The Audubon Societies 



55 



aiiay be dense, that is, thick and pressed together, or, on the contrary, thin 

 and light. 



If the air all over and around the earth was of exactly the same degree of 

 heat or cold (temperature), and of exactly the same weight (pressure), and of 

 -exactly the same wetness or dryness (humidity), as well as of exactly the same 

 thickness (density), it would have no motion at all and there would be only 

 one kind of weather everywhere. 



But this is never the case. The air may be very heavy in one place and 

 very light in another; or, it may be quiet where you live, while moving at a 



l.Ikh l;()\ PROTECTED FROM CATS 

 Photographed by Howard H. Cleaves 



furious rate elsewhere; it may even be freezing cold about the polar seas, and 

 ■at the same time summer-warm near the Gulf of Mexico. 



All of these states, or conditions, of the air, taken at the same time but in 

 •diiferent places and at different heights (altitudes), are what is called the 

 weather, and it is very important to understand all we can about the weather, 

 since we are obliged to live on this earth in weather which constantly changes. 



The air in motion is called wind. When the wind blows hard, we really 

 hear that part of the weather. The air full of water-vapor falls upon us in 

 rain or snow, according as it is warm or cold. When it rains or snows, we can 



