radiation: a general introduction 



89 



The value falls to about 10 per cent of this 

 constant when the sun stands just above 

 the horizon (Kimball and Hand, 1936). 

 Then the rays pass through 14.5 times the 

 air mass that they have to traverse at noon. 

 These radiations were measured at right 

 angles to the rays of the sun. For many 

 ecological purposes, the total amount of 

 radiation, both direct and indirect, is more 

 important. This is better approximated by 

 using the vertical component of the total 

 solar radiation that falls on a given point. 



for a given interval of time can be calcu- 

 lated from the formula: 



Q. = Qo[a +(1.00 -a)S] 



in which S is the percentage of possible 

 hours of sunshine; Qo is the radiation re- 

 ceived from a clear sky, and Q, is the 

 amount received from a more or less over- 

 cast sky; a is a so-called constant the 

 value of which varies with the character 

 of the clouds, with dust in the atmosphere. 



•1000 



900 

 800 

 700 

 600 

 500 

 400 



300 

 200 

 100 



Fig. 7. Smoothed annual variation in the total radiation received on a horizontal surface. 

 A, Oatside the atmosphere at the latitude of Washington. 



Cloudless sky: B, Twin Falls, Idaho; C, Washington. 



Average cloud conditions: D, Twin Falls; E, Washington; F, Chicago. ( Redrawn with modi- 

 fications from Kimball and Hand.) 



The radiation received may consist of (a) 

 direct sunhght, {h) diffuse sky radiation, 

 skylight, and (c) radiation from trees or 

 other objects of the environment. 



The distribution of solar insolation is 

 such that only two-fifths as much heat is 

 received per unit surface at the poles as at 

 the equator, and the polar ice and snow 

 cap may reduce the effective insolation still 

 more (WiUett, 1931). 



All solar radiation is much affected by 

 the amount of cloudiness in the earth's at- 

 mosphere. In general, the proportion of 

 direct sunlight varies inversely with the 

 amount of skyUght. The effect of cloudiness 

 on the vertical component of incident hght 



and also perhaps with surface conditions. 

 A commonly accepted value for a in the 

 eastern United States is 0.22, and the basic 

 equation becomes: 



Q, = Qo(0.22 + 0.78S) 



The maximum amount of sunhght re- 

 ceived at the latitude of Washington is 1.5 

 gm. cal./min./cm."; this is equivalent to an 

 intensity of about 10,000 foot candles. On 

 Mt. Whitney, at an altitude of 4420 meters, 

 the amount may reach 1.67 gm. cal./min./ 

 cm.*, or approximately 11,000 foot candles. 

 The distribution of total radiation from the 

 sun at the earth's surface is as follows: The 



