Evaluating the Surface- Temperatures of the Planets. 753 



£ = 0'1. The absorption without cloud is according to 

 Lowell about 0*3. With cloud much is reflected back 

 without reaching the lower and more absorbing regions. 

 Let us guess that a = 0'2. Of the radiation from the surface 

 we may suppose perhaps that 0'2 passes through, that 7 is 

 reflected, and that 0*1 is absorbed. Of the 0*2 passing we 

 may suppose that 0*1 is absorbed and O'l goes into space. 

 Then ^ = 0*1 and a l = 0'2. 



With these values we get for the different values of n 



(a) J = l; (h) J = 1-08; (c) | = 1-31. 



These guesses, then, make the temperature under a cloudy 

 sky at least as great as under a clear sky. But this is 

 certainly not true in common experience, where, however, 

 we may have clouds accompanied by cold winds and no 

 approach to the steady state here assumed. The results 

 merely serve to show that with certain absorptions and 

 transmissions clouds might actually raise the surface- 

 temperature, and that for the present it is better to neglect 

 them. 



Mars. — If we apply Lowell's data for Mars we have 



* = 0-64, and a = 0*40x 0-65 =0*26,, 



^ = 0'G and a 1 = 0*4, since R is dark radiation. 



W r ith these values we get for the different values of n 

 (a) |'=0-99; (J) |. =1-02; (c) £'=1-10. 



Comparison of the Earth and Mars. — Let us take the 

 temperature of the Earth as 17° C. or 290° A. If it were 

 removed to the distance of Mars its temperature would be 

 inversely as the square root of the distance, which is 1'524 

 that of the Earth or 290/1*235 = 235°. 



With the different values of n the temperature of Mars 

 should be 



qq 



(a) 235 X^ =247° A. or -26° C, 



v ' 94 ' 



{b) 235 x ^=242° A. or -31° C., 



(c) 235 x ii? =231° A. or -42° C. 



Of course the data are very uncertain and the formula 

 used is only an approximation. But with these data it is 



