4 INTRODUCTION 



According to Fowle, on a clear day with the sun at zenith, 6 to 8 per 

 cent of solar radiation of wave length greater than 0.3 m is absorbed in 

 the atmosphere, primarily by the water vapor, which shows some narrow 

 absorption bands in the vicinity of wave length 1.0 m- 



Long-wave Radiation to Space. The loss of heat by radiation is 

 due in part to radiation from the water vapor in the atmosphere and in 

 part to radiation from the earth's surface, which escapes without being 

 absorbed in the atmosphere. At the prevailing temperature on the 

 earth the maximum intensity of the outgoing radiation lies at wave 

 lengths 10 M to 15 ju, and this radiation is therefore called long-wave. 



The water vapor is the only component in the atmosphere which 

 absorbs or emits appreciable amounts of radiation at the temperatures 

 that prevail in the atmosphere, but the radiation and absorption are 

 highly selective. Radiation of certain wave lengths is completely 

 absorbed in a small mass of water vapor, whereas for other wave lengths 

 the water vapor is nearly transparent. Studies of the mechanism of the 

 radiation characteristics of water vapor in the atmosphere are in rapid 

 progress, but a summary of the present status would lie beyond the scope 

 of this brief discussion. It must suffice to state that, owing to the 

 selective absorption of the water vapor, a certain part of the outgoing 

 long-wave radiation comes directly from the earth's surface. This part 

 is of great importance to the heat budget of the earth as a whole, but 

 does not enter in the heat budget of the atmosphere. 



The Heat Budget of the Atmosphere 



Accepting the above values of the solar constant, 1.94 g cal/cm^/min, 

 and the earth's albedo, 0.43, one obtains that, of the incoming radiation 

 from the sun, an annual average amount of 0.276 g cal/cm^/min reaches 

 the surface of the earth or is absorbed in the atmosphere. The same 

 average amount must be lost by radiation to space from the water vapor 

 and clouds in the atmosphere or directly from the earth's surface. As 

 yet it is not possible to separate the amounts that are lost by these two 

 processes, but it is possible to state certain limits between which the 

 direct loss from the earth's surface must lie. 



The surface of the earth radiates nearly as a black body and emits 

 therefore an amount which approximately equals aTe^, where Te is the 

 temperature of the surface. The earth's surface also receives radiation 

 from the water vapor and the clouds in the atmosphere, but this amount, 

 which may be called R, is nearly always smaller than o-T"/. The differ- 

 ence, cTe^ — R, represents the ''effective radiation of the earth's surface, '^ 

 or the "nocturnal radiation" (p. 58). The greatest possible loss of 

 radiation directly from the earth's surface would occur if the effective 

 radiation escaped without being absorbed in the atmosphere, and the 

 smallest possible loss would take place if the effective radiation were 



