CLOUDS, RAIN, SNOW, AND HAIL. 



273 



the margin, the temperature of the earth itself. The directions of the arrows 

 represent the radiation and reflection of the heat 



What are 

 clouds ? 



SECTION II. 



CLOUDS, RAiy, SXOW, AND HAIL. 



607. Clouds consist of vapor evaporated from 

 the earth, and partially condensed in the 

 higher regions of the atmosphere. 



How is mist or When air, saturated with vapor, in imme- 

 fogoccasioued? ^jjg^^g coutact with the surface of the earth is 

 cooled down rapidly, its vapor is condensed ; if the con- 

 densation, however, is not sufficient to allow of its precipi- 

 tation in drops, it floats ahove the surface of the earth as 

 mist or fog. 



Clouds, fog, and mist diflFer only in one re- 

 spect. Clouds float at an elevation in the air, 

 while fogs and mists come in contact with the 

 surface of the earth. 



Mist and fog are also formed when the water of lakes and rivers, or tha 

 damp ground, is warmer than the surrounding air wliich is saturated with 

 moisture. The vapors which rise in consequence of the higher temperature 

 of the water, are immediately recondensed, as soon as they diffuse themselves 

 through the colder air. 



ilist and fog are observed most frequently over rivers and marshes, be- 

 cause in such situations the air is nearly saturated with vapor, and therefor* 



12* 



How do clouds, 

 tf'<K. and mist 

 differ? 



