Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/152

142 METEOROLOGY [ATMOSPHERIC PRESSURE. ing to the minimum in the southern hemisphere. With the solar conditions reversed, a comparison of iigs. 14 and 17 shows that the distribution of atmospheric pressure in July is, considered in a broad sense, the reverse of what takes place in January. In the southern hemisphere atmospheric pressure during the winter season is above the general average of 30 inches between lat. 10 and 40 S. This belt of high pressure encircles the globe, and embraces four regions where pressure rises considerably above this general higli average. These regions are in South Africa, about lat. 20, where it rises to a little above 30 20 inches; in Australia, where it rises on the Murray river very nearly to 30 20 inches ; in South America, where in the basin of the La Plata, about lat. 30, it rises to 30 13 inches ; and in the ocean to westwards, where it reaches 30 02 inches. The point to be noted with respect to the position of these centres of high pressure at this season is that they occur over surfaces between latitudes 20* and 36. As compared with January, pressure in July over nearly the whole of this broad belt of the southern hemisphere is about two-tenths of an inch higher, which is the simple result of season. A comparison of January and July shows that this large accession to the pressure of the southern hemisphere is accompanied by an extraordinary dimi nution of pressure over the continents of the northern hemisphere. Now, just as the greatest excess of pressure during the winter of the northern hemisphere occurs in the continent of Asia, so the greatest diminution of pressure in the summer months takes place in the same continent. The position, however, of these two extremes is far from being in the same region or even near each other. In the Old Continent the maximum occurs in the valley of the upper Amur, where, at Nertchinsk, the normal pressure in January is about 30 500 inches ; whereas the lowest normal pressure in July is 29 412 inches, and occurs, so far as observation enables us to locate it, at Jacobabad on the west side of the basin of the Indus. The difference of these two normals is 1 "188 inch ; and over no inconsider able portion of central Asia the normal pressure of July is an inch less than that of January. In other words, the influence of the sun in summer as exerted on the temperature and aqueous vapour of the atmosphere and atmospheric movements resulting therefrom is so powerful as to remove a thirtieth part of the whole mass of the air from this extensive region. The large extension in recent years of good meteorological stations over the Russian and Indian empires enables us to lay down with much greater precision than formerly the lines of pressure. Of the changes indicated by the new isobars, the most important perhaps is the position of the region of minimum pressure in Asia, which is now seen to occupy the basin of the Indus, and thence stretches over a somewhat broad region to westward nearly as far as the head of the Persian Gulf. The point is of no small importance in atmo spheric physics, inasmuch as it places the region of least normal pressure in July as close geographically to the region where at the time terrestrial temperature is highest as the region of highest normal pressure in January is situated with respect to the region where in that month terrestrial temperature is lowest in Asia. The July isobars of India are of singular interest, and imply con sequences of the utmost practical advantage to the empire. From Cutch southward the normal pressure is everywhere higher, and considerably so, along the whole of the west than it is in the east in the same latitudes, the difference being approximately half a tenth of an inch. This is represented on the map by the slanting of the isobars from north-west to south-east as they cross this part of India ; and it is to be noted that the cast and west coasts of Ceylon show the same manner of distribution of the pressure. The conse quence of this peculiarity in the distribution of the pressure is that the summer monsoon blows more directly from the ocean over western and southern India than would have been the case if the isobars had lain due east and west, and thus probably precipitates in its course a more abundant rainfall over this part of the empire. P>ut a more important consequence follows from the geographical distribution of the pressure over the valley of the Ganges. If the normal pressure there had diminished in the manner it does over India to the south of the Gangetic valley, the winds would have been south-westerly and the summer climate practically rainless. This, however, is not the case, but the normal pressure diminishes westwards along the valley of the Ganges, as the following mean July pressures will show: Calcutta, 29 576 inches ; Patna, 29 535 inches; Lucknow, 29 522 inches ; Roorkee, 29 -;>05 inches ; and in crossing westward into the Punjab pressure falls still lower to 29-439 inches at Mooltan and 29 412 inches at Jacobabad. Indeed pressure in July is 220 inches lower at Jacobabad than at Sibsagar on the Brahmaputra, nearly in the same latitude. It necessarily follows from this distribution of the pressure that the summer mon soon, which blows northward over the Bay of Bengal, is deflected into an E.S.E. wind which fills the whole valley of the Ganges, distributing on its way a most generous rainfall over that magnifi cent region. The influence of the land in lowering the pressure in summer is well illustrated by the course of the isobars over western Siberia and