FEBRUARY 155 



Hail : 



If the water vapor does not encounter freezing air until 

 it has condensed into drops, until, in brief, it has be- 

 come rain, then we have hailstones formed. 



Dew : 



After the sun goes down, the accumulated heat of the 

 day is given back into the air often more rapidly than it 

 can be drawn up from the ground below. This is par- 

 ticularly true of blades of grass, because of their greatly 

 exposed surface. In consequence, they become cold 

 more quickly than the surrounding atmosphere, and on 

 them is condensed water vapor in the form of dew. Peb- 

 bles, however, do not give off heat much more rapidly 

 than they can draw it up, therefore they do not become 

 much colder than the air, and therefore do not have dew 

 formed upon them to any extent. For this reason gravel 

 paths will be dry even in the early morning, while the 

 grass adjoining is wet. 



Anything which will keep the heat from radiating 

 from a surface, will keep that surface free from dew. 

 Thus while cobwebs fairly sparkle with dewdrops, the 

 grass beneath is perfectly dry. Clouds act like the cob- 

 web, preventing rapid radiation of heat. Therefore dew 

 is not formed on a cloudy night. 



Frost : 



This is not frozen dew, but vapor condensed as a solid 

 instead of as a liquid. In its formation it is analogous 

 to snow, not to hail ; hence its crystalline form. 



Effects of Frost : 



Every farmer knows that frost breaks up the ground. 

 For this reason, he often ploughs immediately after a 



