It is obvious that the winds must exercise considerable influence upon the distribution of temperature over the globe. Those which blow from the cold continental interiors during the winter months frequently cause severe weather in comparatively low latitudes ; the trade winds which come from the sea tend to reduce the tem perature of the warm lands towards which they blow ; and on the western coasts of Europe the westerly and south-westerly winds which prevail at all seasons of the year, but more particularly during the winter months, have a modifying effect, reducing the heat of summer and mitigating the cold of winter.
Ocean currents are another factor in the distribution of temperature, and their effects upon climate can most appro priately be considered in connection with the winds through which they make their influence felt. For it must be remembered that the presence of a warm current off the shores of a cold country would have little effect upon its climate, except, perhaps, to keep its ports free from ice, if winds, either warmed or prevented from cooling by the current, did not blow inland. The general circulation of the surface waters of the ocean may best be considered by des cribing what takes place in the Atlantic. There, the heated surface waters of equatorial regions are blown along by the trade winds and gradually acquire a momentum of their own. Two currents, the North Equatorial and the South Equatorial, are thus formed, and these flow westwards till they strike the coast of South America. Here part of the southern current is forced northward and joins the northern current, which, being likewise deflected by the land, turns to the north. Part of it enters the Caribbean Sea and passes into the Gulf of Mexico, issuing to join the remainder, which has made its way northward to the east of the West Indies.
The reunited current follows the coast of the United States as far as the fortieth parallel where it meets the cold Labrador current from Davis Strait and is more or less dispersed. Some of its waters, no longer as a definite current, but as a surface drift, are carried by the westerly winds across the Atlantic. This drift divides off the coast of Spain, part turning south and rejoining the North Equatorial current, while the other part, still under the influence of the westerly winds, makes its way past the British Isles, along the coast of Norway, and into the Arctic Sea. That part of the South Equatorial
current which is deflected southward flows along the coast of South America until it, too, passes under the influence of westerly winds which carry it eastwards until it bifurcates off the coast of Africa, one branch turning north to join the equatorial current, and the other continuing to follow the tract of the westerly winds. In the North Pacific the circulation is, on the whole, similar to that of the North Atlantic, the main differences being accounted for by differences in the configuration of the two basins, while south of the equator the Pacific and Indian Oceans have a circulation like that of the South Atlantic, but modified somewhat by monsoon conditions. On the west coasts of continents, within the trade wind belts, there are cold currents, due in part to the upwelling of cold water to take the place of that blown westwards by the trade winds, and in part to the branches of the easterly drift which turn equatorwards to join the main current. Along the east coasts of North America and Asia there are also cold currents which move southwards until they meet with, and are lost in, the deflected equatorial currents.
Altitude is another important factor in the determination of temperature. With an increase in altitude the atmosphere becomes less dense, and also contains much less water vapour and atmos pheric dust. The sun's rays therefore pass through it more easily and are absorbed to a less extent ; and this is also the case with the heat rays radiated from the earth. Accordingly the atmosphere becomes cooler at an average rate of about 1° F. for every 300 feet of vertical ascent.
Inversions of temperature, however, are frequent, and are often of economic importance. On still nights, when insolation ceases and radiation begins, the air which is in immediate contact with the earth cools more rapidly than that at higher elevations. Conse quently, temperature may slightly increase upwards, even to a height of 2,000 feet and more. Sometimes, also, colder air is found in valleys than on the neighbouring uplands. Valleys frequently receive less sunlight and are warmed to a less extent, while cold air from the mountain sides may slip down into them and remain there. Certain plants which are liable to damage from frost are therefore often planted on the lower slopes of hills rather than in valleys.