92 ANlSrUAL EEPOET SMITHSONIAN INSTITUTION, 1917. 



regions ascends toward space. But convection grows less and less as the air 

 becomes rarer, and must at length cease to be an appreciable factor. It is the 

 water vapor and carbonic-acid gas far above the earth's surface, where the 

 absorption of the rays by the water vapor and carbonic-acid gas lying still 

 higher becomes small, that form the true radiating surface of the earth con- 

 sidered as a planet. * * * With the scanty material at hand, and in con- 

 sideration of the distribution of water vapor in the free air, it seems safe to 

 put the effective position of the radiating surface at fully 4,000 meters above 

 sea level * * * at a probable mean temperature of 263° absolute centi- 

 grade or — 10° centrigrade. 



Some writers have misinterpreted these remarks and understood 

 us as supposing that there is a special layer at 4,000 meters elevation 

 above sea level which prevents radiation escaping from below and 

 whose own radiation passes unhindered to space. Our meaning was 

 quite different. Every layer from sea level to the limit of the atmos- 

 l^here contributes something to the total radiation output of the earth. 

 But, because of the great absorption of superposed water vapor and 

 clouds, the lower solid and liquid and atmospheric layers contribute 

 little, while because of their dryness the higher atmospheric layers 

 contribute little. Eoughly estimating the various factors, we con- 

 cluded that the center of activity of the radiation of the earth as a 

 planet could be set at about 4,000 meters elevation. 



How far are these conclusions now to be altered ? As to the effect 

 of cloudiness, not at all. As to water vapor Mr. Fowle finds the 

 following results on the percentages of absorption of rays from a 

 perfect radiator at the earth's mean temperature in atmospheric 

 columns containing besides carbon dioxide sufficient to produce maxi- 

 mum absorption, water vapor which if precipitated would produce 

 certain depths of liquid water: 



Ppt. water"" 0.003 0.03 0.3 3.0 



Absorption 49 57 66 75 



In order to apply these data I give figures for the average quantities 

 of terrestrial water vapor which, according to Hann, exist in vertical 

 columns from sea level to the limit of the atmosphere over different 

 zones of the earth. 



Latitude 0-20° 20°-30° 30°-40° 40°-.50° 50°-60° 60°-90° 



Ppt. water""___ 4.3 3.1 1'.2 1.6 1.0 0.6 



From these figures it may be seen that the statement, "a tenth 

 part of the average amount of water vapor in the vertical column 

 above sea level is enough to absorb more than half of the radiation of 

 the earth to space," is confirmed. But the conclusion therefrom that 

 " nine-tenths of the radiation of the solid and liquid surface of the 

 earth is absorbed by the water vapor of the atmosphere on clear days " 

 is not confirmed. Mr. Fowle has computed the absorption of the at- 

 mosphere in a state of humidity corresponding to 1.0 cm. ppt. water, 

 and finds it 72 per cent. Considering that the ppt. water in a vertical 



