414 



RECORD OF SCIENCE FOR 1887 AND 1888. 



Dr. Vettiii, or may be specially calculated from the data previously 

 given for the cooling of ascending air: 



Table XXIV. — Average aUitiidc of clouds at Berlin, Germany. 



Month. 



December . 

 January. .. 

 February. . 



March 



April 



May 



June 



July 



August — 

 September. 

 October — 

 November . 



In addition to the intrinsic lightness and ascensive power of every 

 cloud, due to its internal moisture and heat, another powerful external 

 influence comes into play. It has been before said that about 85 per 

 cent, of the radiated solar heat penetrates the atmosphere to within 

 15,000 feet of the earth's surface. But no a[»preciable part of this is 

 able to penetrate through a thick layer of cloud or fog. Consequently 

 all the heat that falls upon the top of a cloud is retained therein instead 

 of reaching the earth's surface, and, both by evaporating the particles 

 of vapor and by warming the adjacent air, contributes powerfully to 

 diminish the specific gravity of the cloud as a whole, increasing the up- 

 draft and disturbing the vertical distribution of heat in the atmosphere. 

 The maximum temperature thus communicated to the top of a cloud is 

 not far from the maximum temperature attainable anywhere at the sur- 

 face of the earth. 



(VI) Special suggestions. — It will therefore be seen that invisible aque- 

 ous vapor disturbs the equilibrium of the atmosphere in the following 

 three ways : 



(1) By its evaporation and subsequent distribution throughout the 

 air it renders the atmosphere, over certain geographical sections, spe 

 cifically lighter than others. 



(2) By its condensation into fog or clouds and rain or snow it transfers 

 to distant regions the heat that had been accumulating in each particle 

 before its evaporation at the earth's surface. 



(3) In the form of fog or cloud it causes to be retained, at the upper 

 surface of the cloud, the total amount of heat that would otherwise 

 reach the earth, thus increasing the up-draft within a cloud, but dimin- 

 ishing the evaporation at the earth's surface, by which the clouds are 

 supplied with moisture. 