Russia, where pressure is seen to fall relatively lowest along the middle line of the Old Continent. la this connexion it is interesting to note the course of the isobar of 29 90 inches over that part of Europe where the breadth of the land is considerably increased between the Baltic and Constantinople. In contradis tinction to this the influence of the Aral, Caspian, and Black Seas in maintaining a higher pressure appears in the remarkable pro longation eastward of the isobars of higher pressure over the region of these seas, being in striking contrast to the lower pressures which prevail to the north and south. The lowering of the normal pressure is very decided in the inland regions of Spain, North Italy, and Scandinavia. The effect is most strongly seen in Spain, the largest and compactest of these regions. Thus, while the normal pressure diminishes between Lisbon and Barcelona from 30 086 to 30 048 inches, the sea-level pressure at Madrid falls nearly to 30 000, and the pressure at Saragossa and Valladolid is nearly as low. This lowering of the pressure over the interior influences materially its summer climate. As remarkable an illustration of the principle as can be pointed to anywhere is seen in the north of Italy ; for, while the normal pressure at Moncalieri is 29 941 inches, at Genoa on the coast the relatively high normal of 29 992 inches is maintained, the distance of the two places being about 40 miles. To the east pressure rises to 29 - 970 inches at Venice, and to westward to 30 023 inches at Geneva. Over Scandi navia, along the west coast from the Arctic circle southward, the normal pressure equals or exceeds 29 80 inches, the variation being comparatively small ; and along the coast from the head of the Gulf of Bothnia to the south-east of Sweden pressure also exceeds 29 80 inches, and the increase from north to south proceeds at a slow rate. In, however, the strictly inland districts to the north-east of Christiania, which lie immediately to the east of the Scandinavian mountains, and sheltered by that lofty range from the winds of the Atlantic, pressure is considerably lower than it is along the east and west coasts of the peninsula. Owing to this peculiar distribution of the pressure, the winds which necessarily result from it give a much finer summer climate to the south-east of Norway and to the strictly inland part of Sweden than would otherwise be the case. The remarkable curving northward of the isobar of 29 80 inches so as to include Lapland within it points probably to the influence of the White Sea and the wonderful lake system of Lapland in maintaining a higher summer pressure over that country, by which the northerly winds that blow towards the low-pressure region of Central Asia, to the serious deterioration of the summer climate of northern Siberia, do not extend so far to westward as Lapland. The distribution of the normal pressure over North America is quite analogous to what prevails over Asia, but, the continent being less, the diminution of pressure in the interior is also correspond ingly less. The highest normal pressure, 30 077 inches, is found in the south-east in Florida, and the lowest, 29780 inches, in Utah, the difference being thus 297 inch. Another region of relatively high pressure is in the north-western States and British Columbia to the north ; the maximum, near the mouth of the Columbia river, reaches 30 062 inches, being thus nearly as high as what occurs in Florida. These two regions are merely extensions of important high-pressure areas which at this season are highly characteristic features of the meteorology of the North Pacific and North Atlantic respectively. Of these two regions of high pressure the one overspreading the Atlantic between the United States and Africa is the more striking, being not only the region where pressure is highest anywhere on the globe during the months of June, July, and August, but where the normal pressure reaches the highest point attained at any season over the ocean. The highest point reached by the normal pressure over the land at any season occurs, as has been pointed out, near the centre of Asia, or approximately in the middle region of the largest continuous land surface on the globe during the coldest months of the year. On the other hand, the highest pressure over the ocean occurs during the warmest months of the year, and not over the largest water surface, but in the middle regions of the North Atlantic, where the breadth is only about half that of the water surface of the North Pacific. From the essential differences between these two sets of pheno mena it may be inferred that the extraordinarily high pressure which is so marked a feature of the meteorology of Central Asia during the cold months of the year is a direct consequence of the lowering of the temperature of the land of Asia and of the atmo sphere resting on it during the time of the year when the effects of solar radiation are at the annual minimum, and of terrestrial radia tion at the annual maximum. But the determination of the place and time of highest pressure over the ocean must be regarded as indirectly brought about. The physical conditions under which it occurs are these : it happens (1) at the time of the year when the earth presents the largest surface of land to the sun, and (2) over that part of the ocean which is most completely surrounded by these highly heated land surfaces. This high summer pressure of the Atlantic has its origin in the upper currents of the atmosphere. Mean Atmospheric Pressure for the Year. The distribution of

the annual atmospheric pressure may be considered as representing