CHAPTER X 

 WEATHER 



Meaning of Term " Weather." The term " weather " is 

 a comprehensive one. It includes all the many conditions 

 of the earth's atmosphere, such as its temperature, its 

 pressure, and the amount of moisture it contains, the 

 variations of the wind, the amount of rainfall, and the 

 appearance of the sky. We have made quite a careful 

 study of the two substances with which we are concerned 

 in. the study of weather. These two substances are air 

 and water air or atmosphere, as the outer portion of 

 the earth ; and water in its many forms : as water vapor 

 in the atmosphere, clouds, fogs floating in the atmosphere ; 

 and rain, snow, hail, dew, and frost as it leaves the atmos- 

 phere again. The weather is almost constantly changing. 

 We say " It is cold to-day " or "It is sultry to-day " 

 according as the conditions of the atmosphere are such as 

 to produce the one effect or the other. The average of 

 the weather conditions for a given place is the climate of 

 the place. 



Functions of the Air. The atmosphere is of vital 

 importance and serves us in a number of important func- 

 tions : (1) It supports life in various ways ; (2) it conducts 

 sound ; (3) it transfers and distributes heat, and also 

 acts as a non-conductor of heat under certain conditions ; 

 (4) it diffuses light ; (5) it supports combustion ; (6) it 

 drives wind-power machines ; (7) it exerts a buoyant 

 force on all objects immersed in it, thus enabling some 



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