CLIMATIC FACTORS : RADIANT ENERGY 



117 



since the December solstice results in diametrically opposite con- 

 ditions, it represents a period without insolation. Conditions in the 

 Southern Hemisphere are, of course, always exactly reversed. 

 Thus, because of differences in insolation, we have seasons 



VERNAL EQUINOX 

 MAR. 21 



SUN 



WINTER 



SOLSTICE 



DEC.2I 



AUTUMNAL EQUINOX 

 SEPT 23 



Fig. 60. Diagrammatic representation of the changing position of the earth 

 with respect to the sun and its relationship to insolation and change of sea- 

 sons in the Northern Hemisphere.— Adapted from Trewartha. 



258 



marked by variation in length of day and temperature. Since the 

 periodic differences in insolation become more marked with dis- 

 tance from the Equator, the seasons likewise become more distinct 

 with increasing latitude. The greatest total insolation, however, 

 occurs at the Equator and decreases with distance from the Equa- 

 tor in spite of the increasing length of day. Toward the poles, in- 

 tensity of insolation is reduced because of the increasing angle of 

 incidence. 



