REPORT OF THE SECRETARY. 93 



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 cm ___ 4.3 3.1 2.2 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 is 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 

 column over most of the earth exceeds 3.0 cm., it now seems probable 

 that the proper figure should be eight-tenths instead of nine-tenths. 



As regards the absorption of carbonic-acid gas Mr. Fowle finds 

 that one-fortieth part of the amount of this gas found in a vertical 

 atmospheric column produces the maximum possible effect. This 

 does not lead to any modification of our conclusions as to the effect of 

 atmospheric carbonic acid gas as stated above. 



With ordinary humidity, at sea level a layer of air 10 meters long, 

 according to Fowle, will absorb 50 per cent of the radiation of a 

 perfect radiator at terrestrial temperatures. Similarly the layer of 

 air above 11 kilometers, or 6 miles, altitude contains enough water 

 vapor to absorb 50 per cent of such radiation. 



In view of what has been said and remembering the presence of 

 clouds, only about one-tenth of the radiation of the solid and liquid 

 surface of the earth escapes directly to space. The atmosphere 

 above 11 kilometers apparently contributes more than half of the 

 radiation of the earth viewed as a planet and prevents half of the 

 radiation of lower layers from escaping. Nearly the entire output 

 of radiation of the earth to space, certainly more than three-fourths, 

 arises from the atmosphere and its clouds as its source. The " ef- 

 fective radiating layer," meaning a layer which if perfectly radiating 

 to space would equal in radiation the actual earth viewed as a planet, 

 may still be thought of as at several kilometers altitude and at a 

 temperature well below freezing. 



The subject of atmospheric absorption is so difficult both theo- 

 retically and experimentally that much more investigation ought still 

 to be done on it. Mr. Fowle's long experience has well fitted him 

 for making further advances. It is hoped to put at his disposal 

 soon the necessary means to make new researches. These include 

 bolometric apparatus of greatly increased sensitiveness, such - ' 



25027—17 7 



