GENERATION, CONTROL, AND MEASUREMENT 153 



earth from the sun. This quantity, known as the "solar constant," has 

 a value of 1.94 cal min"' cm-^, or 1350 w m-^ (Abbott, 1952; Moon, 1940). 

 The solar constant changes +3.5 per cent from the mean owing to vari- 

 ations in distance from the sun to the earth as the earth moves through 

 its orbit. In addition, the solar constant shifts ±2 per cent in an irregu- 

 lar manner owing to deviations in the activity of the sun itself. 



Transmission of the Atmosphere. Large variations in solar irradiance 

 at the earth's surface result from changes in scattering and absorption 

 by the earth's atmosphere. The calculation of spectral-solar-radiation 

 curves and the factors responsible for energy losses in the atmosphere 

 have been extensively reviewed by Moon (1940) and Benford (1948b). 

 Molecules of the atmospheric gases and fine particles of dust scatter much 

 of the incoming energy. Since the dimensions of these particles are small 

 compared with the w^ave lengths of visible energy, the intensity of scatter- 

 ing is inversely proportional to the fourth power of the wave length in 

 accordance with Rayleigh's law, which states that the scattering is 

 inversely proportional to the fourth power of the wave length. Since 

 the shorter blue wave lengths are scattered more than the longer red 

 wave lengths, a clear sky appears blue. During very cloudy weather 

 most of the incoming energy may be reflected back into space by thick 

 layers of small particles of liquid water or ice, which compose the clouds. 

 These particles are relatively large as compared with the visible wave 

 lengths, and therefore there is little selective scattering; consequently 

 clouds usually appear white. 



Most of the absorption of solar radiant energy by the atmosphere in 

 the near infrared is due to water vapor and carbon dioxide, and in the 

 middle ultraviolet, to ozone; the atmosphere of a clear sky is quite trans- 

 parent to the visible and near ultraviolet. The transmittance of an air 

 mass of 1, which occurs only when the sun is overhead (solar angle 90°), 

 is about 80 per cent throughout most of the visible. Water-vapor bands 

 strongly attenuate the near infrared from 720 to 2300 mjj. ; the infrared is 

 absorbed almost completely by atmospheric w^ater vapor and carbon 

 dioxide beyond about 2300 m^u. The transmission of the atmosphere 

 for a heavily overcast midday sky may fall to a few per cent. 



The ultraviolet limit of terrestrial solar energy is at about 300 m/x, 



limited principally by the absorption of ozone in the ionosphere. The 



densest portion of the ozone layer varies in altitude between 22 and 25 km. 



It begins to absorb at about 320 m/i and becomes practically opaque at 



290 m/x, as recorded by Stair (1951). 



Air Mass. The equivalent number of atmospheric thicknesses trav- 

 ersed by the sun's rays is approximately proportional to the cosecant of 

 the solar angle as measured by a tangent to the earth's surface. As the 

 sun appears to move from the zenith position, or 90° solar angle, where 

 the air mass is 1, the air mass increases slowly at first and then rapidly 



