160 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 9 



shine, one may reasonably include half the number of "partly cloudy" 

 days with the number of "clear" days. This arbitrary interpretation 

 of the reports for 10 years shows a consistent distribution of sunshine 

 throughout California. Large variations occur annually and there 

 are pronounced local differences. The effect of a low coastal range is 

 seen in the 10-year average of 204 sunshine daj's per year near the 

 ocean at Santa Ana compared with 310 at Corona 20 miles inland. 

 Another striking contrast is from 215 sunshine days annually at San 

 Francisco to 302 days at Alvarado only 23 miles away across the 

 lower bay. 



Where there is much more cloudiness during the winter than at 

 other times of the year, it can be expected that solar heaters will be 

 used only during the warmer seasons. In these cases the study of 

 number of sunshine days is better restricted to the normal growing 

 season from the average latest spring frost to the average earliest fall 

 frost. In California the major agricultural areas have the equivalent 

 of 7 months or more of sunshine between frosts. 



ABSORPTION OF SOLAR ENERGY 



The absorption and emission of radiant energy depends upon the 

 temperature and surface characteristics of the body in question, the 

 relation of the surface to its surroundings, and the temperature and 

 radiation characteristics of the surroundings. The temperature of 

 the sun's surface (more than 11,000° F.)^ is so great that the predomi- 

 nant radiation is in short wave lengths largely in the visible region. 

 Radiation from substances at terrestrial temperatures is relatively in 

 long wave lengths, the peak being at about 10 microns. In general 

 the absorptivity for solar radiation varies roughly as the visual dark- 

 ness of the surface — any brightness indicating some reflection and 

 consequently a deficiency in absorption or transmission of the visual 

 short-wave radiation. Long-wave emissivity or absorptivity cannot 

 be judged visually. Common building materials, paints, and roofg 

 (except asbestos and metals) have nearly perfect emitting surfaces for 

 long-wave radiation. 



Long- wave radiation from the atmosphere is in strong spectral bands 

 in accordance with the radiation characteristics of water vapor, carbon 

 dioxide, and ozone. The rate of incoming atmospheric radiation varies 

 with moisture content and follows strongly the temperature variation 

 of the air near the ground. In general the net exchange of long-wave 

 radiation between the earth and the sky is outgoing — to balance the 

 incoming short-wave insolation. At night during the winter radiation 

 frosts in Riverside, Calif., the net radiation loss rarely exceeds 0.5 

 B. t. u. per square foot per minute for a black body. 



•Abbot, Charles G., The Sun. 2d ed., 433 pp. (see specifically p. 116). D. Appleton 

 & Co., New York, 1929, 



