Fogs and Clouds 117 
pendent upon the temperature of the air involved. At high 
temperatures the air is capable of holding large quantities of 
water in the vaporized state; but at low temperatures it holds 
much less. The amount of water that warm air over a tropical 
sea can evaporate may be 30 or 40 times that which cold air 
over a polar sea can contain. 
If a warm moisture-laden body of air is suddenly cooled, 
its water-holding ability is reduced and some of the water 
will be condensed to the liquid state as tiny water droplets, 
or to the solid state as ice spicules. Because of their small 
size the tiny particles of water or ice float in the air. The re- 
sult is a cloud. 
HOW A CLOUD FORMS 
The necessary cooling may occur in one of two ways: (1) 
by radiation, which simply means that the heat energy radi- 
ates away or (2) by advection, that is, a sudden change in 
the volume of the air. Expansion of a mass of gas cools it; 
compression warms it. Fogs are caused both by radiation 
and advection; clouds are usually formed by advection alone. 
A mass of moisture-laden air ordinarily forms clouds when it 
moves into lower pressure areas. If you observe clouds on a 
fine day you can see them growing or dissolving away. The 
best way to decide whether clouds are growing, staying 
pretty much the same, or dissolving away is to choose a small 
wisp of cloud and look at it at thirty-second intervals. You 
will be able to judge very quickly the state of the clouds. 
CLASSIFICATION OF CLOUDS 
Clouds occur in so many different forms that it is necessary 
to classify them according to some definite plan, as shown 
in the accompanying table. 
