INTRODUCTION 3 



temperature, in accordance with Planck's law, whereas the emissive 

 power of a gas may correspond to that of a black body of the same 

 temperature at a few wave lengths only and may be nearly nil at other 

 wave lengths. The rate at which radiation is absorbed per unit area, 

 the "absorptive power," is related to the emissive power in a manner 

 that is expressed by Kirchhoff's law: At a given temperature the ratio 

 between the absorptive and emissive power for a given wave length is the 

 same for all bodies. This law implies that, if a body emits radiation of a 

 given wave length at a given temperature, it will also absorb radiation 

 of that same wave length. Therefore, any gas which emits only a radia- 

 tion of certain wave lengths also absorbs only radiation of these wave 

 lengths, and this type of radiation and absorption is called selective. 



Radiation from the Sun. The radiation from the sun is being 

 examined by the Smithsonian Institution at high-altitude stations in 

 California, Chile, and Sinai. By measurements at high altitudes, with 

 the sun at different distances from zenith, it is possible to draw con- 

 clusions as to the solar radiation that reaches the limit of the atmosphere. 

 It has been found that at wave lengths greater than 0.3 /i this radiation 

 has nearly the character of radiation from a black body at an absolute 

 temperature of 5600° to 6000°. The maximum intensity lies close to a 

 wave length of 0.5 n, and at wave lengths greater than about 2.6 /i the 

 energy is negligible. The radiation from the sun is therefore called 

 short-wave radiation. It is continuous except for a few bands that show 

 the effect of absorption in the sun's atmosphere. The intensity of normal 

 incident radiation is, on an average, 1.94 g cal/cmVmin, and this intensity 

 is called the ''solar constant." 



Evidence has been accumulated which indicates that in the ultra- 

 violet, at wave lengths shorter than 0.3 m, the sun does not radiate as a 

 black body but emits a greater and possibly variable amount of energy. 

 This energy is absorbed in the very highest atmosphere and is not 

 measurable. In the following sections it will therefore be necessary to 

 disregard this component of the solar radiation, although it may con- 

 tribute significantly to the heat balance of the earth and the heat budget 

 of the atmosphere. 



Of the incoming radiation a considerable fraction is reflected from 

 the atmosphere, the earth's surface, and the upper surfaces of the clouds. 

 According to Aldrich (Brunt, 1939) the average fraction for the whole 

 earth is 0.43. This figure therefore represents the albedo of the earth. 

 The reflected portion of the solar radiation plays no part in the heat 

 budget of the earth. During one year the average solar radiation that 

 reaches the limit of the earth's atmosphere is SirR^/iTrR^ = S/i, where S 

 is the solar constant and R is the radius of the earth. With S = 1.94 

 g cal/cmVmin, one obtains S/i = 0.485 g cal/cmVniin, of which 0.209 

 is lost by reflection and 0.276 is lost by back radiation to space. 



