Popular Scicncr Moiifl/li/ 



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the eddy that lurks in the lee of such an 

 obstacle; with the downward tendency 

 of the air over lakes, rivers, swamps and 

 forests. "The air is so sensitive," said 

 Mr. Gustav Hamel, the famous tiyer, 

 "that it is affected even by the color of 

 large patches of vegetation. Whether 

 this be entirely due to the different heat- 

 radiating power of differ- 

 ent colors, it is impossible 

 to say, but invariably an 

 aeroplane on passing from 

 grass land to a field cov- 

 ered with yellow flowers 

 experiences a certain 

 amount of air dis- 

 turbance only less 

 noticeable than the 

 inevitable bump ex- 

 perienced in passing 

 from green fields to 

 ploughed land or 

 from ploughed land 

 to meadow." 



When the wind is 

 blowing, the air for 

 at least a few hun- 

 dred feet above the 

 ground is nearly al- 

 ways in a state of 

 turmoil. This is 

 partly due to friction 

 of the moving fluid 

 against the irregular 



surface of the earth, and partly to the 

 ascending and descending currents caused 

 by differences in temperature. The latter 

 effect is illustrated in the rapid rise of air 

 over a bare plain, by day, and its fall 

 over an adjacent forest or body of water. 

 A good pic- 

 ture of the 

 atmospheric 

 ups-and- 

 downs and 

 other dis- 

 turbances en- 

 countered by 

 the airman 

 when flying 

 low is fur- 

 nished by the 

 behavior of 

 the smoke 



from a fac- Clouds Seen from Above 



tory chimney Aeronauts, looking down on the wind-swept surface of the clouds 



;. -L _ _i/-k/1 have observed their surfaces to be thrown into a series of rolls of 



Wlin a mOQ- vapor, which are vast waves of air with crests half a mile apart 



A Solution of the Fog Problem 

 Along the main flying routes landing-grounds will be 

 established at intervals of ten miles. Their loca- 

 tion will be marked by illuminated kite-balloons 



eratewind blowing, forming smoke-waves. 

 The Autograph of a Gust 



These disturbances give rise to the 

 very marked fluctuations in the force of 

 the wind known as y».s7.s-. There are cer- 

 tain forms of anemometer especially 

 designed to record the gustiness of the 

 wind. A record of its force is traced by 

 a pen on a moving strip of paper, and the 

 "anemogram" thus obtained shows a 

 continuous series of irregularities, the 

 extent of which increases with the 

 strength of the wind. The puffs and 

 lulls often alternate 

 at intervals of a few 

 seconds or less, and 

 the actual strength 

 of the wind at a 

 given instant may be 

 many times greater 

 than its average 

 force for, say, five 

 minutes. 



The turbulence of 



the lower air extends 



I to various heights, 



depending upon the 



strength of the wind. 



A rough rule, evolved 



by Zeppelin pilots 



before the war, was 



to expect turbulent 



conditions up to an 



altitude equal to from 10 to 20 times the 



force of the wind in meters per second. 



Thus, for a wind of 10 meters per second, 



the turbulent layer would be from 100 to 



200 meters deep, and so on accordingly. 



With increassd altitude the wind gen- 



e r a 1 1 y in- 

 creases in 

 both strength 

 and steadi- 

 ness, but 

 sometimes 

 very u n - 

 steady air is 

 encountered 

 even at great 

 heights. This 

 brings us to 

 the ■ impor- 

 tant subject 

 of air-layers, 

 or broad 

 streams of air 



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