Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/16

6 come charged with the temperature and moisture of the regions they have traversed, it is evident that isobaric charts, showing the mean pressure of the atmosphere, form the key to the climates of the different regions of the globe, particularly those different climates which are found to prevail in different regions having practically the same latitude and elevation. This principle is all the more important when it is considered that the prevailing winds determine in a very great degree the currents of the ocean, which exercise so powerful an influence on climate. Since winds bring with them the. temperature of the regions they have traversed, southerly currents of air are warm winds, and northerly currents cold winds. Also since the temperature of the ocean is more uniform than that of the land, winds coming from the ocean do not cause such variations of temperature as winds from a continent. As air loaded with vapour obstructs both solar and terrestrial radiation, when clear as well as when clouded, moist ocean winds are accompanied by a mild temperature in wintsr and a cool temperatura in summer, and dry winds coming from continents by cold winters and hot summers. Lastly, equatorial currents of air, losing heat as they proceed in their course, are thereby brought nearer the point of satura tion, and consequently become moister winds ; whereas northerly currents acquiring greater heat in their progress become drier winds. It follows from these relations of the wind to temperature and moisture that the S.W. wind in the British Isles is a very moist wind, being both an oceanic and equatorial current ; whereas the N.E. wind, on the other hand, is peculiarly dry and parching, because it is both a northerly and continental current. Owing to the circumstance of atmospheric pressure diminishing from the south of Europe northwards to Iceland, it follows that S.W. winds are the most prevalent in Great Britain ; and since this diminution of pressure reaches its maximum amount and persistency during the winter months, S.W. winds are in the greatest preponderance at this season ; hence the abnormally high winter temperature of these islands above what is due to mere latitude. The msan winter temperature of Lerwick, Shetland, in respect of latitude alone would be 3, and of London 17, bat owing to the heat conveyed from the warm waters of the Atlantic across these islands by the winds, the temperature of Shetland is 39&quot; and of London 38. In Iceland and Norway the abnormal increase of temperature in winter is still greater. This influence of the Atlantic through the agency of the winds is so pre ponderating that the winter isothermals of Great Britain lie north and south, instaad of the normal east and west direction. This peculiar distribution of the winter temperature of the British Isles has important bearings on the treatment of diseases. Sinca the temperature of tha whole of jthe eastern slope of Great Britain is the same, it is clear that to those for whom a milder winter climate is required a journey southward is attended with no practical advantage, unless directed to the west coast. As the temperature on the west is uniform from Shetland to Wales, Scotland is as favourable to weak constitutions during winter as any part of England, except the south-west, the highest winter temperatures being found from the Isle of Wight westward round the Cornish peninsula to the Bristol Channel ; and from Carnsore Point in Ireland to Galway Bay the tempera ture is also high. The height and direction of mountain ranges form an important factor in determining the climatic characteristics of prevailing winds. If the ranga be perpendicular to the winds, tha effect is to drain the winds which cross them of their moisture, thus rendering the winters colder and tha summers hotter at all places to leeward, as compared with places to windward, by partially removing the protecting screen of vapour and thus exposing them more effectually to solar and terrestrial radiation. To this cause much of the observed difference between the west and east climates of Great Britain is due. In Ireland, on the other hand, where the mountains are not grouped in ranges running north and south, but in isolated masses, the difference between the climates of the east and west is very much less. In the east of the United States the prevailing winds in summer are S.W., and as the Alleghanies lie in the same direction the temperature is little affected by these mountains, and the rainfall is pretty evenly dis tributed on both sides of the range. In its climatological relations the distribution of rain over the globe presents us with a body of facts which lead, when intelligently interpreted, to a knowledge of the laws regulating the distribution of plants more quickly and cartainly than do the facts of temperature. It is to the prevailing winds we must look for an explanation of the rainfall, the broad principles of the connection being these: 1, The rainfall is moderately large when the wind has traversed a considerable extent of ocean ; 2, if the v.-inds advance into colder regions the rainfall is largely increased, and if a range of mountains lie across their path the amount precipitated on the side facing the winds is greatly augmented, but diminished over regions on the othej side of the range ; 3, if the winds, though coming from the ocean, have not traversed a considerable extent of it, the rainfall is not large ; and 4, if the winds, even though having traversed a considerable part of the ocean, yet on arriving on the land proceed into lower latitudes, or regions markedly warmer, the rainfall is small or nil. It is this last consideration which accounts for the rainless character of the summer climates of California, of Southern Europe, and of Northern Africa. The region extending from Alaska to Lower California presents more sudden transitions of climate, and climates more sharply contrasted with each other, than any other portion of the globe, this arising from the contour of its sur face and the prevailing winds. A direct contrast to this is offered by the United States to the east of the Mississippi, a region characterized by a remarkable uniformity in the distribution of its rainfall in all seasons, which, taken in connection with its temparature, affords climatic conditions admirably adapted for a vigorous growth of trees and for the great staple products of agriculture. India and the region of tha Caspian Saa and the Caucasus Mountains also present extraordinary contrasts of climate in all seasons, due to the prevailing winds, upper as well as lower winds, tha relative distribution of land and water, and the physical configuration of the surface of the land. In tha above remarks the only question dealt with has been the average climate of localities and regions. There are, however, it need scarcely be added, vital elements of climate of which such a discussion can take no cognizance. These are the deviations which occur from the seasonal averages of climate, such as periods of extreme cold and heat, or of extreme humidity and dryness of air, liability to storms of wind, thunderstorms, fogs, and extraordinary downfalls of rain, hail, or snow. An illustration will show tha climatic difference here insisted on. The mean wintar temperature of the Southern States of America is almost the same as that of Lower Egypt. Lower Egypt is singularly free from violent alternations of temperature as well as frost, whereas these are marked features of the winter climate of the States bordering on the Gulf of Mexico. Robert Russell, in his Climate cf America, gives an instance of the temperature falling in Southern Taxas with a norther from 81 to 18 in 41 hours, the norther blowing at the same time with great 