dust devils

thermals picking up dirt, crates, and small babies. or, in your neck of the woods, ashes.
 
Hot rising air + cooler/low pressure air above it.
 
The sun heats the ground, the ground heats the air just above the ground. At some point a bubble of hot air will start to rise sucking air in from the surroundings. The air moving in starts to swirl just like the low pressure areas you see on the weather map. As things get going, more and more hot air is sucked in and the next thing you know we aren't in Kansas any more.
 
Jeanie said:
What causes dust devils?
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All I know is that if there is an inversion layer between your aircraft and them, they don't affect you. At least that was my experience when training in a glider a couple years back. One day we were towed up through some bumpy air to find smooth air above about 1300 ft AGL. After release it was a boring sled ride down - zilch lift anywhere. But dust devils were appearing in spots, so we flew over a couple of them thinking we'd find rising air. Nope - dead still air. The yucky haze layer was pretty distinct, so if something had managed to rise up out of it I suppose we probably would have noticed those areas as "hills of haze."
 
Jim Logajan said:
All I know is that if there is an inversion layer between your aircraft and them, they don't affect you. At least that was my experience when training in a glider a couple years back. One day we were towed up through some bumpy air to find smooth air above about 1300 ft AGL. After release it was a boring sled ride down - zilch lift anywhere. But dust devils were appearing in spots, so we flew over a couple of them thinking we'd find rising air. Nope - dead still air. The yucky haze layer was pretty distinct, so if something had managed to rise up out of it I suppose we probably would have noticed those areas as "hills of haze."
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inversion layers cap all mixing in the boundary layer of the atmosphere, unless the temperature on the ground is warm enough that the rising air is still warmer than ambient at the inversion level. if you would've flown to those dust devils below 1300 AGL you probably would've found at least some turbulent air.
 
They are spirits of dead ancestors walking the world once again. If the spirit is good its dust devil will rotate clockwise and if its a bad spirit it will rotate counterclockwise. In the Middleast they are djinn or genie, powerful spirits that when imprisoned can grant wishes.

Seriously, It is formed when a rising column of warm air rushes into a pocket of low pressured cool air. The process causes the air to rotate due to the conserved angular momentum. This rising and rotating column of air picks up dust and debris like a tornado creating a dust devil. Just like other types of weather phenomena dust devils depends on three conditons. The first is flat arid ground or asphalt. This type of ground allows for easy absorption of heat in order to create a constant source of rising warm air. The next thing needed is a clear sky with few clouds. This allows for the maximum amount of solar energy needed to make warm air currents. The final requirement is that there be little or no wind. This is because extreme differences between surface and air temperature are needed to make a dust devil.
 
matt m drove through the crown of the rock house fire when it was just north of marfa. he saw 30 ft fire devils on either side of the road. crazy!
 
If a dust devil put you on your back while taxiing, the NTSB would state "Pilot failed to compensate for the windy conditions"

Here in AZ they can get nasty.

Paul
N1431A
2AZ1
 
Now that we know what causes them, are dust devils hazardous to aircraft?

Once, I was cleared for take off and off to the left of the other end of the runway, I spied two of them. I took off while keeping my eye on them. As I get about 100 ft AGL, the controller then advises me to beware the dust devils ahead...
 
The dust devils out here have been amazing.... I had to swerve to the other lane of the highway to avoid a huge one that was crossing the highway in a most Taz/Genie kind of way. I have seen probably 5 of them all at once in an area -
The wind doesn't seem to bother them - it sure moves them right along

Thanks guys for the explanations - makes good sense
 
skidoo said:
Now that we know what causes them, are dust devils hazardous to aircraft?

Once, I was cleared for take off and off to the left of the other end of the runway, I spied two of them. I took off while keeping my eye on them. As I get about 100 ft AGL, the controller then advises me to beware the dust devils ahead...
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I was getting low too far from the airport to return, but I had a dry lake bed within reach. I noticed a dust devil moving across the lake bed and flew an intercepting course. A few bumps and some sand hitting the canopy (sounds like ice pellets) and I was in 10knts up. Climbed 2000ft and had plenty of altitude to safely reach the airport.

