Page:Popular Science Monthly Volume 92.djvu/739

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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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