Evaluating the Surface-Temperatures of the Planets. 755 



. 2rS 2S 6 . , . 

 energy entering a sq. cm. column is — - = — = - cal./mm. 



°* ° ^ irr it it ' 



Then in 12 hours 1375 cal. enter on the average, and if 

 this heat were all absorbed and retained it would raise the 

 temperature on the average about 13 75/237*5 = 5 D, 8 C. 



As the absorption is only partial and as radiation takes 

 place from the air, the rise cannot really average nearly as 

 much as this. 



Again, consider the radiation during the twelve hours of 

 night. If the air were a black body and of temperature 

 300° A., and these are absurdly exaggerated estimates of its 

 radiating power and of its average temperature, it would 

 only radiate about 1*2 cal./min. per sq. cm. column from its 

 two surfaces, or 864 calories in the twelve hours, and 

 neglecting the radiation from the ground the temperature 

 would only fall about 864/237*5 or 3°'6 C. Obviously, then, 

 the air as a whole cannot undergo much variation in tempe- 

 rature as day alternates with night. It is indeed a flywheel 

 storing the energy of many diurnal revolutions. We may, 

 then, in a rough estimate consider that its temperature and 

 therefore its radiation remain constant during the 24 hours. 



If the total radiation from a sq. cm. column per second is 

 A, there will be a stream D downwards and U upwards 

 where D + U = A. We can find an expression for A by 

 equating it to the average absorption. Considering an 

 equatorial band 1 cm. wide, the average energy entering it 



per sq. cm. in the 24 hours is - Let the average amount 



absorbed be — The value of a at sea-level varies for 



IT ' 



clear sky from perhaps 0*3 with the zenith sun to very 

 nearly 1 with the setting sun. Let the average radiation 

 from the surface during the 24 hours be R, of which aiit 

 is absorbed by the atmosphere. Then neglecting conduction 

 through the air, the constant temperature assumption gives us 



A = — 4-ajR. 



IT 



If a fraction - is radiated downwards 

 n 



D= — +— . 



wit n 



The actual surface temperature depends not only on radia- 

 tion but also on conduction both by ground and air. But 

 we shall neglect this conduction and shall suppose that the : 



