Section 14. Reradiation 



The ocean surface heated by the sun's radiant energy is, in turn, radiating the heat energy 

 back into the atmosphere. This phenomenon is called reradiation (or back radiation). 



The reradiation depends on the physical properties of the surface and its temperature. Rough 

 surfaces reradiate most intensely, white and glossy surfaces with lowest intensity. 



According to Stefan-Boltzmann, the reradiation for an absolutely black surface equals 



E = a (273 + OS 

 in which E = reradiation in g-cal/cm^ min, 

 t = surface temperature, 

 a = reradiation coefficient equalling 8.35-10"-'-^ g-cal/cm min. 



Calculations based on this formula should be considered the extreme case because the re- 

 radiation coefficient of all surfaces of the earth is smaller than that of an absolutely black body. 

 Thus, it is assumed that for black earth the reradiation coefficient equals 87 per cent, for yellow 

 clay 85 per cent, for snow 75 per cent of the reradiation coefficient of a black body. The re- 

 radiation capability of water approaches that of an absolutely black body. 



Because the temperature of the ocean (as well as that of any other surface of the globe) is 

 relatively low, the ocean emits only long-wave rays, the wave-length exceeding 0.2 microns. This 

 reradiation is most thoroughly absorbed by water vapors, and it is assumed that even on clear days 

 the quantity of water vapor m atmosphere is sufficient for the absorption of as much as 90 per cent 

 of the earth's reradiation. 



The atmosphere, heated by the terrestral reradiation, reradiates, in its turn, heat toward 

 the earth's surface. The latter type of reradiation is called cross reradiation. 



Here lies the "hot -bed effect" of the atmosphere; the atmosphere lets the light beams readily 

 through, but detains the thermal reradiation, giving it off to the earth. 



The difference between the terrestrial and cross reradiations is called the "effective 

 reradiation. " 



More often than not, the sea temperature is higher than the air temperature, and the effec- 

 tive reradiation is negative, i.e. , the sea loses heat. But in certain ocean areas, notably In the 

 Arctic Basin, especially above the ice, the temperature of the air In summer at corresponding 

 winds is considerably higher than the temperature of its underlying surface. This results In a 

 positive, effective reradiation which warms the sea surface. 



We have seen that in the arctic, the high values of the intensity of incident solar radiation is 

 caused by the very small absolute moisture content and the small amount of dust in the atmosphere. 

 But the same causes lead to extremely great values of terrestrial reradiation at a cloudless sky 

 and, as a consequence, Intense cooling of the sea surface. 



UTERATURE: 62, 81. 



31 



