CHAPTER XIII 



THE WEATHER 



262. Causes of Weather. We have already learned 

 much about the earth's atmosphere: its composition, 

 density, and pressure (c/. 38 ff.); also about gases: 

 that they expand when heated, or when exposed to a 

 lower pressure; that they contract when cooled, or when 

 put under a higher pressure; that they give off heat when 

 compressed, but absorb it when expanded. We have also 

 studied about water: its three physical states, its great 

 heat capacity, and its presence in the atmosphere. These 

 topics form the basis of our study of the weather, and 

 should be reviewed carefully. By "weather" we mean 

 the changing conditions of the earth's atmosphere, such 

 as its temperature, its pressure, and the amount of 

 moisture it contains; the cloudiness or fairness of the sky; 

 the amount of rainfall or snowfall; the velocity of the 

 wind, and its direction. 



As we look over this list of " conditions of the weather," 

 we find that they are really the properties of two sub- 

 stances : air, as we find it in that great ocean, the atmos- 

 phere; and water, which enters the atmosphere so readily 

 by evaporation from seas, lakes, and rivers, and which 

 leaves the atmosphere again as clouds, rain, dew, snow, 

 and frost. 



The average of all the weather conditions for any given 

 place is the climate of that place. 



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