312 



PHYSIOGRAPHY 



boundaries. If definite poleward boundaries must be assigned, 

 they might be placed at the polar circles, though the westerlies 

 appear to prevail beyond them. 



This definition of zones is less definite and simple than the pre- 

 ceding, but if it leaves the zones with rather vague boundaries, it 

 is because Nature has left them so. 



Zones defined by isotherms. If the zones be defined on the 

 basis of temperature, the dividing-lines between zones are iso- 



Fig. 260. Zones defined by temperature, in degrees Fahrenheit. (Supan ) 



therms. One division which has been suggested makes the annual 

 isotherms of 68 the equator-ward limits of the intermediate zones, 

 while their polar limits are the isotherms of 50 for the warmest 

 month (Fig. 260). On the whole, this seems a fairly satisfactory 

 basis for the definition of climatic zones. 



Subdivisions of the Zones 



Each climatic zone has at least two principal subdivisions, a 

 continental and an oceanic. The oceanic climate of any zone pre- 

 vails where there are extensive areas of water, and the continental 

 climate prevails elsewhere. 



