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quade

Dust Devils

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Attached is a photo of a dust devil. This is a particularly good example of a dust devil with a well developed core. The airspeed on the high speed edge of the core is probably around 50 knots. For the scale on this one, look carefully at the base which has just passed over one the golf cart.
Dust devils are small atmospheric vortexes not associated with a thunderstorms, which are made visible by rotating clouds of dust or debris. Dust devils form in response to surface heating during fair, hot weather; they are most frequent in arid or semi-arid regions. This means they look similar in structure to tornados, but they are formed by a completely different process.
As the sun heats the surface of the earth the air near it becomes warmer as well. Warm air is less dense than the cooler air above it, so this warm air eventually begins to rise. Because of uneven surface heating, the warm air has a tendency to rise in bubbles. As these bubbles of warm air rise, cooler air from above and to the sides of the bubble rush around and below to fill the void. This inrush of air can happen in a somewhat uneven manner and, as it reaches the center of the void filled by the bubble of rising air, can begin to spin. It only takes a small amount of unevenness and a fairly slow inrush of air initially to cause the spin and a dust devil to be formed.
Like an ice skater drawing in her arms to make herself spin faster, as the air gets closer to the center, it rotates faster. This is called the conservation of angular momentum. This same principle can be seen in many places from tether balls to collapsing stars.
Because the air is moving faster at the center of the dust devil, the air pressure at the center is lower than that further away, this causes even more air to be drawn in. Along with the air, it also draws in more dust and debris. As the air rushes in along the surface, it may also be heated somewhat. The combination of lower air pressure and heating causes the air, dust and debris inside the dust devil to rise.
It should be noted that dust and debris do not necessarily have to get drawn into and up a dust devil. For instance, dust devils can form over a clean grassy area and can be almost impossible to see.
Wind speeds near the centers of dust devils can range from just a few miles per hour to perhaps as much as 70 miles per hour. While that's not normally fast enough to do damage to buildings, it can easily cause the upset of a Twin Otter on takeoff or landing. Dust devils can collapse canopies in mid-air. This is particularly bad if you're only a few moments from landing. Dust devils have also been known to pick parachutes up off the ground with skydivers still attached and slam them back into the ground.
quade
http://futurecam.com

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Excellent description... especially as dust devil season starts to crop up here...
One story that our FJC instructor told us was of someone who had landed, and there was a dust devil making a move towards him. People were screaming at him through both radios and vocally, to get the hell on top of his canopy. Apparently, he heard them, but as our Instructor related it to us, "... didn't want to look stupid..." getting his body over the top of the canopy to keep it from getting picked up. Sure enough, Senor dust devil picked him up off the ground by his canopy, and broke some part of him after he fell back down. Ow.
I'm just glad there's plenty of dirt around our DZ to be able to hopefully spot something like that in time.
-Andy
I'm not not licking toads....

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Thanks Quade - impressive photo too. Are there any other clues to tell when this sort of uneven thermal activity may be happening? All the DZs I jump here in UK are grass, and it would be nice to be able to spot symptoms before the plastic chair gets picked up and starts spinning like something out of The Omen (happened last year on an unusually hot Saturday - strangely enough I didn't swoop anymore that day).
Rich M

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Unless there's actually dust and debris that's been taken loft and sometimes even if there has been, they can be almost impossible to spot from the air. The picture of the one that I attached to my first post probably could have been seen easily from the air. About a week ago at Perris, I watched 4 of the Canadian Sky Hawks demo team fly through a little invisible one right over the grass landing area. The first guy whacked in, but got up and was ok. The rest of the team simply had, um, really bad landings.
If you're under canopy try looking for; wind socks blowing in different directions, little swirl patterns on the grass and maybe pieces of paper floating about -- anything higher than your local rooftops should be looked at with suspicion.
I don't know what your local birds are like, but any bird that likes to soar; hawks, falcons, sea gulls -- will probably be circling upward in the thermals.
As for the conditions that make dust devil creation more likely -- look for hot days.
quade
http://futurecam.com

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So, how high can a dust devil take that debris?
Take a look at this pic attached...(white 'something' in the right hand side of the screen).
While training with my team last Sunday, I got a start when something white wipped past my lens! I went back and timed the freefall and this happened at 6,000!
Needless to say, most of my team didn't even see it since they were so intent on that block, but it kinda scared -me-.
Even if it were a paper cup, I wondered, could it have done damage with us going 125 mph through the air if it hit us?
ltdiver
____________________________________________
LightDiverCam

