THE SEASON WHY. 99 



"And Gideon said unto God, * * * Let it now be dry only upon the fleece, and 

 upon all the ground let there be dew. 



formation of clouds, and also by the moisture they impart to 

 the air. 



409. Why does the humidity of the atmosphere sometimes 

 form clouds, and at others form fogs, mists, dews, fyc. ? 



The result depends upon the varying temperature, motion, and 

 direction of the atmosphere. 



A warm light atmosphere, of a few day's duration, will elevate 

 the vapours to the region where they are formed into clouds. 



A chill air, lying upon the surface of the warmer earth, will 

 occasion mists or fogs. 



A cold earth, acting upon the vapours contained in a warmer 

 atmosphere, will condense them and occasion dews. 



410. Why are frosty mornings usually clear ? 



Because, in the cold atmosphere which preceded the frost, there 

 was but little evaporation ; and now .that the frost has set in, the 

 vapours that existed have become frozen in the form of hoar-frost. 



411. "Why are clear nights usually cold? 



Because the "screen" afforded by the clouds does not exist; 

 therefore the heat of the earth escapes, while the vapours of the air 

 are abstracted from it by condensation into dew, thereby imparting 

 great clearness to the nights. 



412. Why are hoar-frosts, or, as they are termed, " white 

 frosts" so frequent, and " Hack frosts" so unusual? 



Because white, or hoarfrosts, result from the coldness of the 

 earth, which, from its great radiating power, is always varying. 

 But black-frosts result from the coldness of the air, which is liable 

 to less variation of temperature than the earth. 



413. What is a black-frost? 



A black frost results from the coldness of the atmosphere, which 

 is at the time overshadowed by a dull cloud, giving a dark- 

 ness to everything, and a leaden appearance to the frozen surface 

 of water. 



414. Why are black-frosts said to last ? 



