22 PLANT SUCCESSION AND CROP PRODUCTION 



from a land surface as compared with a water surface, and (c) the 

 greater efficiency of a vegetative cover, especially a forest, as an 

 evaporating surface when compared with a water surface or a bare 

 soil surface. 



Movements of Air Currents 



One of the most fundamentally important set of facts in 

 aerography deals with temperature and pressure changes by which 

 currents of air are moved horizontally and vertically over the 

 earth's surface. We know that at sea level air is more compressed 

 than on the tops of mountains, and we consequently obtain baro- 

 metric registers of greater pressure at lower altitudes than at 

 higher ones. By simple gravity air tends to collect right at the sur- 

 face and air particles are here crowded and compacted. Higher 

 up the air particles are farther and farther apart, and finally we 

 may get to a region where air no longer exists. 



Above the polar regions of the earth the air is less heated than 

 above the equatorial region. Warm air rises and expands, while 

 cool air tends to settle. There is a general tendency then of air to 

 move inward from the polar regions toward the equator. This ten- 

 dency is considerably offset by another factor, however, the mat- 

 ter of the unequal heating of land and water masses. The difference 

 in the rate of heating was touched on in the previous section. The 

 air over a land mass in summertime will become warm and 

 ascend much more quickly than the air over a mass of water. In 

 winter the air over a body of water is likely to have the higher tem- 

 perature. With the changes of season there come to be established 

 permanent areas of high or low temperature. The high-temperature 

 areas of ascending air are areas of low barometric pressure. The 

 low-temperature areas may be regarded as areas of high barometric 

 pressure. Simple convection movements and tendency to stabiliza- 

 tion and equilibrium would lead to horizontal movements as well 

 as the vertical ones of the rising and descending columns of air. 



The centers of action have come to be regarded by meteorolo- 

 gists as the essential factors in the control of climate. Buchan 

 many years ago prepared maps of winds, temperatures, and pres- 

 sures at sea level for practically the entire globe. It was Teisserenc 

 de Bort at the International Meteorological Conference held in Chi- 

 cago in 1893 who called special attention to the controlling influ- 

 ence which the centers of high and low pressure exerted upon cir- 

 culation. He had named them more than ten years previously "the 



