310 PHYSIOGRAPHY 



than others. The meaning of ''variable climate" is therefore 

 itself variable. 



Classification of Climates 



As in the case of many other topics, climates may be classified 

 in various ways, and each classification helps to emphasize some 

 important point. One classification has already been suggested, 

 namely, uniform and variable. 



Another classification has reference primarily to the amount 

 of heat received from the sun. On this basis the earth is subdi- 

 vided into climatic zones, the borders of which are parallels. These 

 climatic zones may be said to represent solar climate, but the climate 

 which insolation alone would give is much modified by other fac- 

 tors, as we have seen. 



The effect of land and water on temperature has already been 

 noted. It is so important that climates are also classified as oceanic 

 and continental. Continental climates, in turn, may be subdivided 

 on the basis of (1) distance from the sea, (2) height above the sea, 

 and (3) topographic relations. The controlling element in most of 

 these classifications is temperature. 



Climatic Zones 



The climatic zones commonly recognized are (1) the torrid zone, 

 the center of which is the equator, (2) the temperate zones, which 

 occupy the extra-tropical latitudes, and (3) the frigid zones, which 

 lie about the poles. Better names for these zones are the tropical, 

 the intermediate, and the polar zones * respectively, and these terms 

 will be used hereafter. The limits of these zones are defined 

 (1) by latitude, (2) by the direction of the winds, or (3) by tem- 

 perature. 



Zones defined by latitude. Defined by latitude, the tropical 

 (or torrid) zones lie between the tropics (23}^ N. and 23>^ S.), 

 the two polar zones extend from the poles to the Arctic and Ant- 

 arctic circles respectively (90 N. to 66^ N., and 90 S. to 66^ S.), 

 and the two intermediate zones lie between the tropical zones and 

 the polar zones on either hand (23^ N. to 66^ N., and 23^ S. 

 to 66^ S.). 



1 Ward, Bull. Am. Geog. Soc., vol. xxxviii, 1900. 