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Nice picture and explanation. Some friends where I've jumped experienced one of these things a while back. The following was copied from the local papers website:
Skydivers survive after plane stalls
It was not the best of years to be flying a plane or helicopter over Wise County. Three aircraft crashes in the span of a year is the number 10 story of the year.
On March 31, a plane carrying 18 skydivers crashed at Skydive Texas east of Decatur about 10 seconds after takeoff. The plane stalled after hitting what pilot Tom Bishop described as a dust devil at about 300 feet off the ground. Five people were flown to Fort Worth hospitals and six more were taken to Decatur, many with broken bones. There were no fatalities.
James

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Well, not sure a dust devil is the only thing that can pick up debris. Some sort of wind mechanism is necessary to get something off the ground but it could be straight line winds. Once above the ground the conditions could be right for keeping it aloft. I have passed all sorts of things inflight in my airplane. Seen black trash bags the most. Any altitude ranging from 3,000 all the way to 6,000. There's more junk aloft than most people realise. So, please, don't litter.
Chris

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Good thread. Dust devils can easily collapse canopies, and there are numerous instances of jumpers who have safely landed having their canopies (and themselves) picked up by a devil and flung back into the ground, causing serious injury and death.
Dust devils can be very hard to see, particularly when you are in the air and the devil is over grass. On hot days, I typically see at least one per day (not necessarily during a skydive), so they are out there...
I'll throw this question out, just for the purposes of discussion. What do you do if under canopy you spot a dust devil near the intended landing area? I have heard two approaches. One is move away from the dust devil; put as much distance between you and it as possible. The second is that the path of a devil is unpredictable, but if you go to where it was, chances are it won't come back.

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I saw some freaky shit out in Eloy last June. The jump was uneventful, but while under canopy I could see several very developed dustdevils in the distance, like the one in the pic attached to the first post. They must have been several hundred feet high and were just lurking out there, very eire.

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Quote


The second is that the path of a devil is unpredictable, but if you go to where it was, chances are it won't come back.

Maybe not the one you saw, but . . .
There is a very strange little phenomenon that I didn't mention in the original post that might actually be worth mentioning here.
It is possible that like tornados, dust devils can spawn several little mini-vortexes around them. Dr. Fujita, the guy the tornado scale is named after, first discovered this. His theory says that up to four vortexes can surround the main vortex -- kind of like the five on a dice cube. These have been witnessed numorous times in tornados once people knew what to look out for. I've read accounts of people seeing the same thing during dust devils, but I've never witnessed it myself.
My suggestion would be to land away from any dust devil. Landing "behind" it is no guarantee of safety.
http://futurecam.com

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Dust devils usually drift across the ground at about the same speed and direction as the average surface winds of the day. If you observe for a few hours, you see a pattern. All the dust devils on a given afternoon move in the same direction at the same speed.
Try to land upwind of visible dust devils. Try to land as far upwind as possible from dust devils.
The most grisly task I had while working at Rigging Innovations was replacing a harness that had been cut off a student at Elsinore. The student was caught by a dust devil and slammed into the hard-packed gorund. The student died.

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Great post! :)I witnessed a friend of mine coming in under canopy, first one down, and at about 700-800 ft her canpoy collapsed and went into at least 5 line twists causing her to go back into free fall. As soon as this happened you could see her throwing her body in the opposite direction of the line twists, about 300 feet she was out of whatever weird air she got in and was under a canopy with 1 line twist, got out of that and had enough time to flare. SCARY!!! All we can think of was that it was a dust devil that hadn't picked up anything off of the grass landing area.
Tee :)

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How often do these occur in non-arid regions?
I jump in Southwestern Michigan. We're usually pretty humid, and grass pretty much covers everything.
Dust devils were never even brought up in my training. Are they even a concern in regions like these? How often do they occur in arid regions?

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Hey Fred,
Ask Bob Galler at GLS about dust devils.
Our DZ is in the middle of mixed wooded wetlands and farm fields. Tree lines are fairly close on two side. Bob landed and near the peas and immediatley got lifted back into the sky getting tossed around. Was slammed into the ground, picked back up, drug some and finally got to his cutaway handle to chop the main. Wasn't really hurt but it was real scary to watch. We could see the rotation to some extent but don't count on being able to see them here, especially from the air. It's not something to worry about, just something to be ready for.
Terry

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Here is what I have been told in AFF & Coach candidate courses for dust devils.
1. Under canopy, turn way.
2. If caught in a dust devil, or in rough/tumbling air - go to half brakes and work to keep the canopy level.
3. On final, land, maybe minor turn, jump on canopy. (RSL)
4. On ground with canopy layed out, jump on it.
5. On ground w/o canopy, rise one arm to get attention, point with the other arm to the dust devil.
Blue Skies,
Ralf

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>3. On final, land, maybe minor turn, jump on canopy.
And keep flying the canopy until it collapses at your feet!! Most canopies are controllable enough to have a little control even in a dust devil, but you have to _use_ it. Dust devils are bad times to just bring both toggles down and hope for the best.
-bill von

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