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, 
65 
Cloud- 
making. 
