364 C. Abbe — Atmospheric Radiation of 



Akt. XLY. — Atmospheric radiation of Heat and its Impor- 

 tance in Meteorology / by Cleveland Abbe. 



(1.) It is evident that our planet, considered as a whole and 

 on the average of many years, loses all the heat that it receives 

 from the sun ; but all the details of this process have not yet 

 been completely worked out. Fourier, Poisson and their dis- 

 ciples have treated of this radiation in the most general man- 

 ner ; Melloni, Tyndall, Langley and their followers have gone 

 into some of the details as to character and quantity. These 

 physicists have served to awaken meteorologists to the necessity 

 of carefully distinguishing the influence of the absorption and 

 radiation by the soil, the vegetation, the snow and the ocean 

 respectively ; even as to the atmosphere itself we have to dis- 

 tinguish the absorbing and radiating powers of the dry air, of 

 the clouds or haze and of the dust respectively. 



(2.) Heat is thrown into the atmosphere not only by conduc- 

 tion and convection from the immediate surface of continents 

 or oceans, but the radiant heat from these surfaces is in part 

 directly absorbed (just as is solar radiation) by the atmosphere 

 and in part transmitted to outer space. All of the immense 

 quantity of heat thus communicated to the whole atmosphere 

 (viz. 50 per cent of the direct solar radiation, plus about 50 

 per cent of the specular reflection from the earth and water, 

 plus all of the convection from the surface of the earth and 

 ocean, plus a large per cent of the radiation from the earth's 

 surface) must be lost through a process of radiation from the 

 atmosphere as distinguished from radiation through the atmos- 

 phere; judging from the known values of these items one 

 would a priori conclude that this radiating power of the whole 

 atmosphere must average about 0'5 calories per minute per 

 square centimeter of its outer surface. 



(3.) At first thought one would say that, according to Kirch- 

 hoff's law any radiation from a mass of air must be of such a 

 quality as to be immediately absorbed by the adjacent air in 

 the attempt to pass through it and therefore cannot escape into 

 space except when radiated by the very outermost layer of the 

 atmosphere. But in reply to this we note that the processes of 

 radiation from and absorption by layers of air within the 

 atmosphere depend (like those of conduction) upon differences 

 of temperature and possibly on differences in density conse- 

 quently it is quite possible that heat radiated from the warmer 

 and denser, moister and dustier layers below may in part escape 

 directly through the upper layers into ethereal space ; that 

 portion of this heat which is absorbed by the upper layers will 

 in turn, by a similar process, escape from them. 



