Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/149

Rh ATMOSPHERIC PRESSURE.] METEOROLOGY 139 the cold months and low pressures during the warm months of the year. The maximum cloud occurs with winds from the sea and winds advancing into the colder regions of higher latitudes, and the minimum with winds which have traversed an extensive track of land and winds advancing into the warmer regions of lower lati tudes. As the subject, however, is essentially one with rainfall, it is not necessary to prosecute it further. The other atmospheric movements on which the amount of cloud depends are the ascending and descending currents of the atmosphere, the ascending currents wi+li. clouded skies occurring in the belt of calms and over cyclonic areas and regions, and the descending currents with compara tively clear skies over anticyclonic regions. The region of maximum vapour and densest cloud-screen on the globe is the equatorial belt of calms between the trades, which has an annual movement northward and southward with the sun as already explained. To ascensional movements is to be ascribed part of the cloudiness of the southern and eastern sides of the winter cyclonic regions of the North Atlantic and North Pacific, and of the cyclonic regions of low summer pressure in the interior of Asia and other continents. On the other hand the comparatively small amount of cloud in the anticyclonic regions of the Atlantic and Pacific Oceans, and in the high-pressure regions of the interior of Asia and other continents during the cold months of the year, is due to the vast down-currents which occupy the centres of the anticyclones, and which become relatively drier as they descend owing to the increasing pressure to which the air is subjected. Distribution of Atmospheric Pressure. The importance of a knowledge of the distribution of atmospheric pres sure, or of the mass of the atmosphere, over the globe in its varying amounts from month to month is self- evident. Observations teach us that winds are simply the movements of the atmosphere that set in from where there is a surplus towards where there is a deficiency of air ; and observations also teach that isobaric maps (i.e., maps showing the relative distribution of mean pressure) and maps showing the prevailing winds are in accordance with each other. Since prevailing winds to a large extent determine the temperature and rainfall of the regions they traverse, isobaric maps may be considered as furnish ing the key to the more important questions of meteoro- 20 4Q 6O 9O IOO I2O IO I8O ICO FIG. 14. January Isobars of the Globe and Prevailing Winds. logical inquiry. At the time of the first publication of isobaric maps of the globe in 1868, it was impossible to do more than present the subject in its broad general features, owing to the scantiness and quality of the materials then existing. But since then meteorological stations have been largely multiplied in all parts of the civilized world, and the general adoption of the issue of storm warnings has necessitated the use of more accurate barometers and uniform methods of observing. Since there is thus now the means of a more exact representation of this fundamental datum of meteorology, we have prepared a new set of isobaric maps, showing the distribu tion of the earth s atmosphere and the prevailing winds for January (fig. 14), July (fig. 17), and the year. They have been constructed from mean values calculated for the same eleven years (1870-80 inclusive) as the isothermal maps figs. 10 to 13, pressure of 30 inches and upwards being represented by solid lines, and of 2 9 9 inches and under by dotted lines, while the arrows show the directions of the prevailing winds at the localities indicated by the respective arrow-points. Mean Atmospheric Pressure in January (fig. 14). In this month, when the influence of the sun on the northern hemisphere falls to the minimum, the greatest pressures are massed over the continents of that hemisphere, and the least pressures over the northern parts of the Atlantic and Pacific Oceans, over the Antarctic Ocean and southern hemisphere generally. In the southern hemisphere there are three patches where pressure rises to 30 inches, viz., in the Atlantic between South America and Africa, south of the Indian Ocean, and in the Pacific between Australia and South America. _ In the northern hemisphere, on the other hand, pressure rises in Central Asia to upwards of 30 5 inches, the mean pressure for January being at least 30 4 inches at Peking, Semipalatinsk, and Yenisei, and fully 30 &quot;5 inches at Irkutsk and Nertchinsk, in the upper basin of the Amur. This is the region where the normal atmospheric pressure attains to a maximum which is much higher than is reached in any other region or at any other time of the year. It will be observed that this region of highest pressure occupies a position near the centre of the largest continent. The area of high barometer is continued westward through Europe, through the horse latitudes of the Atlantic to Carolina, and thence through the United States to California, whence it crosses the Pacific to Asia. This belt of high pressure thus completely encircles the globe, broadening as it passes the land and contracting as it crosses the ocean. Its greatest breadth is over Asia and its least over the Pacific, or where land and ocean attain respectively their maximum dimensions.