TERRESTRIAL TEMPERATURE. The distribution of heat over the globe is repre sented by isothermal lines, or lines drawn through all places having the same mean tem perature.
The part of the globe having the highest mean annual temperature forms an irregu larly shaped belt, lying along the equator, and comprised between the n. and the s. isothermals of 80°. On either side of this warm belt the temperature diminishes towards the poles; and the lines in a chart showing successively this diminution are, speaking in a very loose sense, arranged parallel to the equator, thus showing the all predominating influence of the snn as the source of terrestrial heat. The coldest portion of the earth's surface is a small oval-shaped patch near to but not surrounding the n. pole, its mean temperature being — 4°. Its narrowest diameter lies n. and s. nearly touching the pole on the one side, and extending on , the other as far s. as 72° '30 n. lat, in 130° w. long. On looking at a chart representing the isothermal of 0°, one might be led to suppose that there are two centers of greatest cold, one n. of Siberia, and the other n. of British America. Such, however, is not the case—the apparent double center of greatest cold being solely due to the isothermals being drawn on Mercator's projection of the earth; for if an isothermal map be drawn on a polar pro jection, the lines of mean annual temperature inclose one connected space of greatest cold, and not two such spaces, as is not unfrequently stated.
While the decrease of temperature in advancing toward the poles corresponds in a general way to what may be called the solar climate, there are deviations brought about by disturbing causes too important to be overlooked. These disturbing causes are (I) the currents of the sea; (2) the prevailing winds; and (3) large surfaces of water which are frozen daring part of the year.
The influence of an oceanic current depends on the temperature of the place it leaves the place at which it arrives. Hence the great equatorial current, flowing from east to w. does not require to be considered here, because the heat remains the same throughout its course; but only those currents which convey the waters of the sea to higher or to lower latitudes. Of these, the most marked and important is the Gulf stream in the North Atlantic, which, by conveying warm water to the arctic regions, pushes the isothermals many degrees to the northward. There is a similar, though much feebler, current passing from the North Pacific to the Arctic Sea through 13ehring's strait, and there, accordingly, the isothermals are pushed a little to the northward. In the southern hemisphere, there are two currents, one discovered by IIumboldt, passing from the Antarctic Ocean northward by the coast of Peru as far as Lima; the other flowing from the cape of Good Hope northward along the w. coast of Africa: these currents, flowing from colder to warmer latitudes, lower the temperature, and thus drive the isothermals toward the equator. Again, the great equatorial current, after hoping ing on the e. coast of Africa, turns southward, and by the warmth it imparts, pushes the isothermals into higher latitudes. For the same reason the current flowing south ward past the coast of Brazil, raises the temperature in the e. of that country. The influence of these great currents is more distinctly marked in the forms of the isother mals for January and July. Thus, in January, when the relative excess of the tempera ture of the Gulf stream is greatest, the isothermals are driven very far to the north; and similarly in the southern hemisphere, the currents from the Antarctic Ocean being coldest in July, the isothermals are deflected more toward the equator during that month. The most remarkable lowering of the isothermals occurs in Labrador and Newfoundland during May and June, and is caused by the icebergs which then descend on these coasts from Davis's strait.
Since winds bring with them the temperature of the regions they have crossed, the equatorial current is a warm wind, and the polar a cold wind; also winds arriving from the ocean are not subject to such variation of temperature during the year as winds from a continent. As an atmosphere loaded with vapor obstructs both solar and noc turnal radiation, it follows that moist winds arc accompanied with a warm temperature in winter, and a cool temperature in summer; and dry winds with cold winters and hot summers. The direction of mountain-ranges is also an important element to be taken into account in estimating the influence of winds on temperature. These considerations explain the position of the isothermals in the north temperate zone, where the prevail ing wind is the s.w. or anti-trade (see WINDS). In January, the western parts of each. continent enjoy a comparatively high temperature, from their proximity to the ocean, whose high temperature the winds waft thither; and they are further protected from extreme cold by their moist atmosphere and clouded skies. But in the interior of the continents it is otherwise; for the winds getting colder as they advance, and being deprived of their moisture as they cross the mountains in the w., the soil is exposed to the full effects of radiation during the long winter nights, and as a consequence, the temperature rapidly falls. In the center of Siberia, the January temperature falls to —40', which is 9° colder than the coldest part of the American continent; and this center of greatest cold lies near the eastern part of the continent of Asia. On the other hand, in July, the interior of continents is much warmer than their western parts. Hence the interior and eastern parts of Asia and America are characterized by extreme climates, and the western parts by equable climates. Thus, at Yakutsk, in Siberia, the July temperature is and the January-43°.8, the difference being 106°. 0; while at Dublin these are respectively 60°.8 and the difference being only 22°.3. This con stitutes the most important distinction of climates, both as respects vegetable and animal life. On man especially the effect is very great—the severity of the strain of extreme climates on his system being shown by the rapidly increasing death-rate as the differ ence between the July and January temperatures increases.
The great fresh-water lakes of North America—lakes Superior, IIuron, Erie, Michi gan, Ontario, Bear lake, etc.—exercise an important influence on the climate of the central parts of North America, for in winter, America, with its frozen lakes, is a truly unbroken continental mass, and its winter climate is therefore continental; whereas in summer its numerous large sheets of fresh water communicate to it many of the features of an insular summer climate.
The whole effect of the disturbing causes is seen at once, if we compare the observed temperature of a place with its normal temperature, that is, the temperature due to it in respect of its latitude. In the northern hemisphere, in January, the sea and the western parts of the continents are in excess of their normal temperature; elsewhere, there is a deficiency. There are two centers of excess, one to the n.e. of Iceland, amounting to 41°; the other in Russian America, amounting to only 18°; and two centers where the temperature is deficient, one at Irkutsk, amounting to 41°; and the other NI . of Hudson's bay, amounting to 27°. In July, the United States, Europe, Asia, the Indian ocean, the n. of Africa, and the extreme n. of South America, have their temperature in excess, while elsewhere it is deficient. The centers of excess are: n. of Siberia, 13°.5; Red sea, 11°.0; and n.w. of the United States, 4°.5; and the centers where the temperature is deficient are: the entrance to Hudson's bay, 11°.0; and the Aleutian islands, 11°.0.