CHAPTER IV 

 CLOUDS AND CLOUD FORMS 



A CLOUD is always a cloud, no matter by 

 what name it may be called or what its form 

 or height above the earth. The fog that 

 knocks about our ears is made up of the same 

 visible vapors as the heaped-up cumulus rising 

 tower-like thousands of feet above us. That 

 one lies along the ground and that the other 

 rises to a lofty altitude is due merely to a differ- 

 ence in temperature and density. 



Clouds are formed by sudden lowerings of 

 the temperature of moist air; and this lower- 

 ing of temperature is usually caused by warm 

 air rising into higher altitudes, expanding as it 

 rises and cooling as it meets with the upper 

 cold-air currents. The simplest and most fre- 

 quent manner of cloud-making is this : The 

 radiation of heat from the earth forms into a 

 column-like current of air, and the natural 

 tendency of this current is to push upward, 

 seeking an exit into cooler regions. It keeps 

 rising, expanding as it reaches thinner air, 

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