HUMPHREYS: HOLES IN THE AIR 245 



is not safe, as just explained, for an aeroplane to take the billow 

 level and thus pass back and forth from the one to the other 

 wind layer. 



6. Wind eddies. Wherever the wind is forced by obstructions 

 markedly to change its direction, and wherever it flows over 

 steep hills and mountains eddies are certain to exist. These 

 generally are most marked during strong winds and on the lee 

 sides of steep hills and bluffs. The upper portion of the eddy 

 moves in the same direction as the prevailing wind to which it 

 is due and the, under portion in the opposite direction. Hence 

 in passing thru such an eddy an aeronaut may, as explained 

 under ' 'wind layers," get into serious trouble. He may also 

 get caught on the foiward side of the eddy and be rapidly carried 

 down. Therefore during strong winds the lee sides of hills and 

 bluffs should be avoided. 



7. Aerial torrents. Air drainage down steep and barren val- 

 leys occasionally amounts to a veritable aerial torrent near the 

 surface t>f the earth while the atmosphere directly above is rela- 

 tively at rest. Hence it necessarily must be dangerous to land 

 an aeroplane under such circumstances, and especially so, as 

 above explained, if it is headed with the torrent. 



8. Aerial breakers. At the onset of thunder storms the wind 

 often is of the breaker nature, violent and irregular, and there- 

 fore so dangerous to the aeronaut that it would be fool hardy to 

 attempt a flight under such conditions. 



CLASSIFICATION 



All the above atmospheric conditions may conveniently be 

 divided into two groups with respect to the method by which 

 they force an aeroplane to drop as if in a hole of some kind: 



a. The vertical group. All those atmospheric conditions, foun- 

 tains, cataracts, cascades, breakers, and eddies that, in spite of 

 full speed ahead with reference to the air, make it impossible 

 for an aeronaut to maintain his level depend for their effect upon 

 a vertical component in the motion of the atmosphere. 



b. The horizontal group. Those conditions of the atmosphere, 



