Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/147

Rh TEMPERATURE.] METEOROLOGY 137 low pressure about Iceland during the winter months (see fig. 14). Since this region of low pressure gives to western Europe its prevailing south-west and south winds, and to North America its north-west winds in winter, it is plain that the temperature of western Europe is thereby abnormally raised by the simple fact of its prevailing winds coming from the ocean and from lower latitudes, and that the temperature of North America is abnormally lowered by its prevailing winds coming from the Arctic regions and from land. The opposite action of these two winds, which are part and parcel of the same atmospheric disturbance about Iceland, is shown from the fact that, while the mean temperature of the south coast of Hudson s Bay in January is 20, in the same latitude in the Atlantic to the west of Scotland it is as high as 44, or 64 higher. A similar though less striking result accom panies the low-pressure area in the north of the Pacific in winter. Another area of low mean pressure which powerfully affects the temperature is the low barometer which over spreads the interior of Asia during the summer months (see fig. 17). Since from this disposition of the pressure the prevailing winds of Europe and western Asia are north west and west, and over eastern Asia south-east and east, it follows that the temperature is abnormally raised on the eastern side and depressed on the western side of the con tinent by the direction from which they severally receive their prevailing winds. This is well shown by the course of the summer isothermals of 80, 70, 60, and 50 across the Old Continent. Since the strongest insolation occurs where the air is driest, the hottest summer climates are met with in those tropical and subtropical regions where no rain falls. The most extensive of the rainless regions during the summer months is perhaps that which extends from the Punjab westwards through Persia, Arabia, and North Africa to Spain. This is the region where the hottest climates of the globe are to be encountered. Similarly no rain falls at this time of the year in lower California and the States adjoining, and this feature of the climate, taken in con nexion with the relatively low temperature of the coast due to the winds and ocean currents from the north which sweep past it, results in sharp contrasts of temperature within short distances such as have no parallel in any other climate. Of the areas of seasonal high mean pressure, the high barometer of Central Asia in winter stands out in character istic prominence (see fig. 14). Now, since the prevailing winds which necessarily form a part of this feature are south and south-west over Russia and western Siberia, the tempera ture of these inland regions is considerably higher than would otherwise be the case. On the other hand, since the prevailing winds are north-west in eastern Asia, the tempera ture of these regions is thereby abnormally depressed. It is this consideration chiefly which explains how it is that, while the mean January temperature in latitude 60 and longitude 120 E. is -30, in the same latitude but in longitude 43 E. the mean temperature is 10, or 40 higher, even though both regions are equally continental in their character. The high mean pressure in the summer in the Atlantic between Africa and the United States has with its system of winds the most decided influence in bringing about the abnormal distribution of the temperature of that and adjoining regions. Since on its west side the prevailing winds are necessarily southerly, the temperature of that region is abnormally raised, and, on the other hand, since on its east side the winds are northerly, the temperature of the region is abnormally depressed. The result of these two opposite winds is seen in the slanting direction of the isothermal of 80 across the Atlantic, which slanting direction is continued far into the interior of North America for the reasons already stated. These important bearings of cyclonic and anticyclonic areas on temperature and climate may be thus summarized. The temperature is abnormally raised on the east sides of cyclonic areas, and abnormally depressed on the west sides ; but, on the other hand, temperature is abnormally raised on the west sides of anticyclonic areas and depressed on their east sides. In the southern hemisphere these direc tions are reversed. Another set of influences, powerfully affecting the tem perature, come into play where the surface of the land rises above the sea into elevated plateaus, lofty peaks, or mountain ranges. Thus it has been observed on Ben Nevis and other mountains that the wind during the day in summer exhibits an ascensional tendency due to the circumstance that the temperature of the surface of the mountain is heated in a much greater degree than the air strata at the same levels all around it. An ascensional current consequently rises from the mountain, which is maintained at a steadily stronger rate than at lower levels, because the drain from the updraught is easily supplied from the free surrounding atmosphere. It is the strong insolation at high elevations in the summer months which explains the excessively high day-temperatures encountered in the Rocky Mountains ; and from the same conditions, viz., the rarity and purity of the atmosphere, by which terrestrial radiation is but little checked, come the low temperatures of the nights of these climates in the same season. From this cause it follows that the elevated lands in the interior of continents tend to reduce mean atmo spheric pressure in summer to a greater extent than would otherwise be the case. In winter, on the other hand, the temperature of elevated regions in the interior of continents is very much colder than that of the surrounding atmosphere at the same heights, because in such regions the air is exceedingly dry and rare, and consequently radiation to the cold regions of space but little checked. Hence down the slopes of these high lands there are poured in all directions descending currents of very cold air, which intensify the rigours of the winters experienced on the low lands round their base, where accordingly the lowest mean winter temperatures occur. These elevated lands thus materially add to the high atmospheric pressure of the interior of continents during the cold months of the year. But it is ocean streams and ocean currents which produce the greatest abnormalities in the distribution of the tempera ture of the air, and a glance at figs. 10 and 11 will show that it is in the North Atlantic where this cause is most strikingly seen. The increase thus accruing to the winter temperature is greatest about the north of Norway. It is also very great in the British Islands ; thus, if no more heat were received than is due to their position on the globe in respect of latitude, the mean winter temperature of Shetland would be 3 and that of London 17. But mainly owing to the heat given out by the Gulf Stream and other warm currents of the Atlantic their mean winter temperatures are respectively about 39 5 and 39, Shetland being thus benefited 36 5 and London 22. The chart of the winter temperature of the British Islands well illustrates the influence of the surrounding ocean in maintaining a higher temperature. It will be seen that the south-west of Ireland is 7 warmer than the east coast of England in the same latitudes. The strong drift current from near Behring s Strait southward along the coast of America has a powerful influence, particularly in lowering the summer temperature of that coast, thus bringing about, in con junction with the dry rainless climate of the interior, what are perhaps the most violently contrasted climates, within XVI. 1 8