CLIMATE IN SOME OF ITS RELATIONS TO MAN 247 



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 lati- 

 tudes. Within the tropics, weather and climate are essentially synony- 

 mous 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 sea- 

 sons, 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 oc- 

 currence for years. 



Over nearly all the equatorial zone the difference between the aver- 

 age 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 tempera- 

 ture 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. 



