CLIMATIC FACTORS : RADIANT ENERGY 



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6AM 8AM 10AM NOON 2 PM 4 PM 6 PM 



TIME OF DAY 



FlG. 67. Intensity of radiation received at different times of day on (A) 

 south, (B) north, and (C) east slopes in the southern Appalachians, on June 

 21 and on December 21. For S. exposure, in summer, the 20 percent slope 

 receives greatest radiation because it forms an angle of almost 90° with the 

 sun's rays at noon. In winter, when the sun is low, the 100 percent slope re- 

 ceives more radiation than the 20 or the 40 percent slope. For N. exposures, 

 in summer, 20 percent slopes receive almost as much radiation as 20 percent 

 south slopes. In December, 100 percent N. slopes are in complete topographic 

 shade but 100 percent S. slopes receive 48 percent of maximum radiation at 

 noon. Curves for west slopes would be mirror images of those for east slopes. 

 —From Byram and Jemison.** 



duced to 15 percent or less) of somewhat different quality (re- 

 duced red and blue light) than those in full sunlight receive. Con- 

 sequently, there are species representing a wide range of tolerance 

 to shade, for no forest is so dense that nothing can grow beneath 

 it, even when there is a reduction to 1 percent or less of full sun- 

 light, as under some tropical forests. The reduction of light in- 



