Chapter 13 — CIRCULATION OF THE ATMOSPHERE 



M* 



fci^gfc 



LOW WIND SPEED 

 (BELOW 20 MPH) 



HIGH WIND SPEED 

 (ABOVE 20 MPH) 



210.37 

 Figure 13-15. — Eddy currents formed when wind 

 flows over uneven ground or obstructions. 



"gustiness" is usually used in conjunction with 

 sudden intermittent increases in the wind speed 

 near the surface levels, and turbulence is used 

 with reference to levels above the surface. 

 Gustiness can be measured by wind instruments, 

 whereas turbulence, unless encountered by air- 

 craft equipped with a gust probe or an acceler- 

 ometer, is usually estimated. 



An eddy is the more or less circular motion 

 of the wind produced by an obstruction in its 



210.38 

 Figure 3-16. — Effect of windflow over mountains. 



path, such as irregularities on the earth's sur- 

 face (hills and mountains), trees, and buildings. 

 The length of an obstacle and the stability of 

 the air are the factors which determine whether 

 the air will flow around or across the object. 

 Turbulence caused by large objects, such as 

 buildings, is usually a combination of horizontal 

 and vertical eddies. (See fig. 13-15.) 



There may be a stationary eddy on the wind- 

 ward side of a mountain if the windward side 

 has a steep slope. The leeward side of 

 mountains has the most pronounced eddy activity, 

 and in most cases violent downdrafts exist. 

 The downdrafts are extremely dangerous to 

 aircraft, and instances are recorded of their 

 having caused aircraft to crash into mountain 

 sides. (See fig. 13-16.) 



311 



