24 PLANT SUCCESSION AND CROP PRODUCTION 



America. The hyperbars or anticyclones are marked "high" and 

 the cyclones or infrabars "low." The arrows indicate the direc- 

 tion of rotation of the air flowing into or away from centers of ac- 

 tion. 



In midsummer there is a large area of low pressure over the 

 central part of the North American continent. On the surface of 

 the earth it is warmer east of a low-pressure area, because the 

 winds blowing spirally, counter-clockwise, inward toward the low 

 are coming from the south, which is usually warmer. Mr. J. War- 

 ren Smith of the United States Weather Bureau, in some private 

 correspondence, has kindly pointed out the two laws which are 

 operating : 



1. The pressure controls the surface winds. 



2. The surface wind controls the temperature. 



The converging air is warm at the surface and remains so un- 

 til it ascends, when it expands and cooling adiabatically, precipi- 

 tates its moisture. It is to the east of the high that the warm 

 weather and sometimes dry weather exists. The amount of rain- 

 fall would depend on the rapidity of the cooling and the total water 

 vapor content of the inrushing air. As far as vegetation is con- 

 cerned, though there might be rainfall, the final effect of the east- 

 ward migration of the low would be unfavorable, since in summer 

 the drying effect of the warm air at the surface would not be bal- 

 anced by the scattering showers. 



Another shifting in the position of a great center of action 

 which would be even more unfavorable in its effect on the vegeta- 

 tion of eastern United States is the westward migration of the At- 

 lantic high shown in the diagram. Where the air is descending it 

 is being warmed adiabatically as it reaches the surface of the earth 

 and is taking up moisture. This air would then have a drying ef- 

 fect on the vegetation and the crops, and since it would be accom- 

 panied by clear skies the water loss from living plants would be ex- 

 tremely rapid. There would be no compensating showers brought 

 in by this type of distribution of the centers, and if the positions 

 were assumed for any considerable length of time the ensuing 

 drouth would be followed by a failure of the plants to maintain a 

 water balance. Wilting and even death would be the effect of a 

 protracted drouth. Besides the continental low and the Atlantic 

 high, there are three other region circulation centers, all of which 



