Duration and Amount of Light 



189 



.080 



.070 



.060 



.050 



E 040 



6.030 



.020 



.010 



.005 



.000. 



Jan. June Dec. 



Fig. 6.2. Mean monthly maximum intensities of solar ultraviolet radiation 

 (2900-4000A) in forest communities of northern Indiana. (Strohecker, 1938.) 



often has a greater effect on the total amount of light received than do 

 differences in the intensity of the sun at noon. 



On the equator the day is always 12 hours long, but in the temper- 

 ate regions the day grows longer as spring progresses. This effect is 

 accelerated at higher latitudes, and the day becomes 24 hours long 

 during the summer in the polar regions. The day becomes cor- 

 respondingly shorter after the summer solstice (Fig. 6.3). Up to 

 moderately high latitudes the increase in length of day during summer 

 has more effect on the total amount of light received per day than the 



