BLAIR— AEROLOGY. 197 



larities of surfaces below them. They are in general less gusty than 

 are west winds. 



Closely related to this arrangement of light and heavy airs is the 

 fact that the two regions of traveling storms, the tropical hurricanes 

 and the high and low pressure areas of middle latitudes, are found 

 where air relatively dense for the level it occupies is flowing over 



Fig. 9. Mean of Wind Observations in " Lows " at 2000 Meters above Sea 



Level, 1907-1912. 



moister and, for its level, relatively light air. These storms are 

 surface stratum phenomena, forming on the boundaries of warm, 

 moist and cold, dry air masses and have approximately the speed and 

 direction of the wind in the stratum immediately above them. The 

 tropical hurricanes have the speed and direction of the antitrade 



