; September 18, 1914] 



SCIENCE 



423 



appears to have been chosen to suit the hypoth- 

 esis.'' In the isothermal layer, by the same 

 principles, there should be negative absorp- 

 tion! Purther comment seems unnecessary. 

 I hasten to add that interchange of radiation 

 between neighhoring air masses is an essential 

 part of radiant progression through the atmos- 

 phere. 



General Results 



I find that vsrith good blue sky there is radi- 

 ated from a land surface near the middle of 

 the temperate zone something like a thermal 

 equivalent of 0.15 gram cal./sq. cm. min. in 

 summer, and 0.22 gram cal./sq. cm. min. in 

 winter; and these correspond, respectively, to 

 transmissions of 26 and 58 per cent., the mean 

 transmission being 42 per cent., when the sky 

 is quite clear. 



In spite of the higher temperature of land 

 surfaces in summer, there is no greater direct 

 outward radiation from these surfaces than in 

 winter, but even a somewhat smaller one, be- 

 cause the radiation has to pass through a more 

 absorbent atmosphere. When the sky is over- 

 east with clouds, direct surface radiation to 

 space ceases, because the seat of action has 

 simply been transferred to the upper surface 

 of the cloud, where a larger proportion of the 

 incoming solar rays is directly reflected back to 

 space, and that which is absorbed is largely 

 transformed into latent heat to reappear else- 

 where after complex atmospheric processes. 

 The whole of the absorbed radiation which, is 

 manifested as heat in either earth or air must 



p. 283). The agreement of the air transmis- 

 sion of 40 per cent, found by these investigatovs 

 in this laboratory experiment with the value which 

 I have given for the entire atmosphere may be only 

 a coincidence; but the fact remains that there are 

 extensive regions of the spectrum which are not 

 emitted freely by air, and which therefore are not 

 much absorbed by the atmosphere and do not take 

 part in its interchange of radiation. 



7 Certain layers in the atmosphere, which are 

 cloud-laden and at various heights, stop all out- 

 going radiation and appear as if at the same tem- 

 perature as the surface, even though they may be 

 20°, or more, colder, and the difference of tempera- 

 ture bears no relation to the absorption by the 

 intervening air. 



ultimately return to space as radiation. There 

 are many steps in this process, and the return 

 is retarded in general, though variously re- 

 tarded, or subject to alternate acceleration and 

 retardation. 



There is room here for some obscurity, and 

 perhaps difference of opinion, as to what shall 

 be considered a direct return of terrestrial 

 radiation to space. All of the processes of ab- 

 sorption of solar radiation and its conversion 

 into and emission as terrestrial radiation, in- 

 volve a thermal mechanism and some delay. 

 If it is insisted that the return must be in- 

 stantaneous, the only " direct " radiation is 

 that which is reflected back to space. But this 

 is not at all what we mean by terrestrial radia- 

 tion. If, then, we grant that there must be 

 some delay in the return of radiation to space 

 after transformation into long waves, the re- 

 turn being so speedy that it may fairly be 

 called direct, there is really no reason (since 

 we permit a little delay in order that heat 

 may be conducted between various por- 

 tions of matter) why we may not include in 

 the direct radiation some of that which is due 

 to heat transferred from the surface to the 

 air by convection, and thence, in turn, radi- 

 ated from the air by the step-by-step process 

 which alone exists in that medium. In fact, 

 by an observer outside the earth in space, this 

 secondarily radiated heat could not readily be 

 distinguished from that emitted after all only 

 a little more directly. When the problem is 

 attacked by purely meteorological methods, we 

 find that these methods increase the computed 

 nocturnal radiation, making it as high as 58.5 

 per cent, of the total loss of surface heat by 

 radiation and convection combined, according 

 to the method developed by Dr. Percival 

 Lowell in his " Temperature of Mars." * 



Lowell's equation for nocturnal cooling may 

 be put in the form 



T -AT i ^ *h 

 T - ATi \l 



y{l — ae) 



ha -be)' 



where T is the average day temperature on the 

 absolute scale, AT^ is the nocturnal cooling 

 with clear sky, aT^ is the same with cloudy 

 8 Proc. American Academy of Arts and Sciences, 

 Vol. XLII., No. 25, March, 1907, p. 660-661. 



