The intensity of incident solar radiation is reduced to the horizontal plane by the formula 



/=/oSina, ■ (1) 



where I^ = radiation striking a plane that is perpendicular to the sun's rays, a = sun's elevation 

 above the horizon. 



Formula (1) explains the relatively intense heating of vertical walls at small elevations of the 

 sun. This phenomenon is of special significance in arctic areas where the vertical walls of hum- 

 mocks and icebergs melt intensely in summer while the horizontal surfaces of ice floes remain 

 untouched . 



The intensity of solar radiation also depends on the sun's elevation because at small eleva- 

 tions the sunbeams, prior to reaching a given object, must penetrate atmospheric strata having 

 considerable thickness or, in other words, pass through considerably greater "atmospheric 

 masses" than at greater sun's elevations. Assume that a sunbeam strikes the earth in vertical 

 direction. When passing through the atmosphere, the energy of the sunbeam, due to absorption and 

 scattering, will decrease, becoming equal to 



where q = coefficient of atmospheric scattering. 



If this sunbeam would penetrate one more atmosphere like this, or one more "atmospheric 

 mass" having the same optical properties, then the preceding formula is transformed into 



but when penetrating m atmospheric masses 



This is known as the Lambert-Bugge formula. 



It is obvious that in order to find the intensity of the solar radiation striking the horizontal 

 surface of the sea, 



/=/o9'"sina. (5) 



As to the coefficient of scattering, it depends on meteorological conditions, the dust content, 

 and, mainly, on the water vapor content of the atmosphere. 



The water vapor pressure in the atmosphere, as we already know, depends on temperature. 

 At -20°, the saturated vapor pressure equals 1.25 mb, at 0°— 6 mb, at 30°— 42 mb. Hence, in 

 arctic regions, despite small elevations of the sun, the intensity of incident solar radiation may 

 exceed the intensity of solar radiation at noon in tropical regions, because in the arctic regions the 

 atmosphere contains not only a small quantity of water vapor but also a small quantity of dust. 

 Thus, the colder the air (with the absence of clouds), the greater is the scattering of the 

 atmosphere . 



27 



