HEAT AND THE ATMOSPHERE 167 



You have already learned that land and water are better 

 heat absorbers than is air. Therefore, it may not surprise 

 you to learn that of the total heat of the air more is acquired 

 by conduction and radiation back from land and water 

 than is acquired by absorption directly from the heat- 

 rays of the sun. The air acquires heat from the sun 

 only when the sun shines on it, but it is always in a position 

 to acquire it from land and water. 



Convection. You learned of this process in studying 

 what happens when a body of water freezes, and you noted 

 then the convection currents of the atmosphere. They 

 are caused by heat, and they in turn are the causes 

 (though not the only ones) of winds and of the distribu- 

 tion of rain. Evidently, then, they are important factors 

 in the environment of life. 



It should not be at all difficult for you now to form a 

 mental picture of the great convection currents in earth's 

 atmosphere. You know that the surface of earth is hottest 

 in the equatorial regions. Hence, the air over these regions 

 will become warmer than the air of the regions to the 

 north and south. So it rises, and, as it rises, cooler air 

 from north and south comes creeping in to replace it. 

 As the warm ascending air reaches higher altitudes, it 

 cools, and then flows away toward the poles. Thus we 

 have a constant great convectional movement or circula- 

 tion of earth's atmosphere (see Fig. 2). This alone is 

 simple enough, but we must remember that the whole 

 matter of atmospheric movements is complicated by the 

 rotation of the earth, and by other factors. These are 

 matters to be taken up in the next chapters. 



