The distribution of air temperatures over large areas can best be shown by a map composed of isotherms. Isotherms are drawn to connect all points having the same temperature. Usually, isotherms representing 5° or 10° differences are chosen, but they can be drawn for any selected temperatures. The isotherms pass through the observing stations only when the station readings coincide with the value selected for an isotherm. Otherwise it is necessary to draw the isotherms by estimating their proper position between stations. The value of isothermal maps is that they make clearly visible the important characteristics of the prevailing temperatures.
Throughout the year, isotherms shift through several degrees of latitude, following the declination of the sun but lagging behind a month or so in time. Over large water areas, such as the south Pacific, the annual shift amounts to only about five degrees of latitude, whereas over land masses, such as Africa, this shift is as much as 20° latitude. This difference in amount of latitude shift is also explained by the rapidity and intensity with which lands are heated and cooled as compared with ocean areas.
Certain definite centres of high and low temperature occur and are shown by isotherms which are completely closed to form oval or irregular-shaped enclosures. Notice that all of them are over landmasses. In July, high-temperature occurs over the southwestern United States, North Africa, and southwestern Asia. In January, a continental centres of low temperatures occurs over Siberia and is strongly developed with the average January temperatures lower than 50°F (-46°C). A corresponding region of low temperature, marked off by the closed isotherm of -30°F (-34°C), occurs in northernmost North America. It is not so well developed as that of Asia because of the presence of considerable areas of Arctic Ocean among the islands of the northern fringe of the landmass and the smaller size of the North American landmass.
Permanent centre of low temperature exist over Greenland and Antarctica, the two regions of massive icecaps. Temperatures over Greenland do not, however, reach the extreme low of northern Siberia in January, although the annual average temperature of the icecap is much lower.
Annual temperature range has environmental significance, for it is an indicator of thermal stress (or the lack of it) placed upon plants as well as upon man and the other animals. Thermal stress is small in the equatorial zone but severe in the sub arctic zone. Climates having a small annual temperature range are described as equable.
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