Understanding the Weather [ 427 



they have mass, and gravitation pulls them downward, giving them 

 weight. As a rule you do not think of the pressure of air against 

 your body; but when you drive from mountain heights to a lower 

 level the change to "heavier air" is very noticeable especially on 

 your eardrums. 



How WINDS ARE FORMED 



For a number of reasons we have high-pressure areas, in 

 which the air is cooler and therefore more compact, and low- 

 pressure areas, in which the air is warm and expanded. (One 

 reason for this is that different portions of the surface of the earth 

 heat up or cool off at unequal rates.) Wind is air moving from 

 high-pressure areas to areas of low pressure nature's effort to 

 equalize pressure differences in the atmosphere. 



A high or low-pressure area may range from a few hundred to 

 a thousand miles in diameter. In a "high," the pressure increases 

 steadily toward its center, and in a "low" it gradually decreases. 

 The speed of a wind depends on the degree of pressure differ- 

 ence between a low-pressure area and the high-pressure area next 

 to it. When we have stormy weather these variations show up 

 sharply on the weather map in your daily newspaper; this is an 

 especially good time to study the weather map with your child. 



Why It Rains 



"Look at the size of those raindrops!" is an exclamation that may 

 start a child wondering about why we have raindrops at all, and 

 why they are of varying sizes. 



The "makings" of rain are around us all the time tiny droplets 

 of water that we know as vapor and bits of water-attracting dust. 

 These dust particles two important kinds are salt from the sea, 

 and smoke become the center of the droplets when the vapor in 

 the air takes the form of moisture. Air can hold only a certain 

 amount of vapor (warm air can hold more than cold air) , and 

 when there is too much vapor, the droplets of water join together 

 and form raindrops. 



