278 Common Science 



When air or any gas expands, it cools. Do you remem- 

 ber Experiment 31, where you let the gas from a tank 

 expand into a wet test tube and it became so cold that 

 the water on the test tube froze? Well, it is much the 

 same way with rising air. When air rises, there is less 

 air above it to keep it compressed; so it expands and 

 cools. Then the water vapor in it condenses into drop- 

 lets of water, and these form a cloud. 



Each droplet forms a gathering place for more con- 

 densing water vapor, and therefore grows. When the 

 droplets of water in a cloud are very close together, 

 some may be jostled against one another by the wind. 

 And when they touch each other, they stick together, 

 forming a larger drop. When a drop grows large enough 

 it begins to fall through the cloud, gathering up the 

 small droplets as it goes. By the time it gets out of the 

 cloud it has grown to a full-sized raindrop, and falls 

 to earth. The complete story of rain, then, is this : 



How rain is caused. The surface of the oceans and 

 lakes is warmed by the sun. The water evaporates, 

 turning to invisible water vapor. This water vapor 

 mingles with the air. After a while the air is caught 

 in a rising current and swept up high, carrying the 

 water vapor with it. As the air rises, there is less air 

 above it to press down on it ; so it expands. When air 

 expands it cools, and the water vapor which is mingled 

 with it likewise cools. When the water vapor gets cool 

 enough it condenses, changing to myriads of extremely 

 small drops of water. These make a cloud. 



A wind comes along; that is, the air in which the 

 cloud is floating moves. The wind carries the cloud 



