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Science Frontiers ONLINE No. 39: May-Jun 1985 |
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Recipe For Dust Devils
R.H. Swinn, formerly Chief Instructor for the Egyptian Gliding School, has had much practical experience with those fascinating little (sometimes not so little) swirls of hot air called "dust devils." Under the broiling Egyptian sun, dust devils launched themselves naturally every few minutes from a tented camp near the airfield where Swinn taught. Curiously, the devils often were born in pairs; a big one followed by a modest little chap following behind by 100-150 yards. The devils ranged from just a foot or so in diameter to 500 yards and more. The giants were majestic masses of swirling sand that moved along at leisurely paces. These appeared harmless enough, but stepping through the outer wall into the vortex sucked the air out of the lungs.
"Outside our hangar there is a large stretch of wind-sheltered concrete which becomes intensely hot. In this area, close to the foot of the hangar, one can start up one's own little devils on occasions by a quick sweep of a signalling bat (which is shaped like a large ping-pong bat) from shoulder level in circular and downwards direction to a point almost touching the ground; one must step rapidly back or the vortex that is set up is spoilt. Such a miniature thermal starts about a foot in diameter and quickly assumes a conical shape about two feet high, moving along the ground at a walking pace. Its rotation increases very rapidly, until one has the impression of a whirling snake in front of one. As it reaches the edge of the concrete a little sand is thrown up and the thermal dies away."
(Swinn, R.H., "On Flying Gliders into Wind Devils," Journal of Meteorology, U.K., 10:17, 1985.)