Mingling of Molecules 287 



range, it either would have to stop, or rise and go over 

 the mountain. The pressure of the air behind it, mov- 

 ing in the same direction, keeps it from stopping, and 

 so it has to go up the slopes and over the range. But 

 as it goes up, there is less air above it to push down on 

 it ; so it expands. This makes it cool, and the water 

 vapor in it begins to condense and form snow or rain. 

 Therefore the side of mountain ranges against which 

 the wind usually blows, almost always has plenty of 

 rainfall. 



It is different on the farther side of the mountain 

 range. For here the air is sinking. As it sinks it is 

 being compressed. And as it is compressed it is heated. 

 If you hold your finger over the mouth of a bicycle 

 pump and compress the air in the pump by pushing down 

 on the handle, you will find that the pump is decidedly 

 warmed. When the air, sinking down on the farther 

 side of the mountain range, is heated, the water vapor 

 in it is not at all likely to condense. Therefore rain 

 seldom falls on the side of the mountains which is turned 

 away from the prevailing winds. 



How dew and frost are formed. The heat of the 

 earth radiates out into the air and on out into space. 

 At night, when the earth loses its heat this way and does 

 not receive heat from the sun, it becomes cooler. When 

 the air, carrying its water vapor, touches the cool leaves 

 and flowers, the water vapor is condensed by the cool- 

 ness and forms drops of dew upon them. Or, if the 

 night is colder, the droplets freeze as they form, and in 

 the morning we see the grass and shrubs all covered 

 with frost. 



