1921] 



on Cloudland Studies 



421 



the earth temperature is known, and the amount of the atmospheric 

 radiation follows from the observed ratio. The value deduced 

 is the equivalent black body temperature of the radiating zenith. 

 To obtain this readily, curves were drawn, based on Stefan's law, for 

 the radiation into a liquid oxygen sink at 90° Abs. from a black body 

 at certain definite ordinary earth temperatures. These show as 

 ordinates the proportion to the unit black radiation at earth tem- 

 perature that would be emitted by a black body at lower temperatures 

 indicated as abscissas. The observation ratio Z/E is then sought on 

 the curve drawn corresponding to the known earth temperature ; and 

 from this on the scale of abscissae the theoretical black body tem- 

 perature of the observed sky radiation is obtained. Fig. 5 shows a 



RaHoi 



-i)0°C -70 



■50 -40 -30 -20 -10 

 Zenirh Temperahurc 



Fig. 5. 



o*c 



Black 

 Standard 



20°C 



set of such curves :: ' at 5° intervals of earth temperature from 0° C. to 

 20° C. As an example, with earth temperature of 15° C. and obser- 

 vation ratio of 0*6, the corresponding black body temperature is 

 - 20° C. Interpolations are readily made between the curves. 



Some specimens of the values obtained in this way are given in 

 Table III. The first column gives the date and time of the obser- 

 vations ; the second the prevailing surface temperature t ; the third 

 the observation ratio Z/E ; the fourth the deduced black body zenith 

 temperature T z ; and the fifth the difference, Af, between the earth 

 and the deduced zenith temperature. Xo measurements of surface 



* The equation of these curves is x A 

 x = T z ; and y = Z/E. 



y t\ where t = 0°, 5°, 10°, 15°, 20° ; 



