Page:Popular Science Monthly Volume 76.djvu/251

Rh that these zones are really zones of sunshine, or of solar climate, a subject which Professor Libbey has already considered in his lecture on "Astronomical Climate" (January 19, 1909). The so-called "torrid" zone has the greatest annual amount of sunshine. It is the summer zone. The polar zones have the smallest amount of sunshine. They may well be called the winter zones. The temperate zones are intermediate between the tropical and the polar, in the matter of the annual amount and of the annual variation of sunshine.

The Temperature Zones.—The usual classification of the climatic zones on the basis of the distribution of sunshine serves well enough for purposes of simple description, but a glance at any temperature-chart shows at once that the lines of equal temperature (isotherms) do not coincide with the lines of latitude. In fact, in the higher latitudes, the lines of equal temperature often follow the meridians more closely than they do the parallels of latitude. The astronomical zones—i. e., the zones of light—therefore differ a good deal from the zones of heat. Hence, in recent years, it has become quite customary, at least in climatology, to limit the zones by lines of equal temperature, thus making a closer approach to the actual conditions of climate.

Characteristics of the Tropics.—The dominant characteristic of the great equatorial zone is the remarkable simplicity and uniformity of its climatic features. The tropics lack the proverbial uncertainty and changeableness which characterize the weather of the higher latitudes. Within the tropics, weather and climate are essentially synonymous terms. Eegular conditions, which depend upon the daily and annual march of the sun, are dominant. Irregular weather changes are wholly subordinate. In special regions only, and at special seasons, is the regular sequence of weather temporarily interrupted by an occasional tropical cyclone. These cyclones—the hurricanes of the West Indies and the typhoons of the China seas belong to the group—although infrequent, are notable features of the climate of the areas in which they occur. The devastation produced by one such storm often affects the economic condition of the people in the district of its occurrence for years.

Over nearly all the equatorial zone the difference between the average temperatures of the warmest and coldest months is less than 10°, and over much of it it is less than 5°. At Equatorville, in the interior of Africa, on the Congo, the difference between the average temperature of the warmest and coolest months is only a little over 2°. The variation in temperature during the day is usually larger than this seasonal difference. Thus, at Equatorville it is seven times as large. It has been well said that "night is the winter of the tropics." Over much of the equatorial zone the lowest temperatures usually do not fall below 60°. Maximum temperatures of 115°-120° occur over the deserts of northern Africa.