2O 



ELEMENTARY SCIENCE 



So the sun explains how the air starts to circulate. But 

 that is not enough. We want to know about the winds 

 that blow from every direction. Sometimes they are very 

 gentle and pleasant, sometimes they are very strong and 

 destructive. What makes the air behave this way? 



The air behaves in this way because it is always getting 

 stirred up, and then tries to smooth itself out again. A 

 high wind is just the rushing in of a lot of air to take the 

 place of other air that has moved away. The thing that 



keeps the atmosphere 

 stirred up is the heat 

 of the sun. You know 

 how heat keeps the 

 water in a kettle 

 stirred up. Somewhat 

 in the same way the 

 heat of the sun keeps 

 the air stirred up. 



The heat of the sun 

 comes down on the 

 air, and the air catches 

 some of this heat and 



holds it. The air is a sort of trap for heat. But some 

 parts of the world get more heat than other parts. Also 

 the amount of heat received from the sun varies at dif- 

 ferent times of day. So, always, some parts of the air get 

 hotter than other parts, and that is what keeps the air 

 moving. For, just as soon as any air gets warmer than 

 the air around it, it begins to move. It begins to move 

 up, because it is lighter than the rest of the air (see Fig. 2). 

 Now, because the air is always stirred up in this way, 

 the amount of air over your head keeps changing. If a 



FIG. 2. Diagram to illustrate how the air be- 

 comes heated at the equator and moves up- 

 ward. After SALISBURY. 



