382 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



If we compare the value of the evaporation here given as the 

 equivalent of the total insolation with the observed rainfall of the 

 globe we come to the conclusion that either the value 5 = 2.5 is still 

 far too high or that out of the total insolation falling on the boundary 

 of the atmosphere only a much smaller portion arrives in the lower 

 stratum than one might expect from the measures of absorption 

 made on very clear days. 8 



This fractional portion could be easily estimated if the average 

 precipitation were known for the whole earth; since, as just stated, 

 the re-evaporation of the fallen precipitation represents the principal 

 work that the sun's heat has to perform. 



Unfortunately we are not able to give even an approximately 

 reliable value of this precipitation, since observations of the quantity 

 of rain are almost wholly wanting for the greater part of the globe, 

 namely for the ocean. 



If the average precipitation be 55 centimeters and if 5 = 2.5, 

 then the heat consumed in the evaporation of this amount of water 

 would be one-tenth of the total furnished by the sun and we should 

 therefore have to assume that the heat which reaches the lower 

 strata of the atmosphere amounts to not much more than one- 

 tenth. If the average precipitation were no centimeters, which 

 certainly seems to be too high an estimate, then we should conclude 

 that about one-fifth of the total solar radiation reaches the lower 

 strata. 



In any case the quantity of heat reaching the surface of the earth 

 is a much smaller fraction of the total insolation than has been given 

 by the measurements hitherto made on perfectly cloudless 

 days. 



Of course a very considerable fraction of the incident radiation 

 is absorbed by the clouds and certainly a still larger part of it is 

 reflected by their upper surfaces, and thus a quantity of radiant 

 energy is rejected at the very threshold. 



" It needs but a single glance from the summit of a mountain down 

 on the ocean of cloud lying below one and illumined by the sun to 

 convince one that the diffuse reflection from that surface is incom- 

 parably greater than the similar reflection from the ground or from 

 a water surface and that therefore this must play a very important 

 part in the thermal economy of the earth." 



How strong this reflection is (and to it I have moreover often 

 referred, although it seems to have received but little attention) may 



See Angot-Pernter: Met. Zeit., Ill, 1886, pp. 540-546. 



