540 F. H. KNOWLTON EVOLUTION OF GEOLOGIC CLIMATES 



meiited by carbon dioxide^ acts as a trap to imprison the heat rays, thus 

 producing the so-called blanketing effect. Any increase or decrease in 

 the efficiency of this thermal blanket may have very profound and far- 

 reaching results. 



POWER OF CLOUDS IN REFLECTING THE SUN'S RAYS 



Some very recent observations (1918) were made by Dr. L. B. 

 Aldrich/^ of the Astrophysical Observatory, on the power of clouds to 

 reflect the rays of the sun. By means of a captive balloon he was able 

 to secure readings from a pyranometer from the surface of a fog bank 

 when the upper and lower surface of the fog lay respectively about 2,800 

 feet and 1,000 feet above the ground. He says : 



"The measurements were singularly concordant and satisfactory, and gave 

 as the mean reflecting power of the fog during the interval from 7 until 11 

 o'clock 78 per cent. No apparent change owing to the change of the height 

 of the sun during that time was observed. However, it is hardly question- 

 able that if the measurements had been made nearer sunrise the reflective 

 power of the fog would been found somewhat greater. Accordingly, we must 

 suppose that if there should be a planet completely covered with smooth 

 clouds it would reflect upwards of 78 per cent of the solar rays otherwise 

 available to heat its surface. In the case of the earth, the cloudiness is about 

 50 per cent, so that if the clouds were as smooth on this surface as the clouds 

 observed, the result would be that they would reflect away about 39 per cent 

 of the solar rays and make ineffective to warm the earth. Taking this result 

 in connection with the consideration of the other parts of the earth's surface, 

 it appears that the reflecting power of the earth as a whole for solar rays of 

 all wave lengths should be in the neighborhood of 43 per cent." 



The value of these observations is not to be underestimated. If the 

 earth was surrounded by a complete cloud envelope through which only 

 approximately 20 per cent of the sun^s rays could penetrate and become 

 available for heating the earth's surface, it can hardly be questioned that 

 this source of heat is inadequate. 



SOURCES OF HEAT 



In general. — According to the late S. P. Langiey, the earth, on the 

 average, is still over 52 per cent cloud-covered, and this condition, be it 

 recalled, is with the sun as the direct controlling factor in heating the 

 earth's surface. It was long ago suggested that this cloud envelope, or 

 thermal blanket, was much more extensive in the ages before the Pleisto- 

 cene than it is now ; in fact, it is held that the cloud mantle was prac- 

 tically complete. Let us examine the data and see what warrant there is 



26 Smith. Misc. Collections, vol. 70, 1919, p. 28. 



