the Atlantic is bounded to southward by systems of much higher pressures than are to be found in the Pacific. The result of this arrangement is that much stronger winds blow northward over the Atlantic and round upon Iceland ; and, as these more quickly advance into colder latitudes, there is thus a greater and more frequent concentration of vapour and lowering of the barometer in the north of the Atlantic. The heavy rainfall of north-western Europe may be referred to as con firming this view. A belt of low pressure passes through the equatorial regions quite round the globe. This marks the well-known region of calms towards which on either hand the trade winds blow. In the Atlantic it lies quite north of the equator even in January, when the sun s course is farthest to southward, and it lies nearly parallel with the equator. On the other hand, in the Indian Ocean the position of the line of lowest pressure is to the south of the equator and not parallel with it, but taking a slanting course from near the north of Madagascar towards Sumatra, thence towards the low pres sure which prevails at this season in Australia ; its course is then a little to northwards, and crosses the Pacific to the central regions of South America. Its path is thus a devious one, being north of the equator only in the eastern part of the Pacific and in the Atlantic, but elsewhere to the south of it, being drawn farthest southward when under the influence of the regions of low pressure which now occupy central Australia, central and southern Africa, and central South America. In this trough of barometric depression nearly all the tropical storms of the Indian Ocean have their origin. There are several important modifications of the isobaric lines as originally published. In 1868 the region of lowest pressure in the northern hemisphere in winter was represented as extending from Iceland to north-eastward ; now the area of lowest pressure is seen to extend from Iceland south-westward to Greenland. In connexion with this point Captain Hoffmeyer discussed the weather of the North Atlantic during several winter months, and published the results in 1878, which conclusively showed that the meteorology of Greenland and Iceland exerts on the distribution of atmospheric pressure a powerful influence not before properly recog nized, resulting in the mean minimum of pressure being localized distinctly to the south-west of Iceland, and that in addition to this minimum there are two subordinate minima, one in Davis Straits and the other in the Arctic Ocean midway between Jan Mayen and the Lofoten Isles. The investigation further established the fact that, when any particular one of these three minima plays an important part, the other two either do not appear at all or occupy quite a subordinate place, and that according as one or other of these minima of pressure predominates so is the character of the weather, as regards mildness or severity, of the winter of north western Europe and regions surrounding the North Atlantic. As regards the British Islands, the displacement of the minima to west ward of the position shown in fig. 14 means milder winter weather, whereas a position more in the direction of the north of Norway means severer winter weather. Another change implying important consequences is seen in the United States, where, instead of one, two distinct centres of maximum pressure occur, or rather the high pressure of the western and central States is separated from that of the south eastern States by a region of lower pressure occupying the region of the Mississippi States. Professor Loomis first drew attention to this peculiarity in 1879 in an inquiry into the distribution of pressure over the United States, and established the fact that there are two distinct areas of high pressure, the larger having its centre in Utah, and the less overspreading the greater portion of the south eastern and southern States, and that these two areas of high pres sure are clearly separated from each other by a broad extensive region of lower pressure stretching in a south-western direction from the region of the great lakes to western Texas. The reason assigned by Professor Loomis is undoubtedly correct, that the relatively low normal pressure of the Mississippi States is clue to the fact that the path usually taken by the barometric minima of American storms in the earlier part of their course is from Texas to the lakes. Since, on the other hand, the centres of comparatively few storms, with their low barometer readings, cross the southern and south-eastern States, the normal winter pressure is higher there than it is along the Mississippi. Another important modification occurs in India, where the isobar of 30 inches is deflected to the south-east toward Madras and thence towards the north-east to near Akyab in Arakan. This remarkable deflexion well shows the important influence exerted on the course of the isobar by large well-defined sheets of water and extensive L ncts of land. The distribution of pressure here indicated, by which south of lat. 22 the normal pressure is considerably higher in the east than in the west of India, has, through the agency of the winds resulting from it, the most intimate and vital bearings on the distribution of the winter rains and temperature over con siderable portions of India ; and the same relations hold, but in a degree still more striking, in the meteorology of Ceylon. The remarkable effect in interrupting or changing the course of the isobars is particularly well illustrated by the lines in the region of the Aral, Caspian, and Black Seas. As the point is of no small importance in meteorology, and is best illustrated by the Mediter- FIG. 15. Isobars of Europe for January. ranean and the countries to the north of it, we subjoin a map of this part of Europe (fig. 15), showing the normal pressure with greater distinctness and fulness than could be shown on fig. 14, the isobars being drawn for every half-tenth of an inch. Here we see two distinct areas of high pressure, the one in Hungary and the other in the Peninsula, where the normal pressure exceeds 30 - 20 inches. The latter is the larger of the two, and may be regarded as the prolongation of the region of high pressure which characterizes the Atlantic immediately to the south-west at this season. The high-pressure area included within the isobar 30 15 inches is of peculiar interest. In the Peninsula it covers a pretty broad area, but to the north-east it contracts to a narrow nt -k between the Bay of Biscay and the Gulf of Lyons, and again expands to north-eastward covering the distance from Carlsruhe to Modena, its prolongation eastward being there somewhat suddenly inter rupted. At some distance to the eastward the second region of high pressure is met with, which is properly a part of the high pressure that overspreads the interior of the Old Continent in the winter months, its western limit being the isobar of 30 15 inc-hes, which passes round by Pinsk, Cracow, Vienna, Laibach and the upper southern slopes of the basin of the Danube, Scbastopol, and thence southward in the direction of Cyprus. The position of the latter of these regions of high pressure is approximately midway between the south coasts of Asia Minor and the Baltic. In other words, its position occupies the interior of this part of the Old Continent ; and it is instructive to note that the position of the Black Sea and the Greek archipelago in the south portion of this region pushes the isobar of 30 15 inches a good deal to northward. The position of the region of high pressure in the Peninsula, France, and Switzerland is also decidedly inland. It does not, however, exactly occupy the middle space of the land lying between the Mediterranean and the North Sea, owing no doubt to the circumstance that the very steep barometric gradient from France to Iceland greatly lowers the pressure over the whole of the northern half of France. It follows that the abnormally high pressure which so remarkably characterizes the interior of the Old Continent during the cold months of winter is represented, though in a greatly reduced form, westwards through the central districts of that continent. These two regions of high pressure are separated from each other