172 



BIOLOGICAL EFFECTS OF RADIATION 



tribution is greater for lower sun, when the direct radiation travels a 

 greater air path, i.e., through a greater air mass. The contribution of sky 

 radiation is shown by the dash-dot curve. At noon, it amounts to 

 12.5 per cent of the total, i.e., the direct is seven times the sky radiation. 

 At midmorning or midafternoon it amounts to 20 per cent of the total, 

 or one quarter of the direct, and exceeds the direct near sunrise and 



Apparent or True Solar Time 



PiQ, 18. — Record of total solar radiation received on a horizontal surface, Washington, 

 D. C, on July 11, 1924. (Kimball, 43.) Dotted curve shows total radiation. Dash-dot 

 curve shows sky radiation. 



sunset. Representative values from the same source are given in Table 

 12. 



Table 12. — Ratio of Direct Solar Radiation Received on a Horizontal Surface 



TO Diffuse Sky Radiation 



The maximum intensity commonly attained at sea level for the lati- 

 tude of Washington is 1.5 gm. cal./min./cm.^ or 105 mw./cm.^. For high 

 altitudes, it may be as high as 1.64 gm. cal./min./cm.^ (Mt. Whitney, 

 4420 meters). Even a light cloud over the sun on an otherwise clear 

 day may, however, reduce the total radiation to one-third of its clear 

 value. 



