22 WATER 



and have been transpired; that is, breathed out, by the leaves. 

 On cloudy nights dew is not likely to form because the clouds act 

 like a great blanket and prevent rapid cooling of the ground, so 

 that the temperature of the earth and the air in contact with it do 

 not differ enough to favor the condensation of moisture. Again, 

 when high winds prevail, moisture is evaporated as fast as it con- 

 denses and hence no dew is formed. Frost may be regarded as 

 frozen dew; that is, the surplus moisture in this case is frozen as 

 it reaches the point of condensation. 



Clouds prevent frost by holding in the heat which has accu- 

 mulated during the day, because any heat radiated into space is 



Snowflakes. 



caught by the surface of the clouds and is thrown back to the earth. 

 We often effect the same result by placing coverings over plants 

 on cold nights to protect them from frost. High winds cause rapid 

 evaporation and so prevent frost. Again, the warm air, being 

 lighter than the cold air, which drives it out of the valleys and low- 

 lands, covers the hills and ridges and so often protects plants 

 from the damage done by frost, while in the low places vegetation 

 suffers severely from its effects. Dust and smoke also act like 

 clouds in stopping radiation and preventing frost. 



If a few feet or a few hundred feet of the lower air cool to the dew- 

 point, a fog is formed in place of dew. The particles of invisible 

 vapor unite and become visible, but are so light that the air still 

 supports them. Clouds do not differ very materially from fogs 

 except that they are formed higher in the air. They are seen at 

 heights that vary from a few hundred yards to distances ranging 

 from five to ten miles above sea level. Finally, these fine particles 



