In January the northern water areas of the continent are frozen and snow-covered; Hudson Bay becomes unduly cold, and the greatest southward bending of the isotherms is somewhat east of the continental axis, with an extension of its effects out upon the Atlantic; but the southward bending isotherms are somewhat looped back about the unfrozen waters of the lower Great Lakes.
With the exception of the Lysa Gora hilly tracts (Kielce and south Radom), which lie within the isotherms of 41° and 42°, Poland is situated between the isotherms of 42° and 46°.
The extreme range of the monthly isotherms crossing the state is from about 35° in winter to 81° F.
In summer, and the range of annual isotherms from about S4° to 60° F.
In the same midsummer month all the eastern half of the United States is included between the isotherms of 66 and 82; the contrast between Lake Superior and the coast of the Gulf of Mexico, 1200 m.
The normal trend of the annual isotherms of the state is very simple: a low line of about 40° circles the angle in the Nevada boundary line; 50° normally follows the northern Sierra across the Oregon border; lines of higher temperature enclose the Great Valley; and lines of still higher temperature - usually 60° to 70°, in hotter years 60° to 75° - run transversely across the southern quarter of the state.
The distribution of life-zones is primarily a matter of altitude and corresponds to that of the isotherms. The mountain goat and mountain sheep live in the Sierran upper-land, though long ago well-nigh exterminated.
The country lies thus between the annual isotherms of 41 ° and 28° Fahr., which run in a W.N.W.-E.S.E.
The same effect is well shown by the linguiform isotherms. In January, for example, the isotherm of 14°, after skirting the north coast of the Scandinavian peninsula, turns southward along the Keel, crossing the upper part of the district of the great northern lakes.
Although the temperature of the entire lower peninsula is considerably influenced by the lakes, yet, the prevailing winds being westerly, it is in the west portion of that peninsula that the moderation is greatest, both the summer and winter isotherms being there deflected more than half the length of the peninsula.
The inflexion is still greater for the winter isotherms. Closely following one another, they run almost N.
The summer isotherms cross the winter isotherms nearly at right angles, so that Kiev and Ufa, Warsaw and Tobolsk, Riga and the upper Kama have the same average summer temperatures of 64°, 622° and 61° respectively.
It is comprised between the summer isotherms of 59° and 63°, being bounded on the S.
The isotherms are exceedingly interesting.
Quite contrary is the course of the January isotherms. That of 14° F., which passes in Europe through Uleaborg in Finland only touches the southern part of West Siberia in the Altai Mountains.
The arrangement of the isotherms thus affords a basis for valuable deductions as to the direction of ocean currents.
Corner it was 52.2° F., and as the isotherms cross the state, especially in the N., their tendency is to move S.W.
As a result, the isotherms of July are strongly convex poleward as they cross the United States, the isotherm of 70 Sweeping up to the northern boundary in the north-west, and the heat equator leaping to the overheated deserts of the south-west, where the July mean is over 90.
Conversely, the isotherms of January are convex southward, with a monthly mean below 32 in the northern third of the interior, and of zero on the mid-northern boundary.
The seasonal bending of the isotherms is, however, unsymmetrical for several reasons.
The continent being interrupted on its eastern side by the Gulf of Mexico and Hudson Bay, with the Great Lakes between these two large water bodies, the northward bending of the July isotherms is most pronounced in the western part of the United States.
Indeed the contrast between the moderate temperatures of the Pacific coast and the overheated areas of the next interior deserts is so great that the isotherms trend almost parallel to the coast, and are even overturned somewhat in southern California, where the most rapid increase of temperatures in July is found not by moving southward over the ocean toward the equator, but north-eastward over the land to the deserts of Nevada and Arizona.
In July, on the other hand, the isotherms show an almost constant temperature all over the country, and the linguiform curves are wanting.