CIRCULATION OF TEE ATMOSPHERE 4 45 



from the tropics over the United States from the south, quite 

 contrary to the general principle; and similarly, though not so 

 conspicuously, a case is found in South x^merica and south Africa. 

 On the contrary, the warm air in the lower levels over the Indian 

 Ocean, whose winds are called monsoons, simply beats upon the great 

 mountain ranges to the northward of India without penetrating the 

 temperate zone in Siberia. In this way certain great circulations 

 called centers of action form in each hemisphere. There is one over the 

 middle Atlantic Ocean; another over the middle Pacific Ocean of the 

 northern hemisphere; and there are other corresponding centers of 

 action in the southern hemisphere. These are especially well marked 

 during the summer time when the ocean is cool and the land air is 

 warm. In the winter time the tendency is to form centers of action 

 over the land areas instead of over the ocean areas, but the process is 

 much more irregular, and in the United States it is exhibited chiefly 

 by a succession of cold waves which traverse the United States from 

 west to east. Eeferring to the center of action over the middle Atlantic 

 Ocean in summer, we know that the winds near the American side are 

 from the south or southwest. On the Atlantic Ocean in latitude 35° 

 to 40° north the winds are blowing eastward, and in latitudes 25° to 

 30° they are blowing westward; on the European side they are blowing 

 from the northwest and north. The consequence is that the United 

 States is bathed during the summer with warm, moist winds in the 

 eastern half, and with warm, dry winds in the western half of the con- 

 tinent. In Europe, on the contrary, the northerly winds prevail, and it 

 follows that the American continent is warm during the summer while 

 Europe is cool, and this is the cause of the annual migration of tourists 

 from America to Europe instead of from Europe to America. The con- 

 trol of the climate of Europe by the American Gulf Stream is a myth. 

 As a matter of fact the European climate is controlled by the great 

 currents of circulation referred to these centers of action. 



More generally, warm masses of air find their way from the 

 tropics into the temperate zones by very irregular paths, and cold 

 masses find their way from the northern latitudes into the temperate 

 zones by very irregular paths. A similar statement applies to the cir- 

 culations of the southern hemisphere. The disturbances in the general 

 circulation which are produced by the land and ocean areas make it 

 well-nigh impossible to reduce meteorology to any simple scientific 

 system. The irregularities produced by the interaction of these warm 

 and cold masses are so great that the problem of forecasting seems to 

 bid defiance to any clear classification. The eastward drift over the 

 United States is, of course, the basis of any possible forecasts, and the 

 irregularities caused by the interpenetration of the warm and cold 

 masses, one after the other under the action of gravitation, produce 



