From Sun Jan 29 09:10:35 1995
Date: Sat, 10 Dec 1994 08:01:24 -0800 (PST)
From: William Beaty <>
To: Forum for Physics Teachers
Subject: Pool devils solved?!

Long ago there was a discussion here about small v-shaped wakes which zip across swimming pools as if an invisible insect were moving across the surface. Was the source of these ever discovered?

I had a chance to see these in great numbers close up. I was standing in line in a hotel, near the glassed-in jacuzzi near the swimming pool. The sun was shining into the pool, and the water was almost still. The shadows at the bottom showed many lens effects from slow turbulence, and the motion of many small terminations of underwater vortex filaments on the surface (whirpools!) were forming and disappating.

(Someone else has photographed the whirlpools:
http://www22.pair com/csdc/car/carfre3.htm)

Then I started noticing pool devils! They were zipping back and forth over the surface, leaving the classic v-shaped wakes. Some of the patterns turned into momentary spirals as the invisible "objects" slowed and changed direction. AHA! Pool devils are "devils" indeed. They are tornadoes. They are invisible dust devils. They must be arising as the hot water in the pool causes convection. The v-shaped tracks come about when the mouth of the tornado moves quickly across the pool. The tornado mouth would be at slightly decreased pressure, so it would be creating a small mound in the water where it attaches to the surface, and the movement of this mound creates the wakes.

I have not tested this theory, but the evidence I saw points strongly to the tornado explanation. If I was a smoker, I could have taken a lit cigarette to the pool to trace air patterns. If anyone has an indoor hot tub in a space which is large enough to support pool devil formation, you might try dumping some dry ice into the water to form a pool of dense mist, and see if tornado mouths do indeed dance in the air across the surface. The tornados would draw the mist upwards into the vortex strings. Don't drink the carbonated water! ;)

-------------------------------------------------------------------------- Subject: Wind on water effect is 'pool devils'
Newsgroups: sci.physics computational.fluid-dynamics,sci.physics
Followup-To: sci.physics computational.fluid-dynamics,sci.physics
Organization: Eskimo North (206) For-Ever

R. Sukumar (rs a cfdrc com) wrote:
: Oz writes
: > >>In article <4p1n7t$582 a arl-news-svc-3 compuserve com>
: > >>mlane a csi compuserve com (Mickey Lane) writes:
: > >
: > >>> While standing near a pond during a recent windy day, I noticed that
: > >>> from time to time, a wake-like pattern would race across the surface
: > >>> of the water for perhaps a meter or two. The leading edge of the wake
: > >>> is a sharp point and I can see that it's not caused by fish.
: > >
: > >>> Anyone know what causes this? And what they're called?
: > >
: > >>This almost certainly was the result of a ``water-strider'' bug
: > >>zipping across the surface, rather than an effect due to wind.

A long discussion broke out about this on phys-L list. The phenomena, when it occurs on the surface of a swimming pool, is called a "pool devil," and is (most probably) caused by the motion of the termination points of vortex threads across the surface of the water.

I was once luckily in a position to observe these "pool devils" in detail. While between sessions at a conference at a hotel, I was standing outside the glass wall enclosing the jaccuzzi. The sun was shining through skylights and brightly illuminating the pool bottom. The jets were off, there was no one in the pool, and the slightest wave was starkly visible projected on the pool bottom by lensing on the surface. Mist was rising from the surface of the hot water.

The whole surface of the pool was covered by little zipping "V" shape wakes! They were barely visible directly, but as projected on the pool bottom by sunlight they were totally obvious. It was also possible to see the cause of the wakes: the mist above the water surface was occasionally swirling in patterns resembling small dustdevil tornadoes.

I don't think we can escape the conclusion that the phys-L group's speculation is correct. The "v" shaped wakes are caused by the moving mouths of small "dustdevil" vortices. If the water is warmer than the air, then the rising heated air provides energy to drive numerous vortices. And if conditions across the landscape instead are generating fairly large "dustdevils," occasionally one may pass over water and generate not only a "V" shaped wake, but a small travelling patch of spray where the low pressure of the vortex core sculpts the water's surface into a point which emits droplets.
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