Intensity of Light 



187 



only a small percentage of the total radiation, and it may be reduced 

 to immeasurably small quantities under certain circumstances. 



Intensity of Light. The intensity of light reaching the earth's 

 surface varies with the angle of incidence and with the amount of ab- 

 sorption by the atmosphere and by obscuring features. The lower the 

 altitude of the sun, the smaller is the angle of incidence and the longer 

 is the path of the light through the atmosphere with corresponding 

 reduction in intensity. Changes in the sun's altitude result from dif- 

 ferences in latitude as well as from changes in the season and in the 

 time of day. The greatest intensity of sunlight occurs at positions 

 on the earth's surface and at times at which the sun is most nearly 

 overhead. At higher latitudes, the intensity of light is correspond- 

 ingly reduced. At 50° N latitude, for example, during the period of 

 the equinox in March and in September when the day is everywhere 

 12 hours long, the intensity of sunlight is only about one-half of what 

 it is at the equator (Fig. 6.1). 



Fig. 6.1. Total solar radiation ( g-cal/day/cnr of horizontal surface) on March 

 21 with average cloudiness. ( Modified from Kimball, 1928. ) 



Latitude thus has a definite effect; but other factors may have much 

 greater influence upon the light factor. Moisture, clouds, and dust 

 in the atmosphere have a profound and irregular effect in reducing 

 illumination. Living organisms may also act to diminish the in- 

 tensity of daylight, as is clearly shown by the forest vegetation, and 

 this represents a reciprocal action in which the inhabitants modify 

 the light factor in their own environment. 



Different forest communities vary widely in the degree to which 

 they diminish the sun's radiation. Cottonwood (poplar) trees tend 



