CHAPTER XXII 

 HEAT AND THE ATMOSPHERE 



Now we have a picture in mind of the atmosphere, and 

 we know something of the nature of heat. We know that 

 it is a form of energy that tends to increase molecular 

 motions (see Chapter XVI), and that what we perceive 

 as heat or cold is simply the effect on us of different rates 

 of these molecular motions. So now our problem is to 

 consider the heat of the sun as applied to our atmosphere, 

 to see what effects this atmosphere produces upon the 

 distribution of this heat, and to realize what relations the 

 resulting phenomena have to life. 



You have learned that heat may be absorbed. So it 

 is easy for you to realize that the amount of solar (sun) 

 heat that reaches the solid and liquid surfaces of earth 

 is much less than the original amount that comes to earth 

 from the sun. For the heat "rays" coming to earth 

 first encounter its gaseous surface; i. e., the outer limits 

 of the atmosphere. In passing through the atmosphere 

 much of the heat is absorbed, so that the amount of it 

 which finally reaches the land and water surfaces (at the 

 bottom of the great enveloping sea of air) is far less than 

 that originally received at the atmospheric surface. 



In view of what has just been said, why is it that, gen- 

 erally speaking, the higher up we go the colder it gets? 

 You know that mountain tops, although nearer the source 

 of heat, are generally colder than the land which lies be- 

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