THE ENGINE OF THE AIR 149 



nothing but this to act as motive power for every drop of rain 

 that ever fell, and for every wind that ever filled a sail or 

 wrecked a ship since the world began. This is the effect of 

 the climate engine. 



A relatively simple explanation of the forces that make the 

 air engine of climate work is given by Rossby in the U. S. 

 Agricultural Department Year Book of 1941. It goes like this: 

 The earth's atmosphere is heated not from above but from 

 below, by radiation of heat from the ground, caught largely 

 by water vapor at low levels. The atmosphere acts as a pro- 

 tective blanket to hold and raise the mean temperature of 

 the earth's surface. This heat, trapped before it can radiate 

 into space, is not evenly distributed, and therefore, seeking 

 to establish equilibrium, indirectly creates winds that in turn 

 help create and maintain the heat-circulating system of the 

 Ocean River. 



But the power of the engine, the prime source of the earth's 

 radiant heat, is the sun. About sixty percent of the total radi- 

 ation from the sun reaches the surface of the earth. Though 

 this short-wave radiation passes with little change through 

 the lower atmosphere, it is nearly all absorbed by land and 

 water, turned into heat energy, and eventually radiated back 

 again. The radiations leaving the earth are heat rays with a 

 longer wave length than solar radiation and are easily absorbed 

 by the atmosphere. It is obvious that any changes in radiation 

 from the sun or in its distribution over the earth would be 

 translated into climatic changes. This transfer of heat moti- 

 vates the constant battle between high- and low-pressure areas 

 partly because gases and liquids expand when they are heated 

 and lose weight and rise — just as when they are cooled they 

 contract and settle. 



The land and the sea have very different actions in this 

 machinery of climate. The sun's energy, absorbed by rocks and 

 the soil, cannot penetrate far because they are notably poor 



