190 Light 



I II III IV V VIVIIVIIIIX X XI XII I II III IV V VI 

 80° N 



60° N 



40° N 



20° N 



20° S 



40° S 



60° S 



80° S 



Fig. 6.3. Variation in length of day at the indicated latitudes during the indicated 

 months. Arrowheads indicate periods of decreasing daylength. The heavy line 

 shows the course of zenith sun (Baker, 1938.) 



Fig. 6.4. Total solar radiation ( g-cal/day/cnr of horizontal surface) on June 21 

 with cloudless sky. ( Modified from Kimball, 1928. ) 



