Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/151

Rh PRESSURE.] METEOROLOGY 141 by a large area of comparatively low pressure overspreading the greater portion of the Mediterranean Sea, marked oil in fig. 15 by the isobar of 30 10 inches, within which pressure is everywhere less than 30 10 inches. This region includes an area of still lower pressure within the isobar of 30 05 inches, bounded by Sicily, Corfu, Athens, and Crete. Hence the singularly low pressure which characterizes the northern part of the Atlantic at this season has its analogue in the south of Europe, which is unquestionably due to the higher temperature and larger humidity of the climates of southern Europe which they owe to the Mediterranean. It is deserving of special notice that, while the increase of the normal pressure of January from Genoa to Geneva is 081 inch, it is only 021 inch from Trieste to Kiva, and that to the north of the Adriatic as far as latitude 50 pressure is considerably lower than obtains to the west and east of that region. An examination of the daily weather maps of Europe shows that not unfrequently the storms of north-western Europe on advancing as far to east ward as Denmark seem to connect themselves in some degree with Mediterranean storms prevailing at the time through a north and south prolongation of a system of low pressures. The comparative frequency with which this occurs is probably occasioned by the general drift to eastward of the atmosphere of Europe, considered as a whole, taken in connexion with the high mountainous ridge which bounds the Adriatic on its eastern side, from which it follows that the air overspreading the deep basin of the Adriatic is often highly saturated with vapour, and this highly saturated air is drawn northwards through central Europe when north-western storms of Europe with low barometric depression centres pass across Denmark and the Baltic. Thus the low normal pressure to the north of the Adriatic, separating the two regions of high pressure to the east and west of it, is in some respects analogous to the low normal pressure of the Mississippi valley, which separates the higher normal pressures of the Rocky Mountains and of the south eastern of the United States. The influence of land and water respectively in the cold season of the year is well shown in fig. 16, which represents for every 020 inch the normal pressure over the British Islands in January, drawn from means calculated for two hundred and ninety-five stations. 1 It is in the winter months that the isobars of the British Islands crowd most closely together, and in accordance therewith strong winds are then most prevalent. The crowding of the isobars reaches the maximum in January, forming what is probably the steepest mean monthly barometric gradient that occurs at any season anywhere on the globe. The point, however, to which attention is here drawn is the remarkable influence of St George s Channel and the Irish Sea in diminishing the pressures as they cross these seas, and of the land in increasing the pressure, which is seen in the curves occupying approximately the central districts of Great Britain from the Isle of Wight to Cape &quot;Wrath. This shows on a comparatively small scale the influence of the land in FIG. 16 Isobars of the British Islands for January, raising the normal pressure, and of the sea in lowering it, during the FIG. 17. July Isobars of the Globe and Prevailing Winds. cold months of the year, just as is seen on the grand scale in central Siberia and the north of the Atlantic. 1 See Journal of Scot. Meteorological Society, vol. vi. pp. 4-21. Mean Atmosplicric Pressure in July (fig. 17). In this month the physical conditions are the reverse of what obtains in January, the effects of the influence of the sun on the temperature and humidity of the atmosphere rising to the maximum in the northern and fall-