71 



The lows are regions of ascending, converging, cooling air, with in- 

 creasing moisture and clouds on their western side, but are the re- 

 verse on their eastern side (Moore, '10: 153). These same character- 

 istics apply to the small highs and lows which we are accustomed to 

 see on the daily weather maps. 



If, now, we consider these large centers of action, such considera- 

 tion will do much toward giving us a graphic idea of our climate. Dur- 

 ing the winter, because of the small amount of heat received in the 

 Mackenzie basin, the temperature becomes very low, and a powerful 

 high barometric area is formed ; then the descending air blowing from 

 the eastern part of this high, or from small highs originating from the 

 larger one, produce the cold winters and cold waves in winter which 

 characterize the northeastern United States. If, however, the Atlantic 

 high wanders on the eastern coast of the United States in winter, the 

 zvestem part of this high, with its descending, diverging, warming, and 

 drying air, produces a mild winter. The climate of the eastern United 

 States is thus, in the cold season, under the alternate invasion of these 

 two powerful centers of action. During the warm season the conti- 

 nental winter high is replaced by a low, due to the accumulating warm 

 continental temperatures which thus have produced an inversion or 

 seasonal overturning. But the Atlantic high is permanent and exerts 

 its influence continuously. If the zvestem part of this high encroaches 

 upon the eastern United States during the summer, with its descend- 

 ing, drying, and clear air, it may produce drouth, this depending, of 

 course, on its degree of development. The continental low of sum- 

 mer, with the drying influence of its eastern side, has a similar ten- 

 dency. Thus the character of the summer is determined, to an im- 

 portant degree, by the interplay and relative balance between these tzvo 

 zvarming and drying centers. The activity of these centers has a pow- 

 erful influence upon the moisture-bearing winds, which influence hu- 

 midity and evaporation in Illinois, and in the eastern United States. 



4. Relative Humidity and Evaporating Power of the Air 



We are now in a position to examine the facts of relative humidity 

 and the relative evaporating power of the air in the eastern United 

 States. The relative aridity on the plains east of the Rocky Moun- 

 tains is due primarily to the removal of moisture from the prevailing 

 westerlies in their passage from the Pacific over the various western 

 mountain ranges which extend across their path, combined with the 

 excessive summer heating of the continental mass. Here, then, is the 

 influence of the continental summer low. Farther east the Atlantic 

 high tends to supplement the continental low and to cause the Gulf 



