Mingling of Molecules 277 



How water vapor gets into the air. The water vapor 

 gets into the air by evaporation. When we say that 

 water evaporates, we mean that it changes into water 

 vapor. As you already know, it is heat that makes 

 water evaporate ; that is why you hang wet clothes in 

 the sun or by the fire to dry: you want to change tin- 

 water in them to water vapor. The sun does not suck 

 up the water from the ocean, as some people say ; but 

 it warms the water and turns part of it to vapor. 



What happens down among the molecules when water 

 evaporates is this : The heat makes the molecules dance 

 around faster and faster ; then the ones with the swiftest 

 motion near the top shoot off into -the air. The mole- 

 cules that have shot off into the air make up the water 

 vapor. 



The water vapor is entirely invisible. No matter 

 how much of it there is, you cannot see it. The weather 

 is just as clear when there is a great deal of water vapor 

 in the air as when there is very little, as long as none of 

 the vapor condenses. 



How clouds are formed. But when water vapor 

 condenses, it forms into extremely small drops of real 

 water. Each of these drops is so small that it is usually 

 impossible to see one; they are so tiny that you could 

 lay about 3000 of them side by side in one inch ! Yet, 

 small as they are, when there are many of them they 

 become distinctly visible. We see them floating around 

 us sometimes and call them fog or mist. And when 

 there are millions of them floating in the air high above 

 us, we call them a cloud. 



The reason clouds form so high in the air is this: 



