GENERATION, CONTROL, AND MEASUREMENT 



155 



sun and sky in the temperate zone in the vicinity of a large city varies 

 between 5000° and 6500°K (Taylor and Kerr, 1941). 



Intensity. The solar disk has an apparent intensity, when observed 

 at the earth's surface, of 50 w m~- and a brightness of 1600 c mm~^. 

 Few artificial sources can equal the intensity of the sun. Terrestrial 

 solar irradiance is very variable and is determined primarily by solar 



Table 3-8. Relative Solar Irradiance, Luminous Efficiency, and Color 



Temperature at Sea Level for Various Air-mass Values 



(Adapted from Moon, 1940, Table IV.) 



" Multiply by 100 for microwatts per square centimeter and by 0.0014 for calories 

 per minute per square centimeter. 

 '' Value of solar constant. 



angle and climatic factors. Benford (1947b, 1948a, b) and Hand (1950) 

 have plotted the manner in which the irradiance varies with solar angle 

 in relation to a normal and a horizontal plane. Kimball (1924; Kimball 

 and Hand, 1936) and Hand (1937, 1941) of the U.S. Weather Bureau 

 have reported detailed summaries of the daily and seasonal variations in 

 solar irradiance at many locations in the United States, including Alaska 

 and Puerto Rico; Crabb (1950) has summarized the data for the state of 

 Michigan. 



The highest total irradiance attained at sea level in the temperate zone 

 is about 1.5 cal min~^ cm^^, or 1000 w m~-, and a visible irradiance of 

 about 10,000 ft-c. The values at high elevations and in dry, dust-free 

 climates occasionally may be as much as 20 per cent higher. The average 

 daily total of energy in the United States during the summer months 