Sure dust devils are bumpy and should be avoided when close to the ground. But when you need that lift they are wonderful.

We do not take off on tow when a dust devil is visible near then runway. Landing you don't always have a choice. The bad ones are the thermal twisters with not enough dust to show themselves.
 
We have noticed since the first day following the fire that dust devils are particularily vicious, and a lot more common now. At first we thought we could simply see them better because their was so much more particulates available now that the grass is gone and the soil (and ash) is so exposed. (the fields look completely nekkid right now). But the other, and perhaps stronger theory is that the grass is not protecting the ground from heating as much - so we are seeing more dust devils, and dd's of greater rotational speed and magnitude are being formed as a result.
They are definitely a thing to watch for while driving around here right now as Jeanie says.


Now that we know what causes them, are dust devils hazardous to aircraft?
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Dust devils can be fatal to pilots
Oh. sorry. But they really can toss an airplane about. You haven't fully enjoyed west Texas flying til you find yourself in one during the flare.


some wiki info


Dust devils occasionally are strong enough to cause minor damage (up to F1 on the Fujita scale)
 
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skidoo said:
Now that we know what causes them, are dust devils hazardous to aircraft?
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Last summer I had one almost shove my 10,000 lb motorhome off the road at about the same speed I am when on final in a 152 or 172. I turned the steering wheel 180 degrees left and it was still drifting to the right. I would not want to try that in a 1200-1700lb airplane. No good could come from it.

You gotta love AZ though. Dust devils all over the place. The other day I was trying to go to Oz. I rode my motorcycle through two of them kinda, um, shall we say, very deliberately. The first one was picking up debris and gave me a moderate shove. The second was very well established and I centerpunched it. It wasn't the best of ideas..I'm pretty sure I saw Toto in that one. The third one, nooooooo flipping way. It looked like a tornado, probably 100 feet wide and went up way too many hundreds of feet.
 
BillTIZ said:
The bad ones are the thermal twisters with not enough dust to show themselves.
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Yeah... it's the irony of having a day with big lift. I used to think my seat belts were tight enough on every flight... until I almost wound up upside-down against the canopy of the 2-33 when I got towed through a super-strong thermal just beyond the runway. Saw the tow plane get hammered, and thought "Eh, that doesn't look so- OMFG!!!" :D
It was like a giant fist coming up from the ground; never had hit a bump quite like that in 13 years of flying.

Now I cinch the lap belt until it hurts a little. :D
 
I was tow pilot today, winds were 20G30 and about 30degrees off runway heading. Made for sporty takeoffs and landings but well under control. Good for the students too!

Thermal, ridge and wave lift today, you know it's strong when the Pawnee gets 600fpm in calm air with the Grob 103 on tow, and all of a sudden we have 2000fpm!! Time for the Grob to let go!

Toward the end of the day the crosswind factor increased. I decided to let the pawnee nose into the wind and roll into the infield (bladed level with runway for just this reason) when I noticed the "dust" twitch and come alive. I had been rolling with the tailwheel down when all of a sudden I was 4 feet up. A dust devil formed and lifted me, time to power up and fly!!

It's nice to have 250HP react to a smooth throttle. I let the dust devil pass, and landed on the next pattern.
 
I have seen an almost instant +20 knot airspeed increase and why-are-we-so-high-now-guess-we-are-going-around effect from a dust devil.
 
one landing in the 425, I was about 30' up & the airplane yawed hard (it felt like 30° but was probably only 10°) -for an instant I was thinking why such hard yaw from an engine failure at only 300lbs torque, then the next millisec it yawed the other way and I realized the invisible devil got me. Happened so fast I had no time to power up and as it turned out, didn't need to after all.
 
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