BLAIR— AEROLOGY. 



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thus become gusty, just as do winds flowing over irregularities in 

 the earth's surface. These gusts are accompanied by appropriate 

 changes in pressure and temperature, and progress in the direction 

 and with the speed of the wind in which they occur. They carry 

 with them the self-sustaining disturbances of the lower or surface 

 stratum which would otherwise be practically stationary phenomena. 

 Figs. 4 to 15 inclusive show the direction of the winds about 

 centers of high and low pressure at the earth's surface and at 

 levels above these centers. All winds, whatever their direction at 

 the earth's surface, change direction with altitude in such a way as 

 to become westerly by the time the four kilometer level has been 

 reached. This tendency is shown by a comparison of surface winds 



Fig. II. Mean of Wind Observations in "Lows" at 3000 Aleters above Sea 



Level, 1907-1912. 



