204 



SYMPOSIUM ON AERONAUTICS. 



TABLE II. 



Relative Frequency (Per Cent.) of Winds from the Different 

 Directions Observed at Each Level. 



operation. They are set up locally because of peculiarity of topog- 

 raphy of the earth's surface or in its nature so far as ability to ab- 

 sorb and radiate heat is concerned. The variation in the intensity 

 of insolation during the twenty-four-hour period also gives rise to 

 a convective system which is of especial interest to the aeronaut. 

 Figs. 19 and 20 show the temperature distribution up to the three- 

 kilometer level accompanying the diurnal convective system, as it 

 has been observed at Mount Weather on clear days. Fig. 19 is 

 based on data for the summer half of the year and Fig. 20 for the 

 winter half. The horizontal circulation that obtains in this convec- 

 tive system is not often in direct evidence. It usually manifests 

 itself as a modification in the direction and speed of the wind pre- 

 vailing at the time and need not now be further considered. The 

 height to which turbulence in the air, caused by the heating of the 

 earth's surface during the day, extends and the time of greatest ac- 

 tivity in this stratum are shown to be, on the average, between 1.5 

 and 2 kilometers above sea level in the summer months, between 

 I and 1.5 kilometers in the winter months. The height of the ob- 

 serving station on the Blue Ridge was 526 meters above s^a level. 



