VISIBLE AND NEAR-VISIBLE RADIATION 



179 



41 to 45 per cent lies in the visible, and from 50 to 58 per cent lies in the 

 infra-red. With high sun, the wave-length of maximum intensity per 

 unit wave-length occurs at about 5000 A, corresponding roughly to a 

 black-body temperature of 6000°K. Abbot's (6) estimates of total 

 radiation outside the atmosphere of 1.90 to 1.97 gm. cal./min./cm.^ 

 correspond, however, to the total radiation of a black body with tempera- 

 ture nearer 5000°K. The energy reaching the earth for high sun com- 

 monly varies from 0.5 to 1.5 gm. cal./min./cm.^. With decreasing alti- 

 tude of sun, the increasing air mass reduces the relative intensity of 

 ultra-violet and blue, as compared with the red and infra-red, so that the 



G cal/cm^/min. 



0.00135 

 u.v. Q 



Q00120 



0.00105 



c 



■2 Q00090 



o 



I 0.00075 



"5 0.00060 



> 



E 0.00045' 



3 



Q00030 



0.00015 



Microwatts,/JW 



94.5 



84 



735 



63 



'52.5 



42 



31.5 



21 



10.5 



Jcin. Feb. Mar Apr. May Jun. July Aug. Sept. Oct. Nov. Dec. Jan. Feb 



Fig. 26. — Average ultra-violet solar radiation intensities of wave-lengths less than 0..313/X. 



{Cohlentz and Stair, 46.) 



distribution corresponds to a lower black-body temperature. Thus 

 Abbot computes the distribution of direct solar radiation, equi\'alent to a 

 black-body (or color) temperature varying from 6300°K. for air mass zero 

 to 2760°K. for air mass 10. The sky radiation, however, is much stronger 

 in the blue and ultra-violet, so that its color temperature is much higher. 

 Priest found for Washington that this varied from 6000° to 24,000°K. 

 On overcast days, it is near the lower limit, 6300° to 7000°K. On clear 

 days, the usual range is from 8000° to 15,000°K. Thus, the sky con- 

 tribution raises the color temperature of total radiation over the 

 direct. 



Since the color temperature varies markedly with air mass, or height 

 of the sun, the luminous equivalent varies correspondingly. Kimball 

 gives the values in foot-candles for 1 gm. cal./min./cm.-. 



