Dust devils are swirling columns of air that we can sometimes see because of the dirt they pick up from the ground. There have been a few fatalities worldwide that could be attributed to dust devils, and they cause a lot of serious injuries.
Photo by Willy Boeykens
If your country’s climate doesn’t include dust devils, be extra aware of this hazard if skydiving abroad in potential dust devil formation areas. This article will explain the warning signs to look out for and conditions that make them more likely, with some real examples to learn from.
On a 4-way training jump, we were on a back to back so I wanted to land close to the spot where I could run across the runway to get my gear on for the next load. Coming in on the base leg I could see no indication of wind on the landing area at all. Then by the time I set up on final the flags on the landing area were going crazy. The runway was on my left, my teammates were on my right, so there was nowhere to go other than straight ahead. I went shooting over the grass and just as my feet touched the ground the dust devil came from over the runway and hit me from the side. There was no dust in it yet so there was nothing to see. My canopy went from straight flight to sideways instantly, I was “all arses and elbows” to quote Dan off manifest. It wasn’t hitting the ground that broke me; it was the sudden twisting force that snapped the Transverse Process off my L1. Put a stop to jumping for a few weeks”
Sarah Smith, female 4-way World Champion
How do dust devils form?
Dust devils are formed usually in flat, desert landscapes during spring and early summer. They begin when areas of ground heat up in the warmth of the sun to different extents on different surfaces. In some places there will be warmer air, such as over desert, which reflects the heat more than grassy areas due to lack of moisture. Hot air convection rolls begin and, if they can find a passage through the cooler air above, the hot air will rise rapidly, making a column through the colder air.
The column stretches quickly upwards to provide an escape for the air, which pulls the mass closer to the center of rotation, making the vortex spin faster. (like ice skater pulling in her arms in a spin to turn more quickly). The speed creates an area of low pressure in the center of the vortex, pulling in more hot air from the surrounding surface, which is then drawn rapidly upward by the lower pressure hundreds of feet above. The spinning effect is intensified and becomes self-sustaining. The rotating air usually causes the dust devil to travel horizontally along the ground, which means it can sustain itself longer by pulling in more sources of hot surface air.
Fully formed, the dust devil is a ‘chimney’, through which hot air is channeled upwards, in a circular motion, and eventually outwards. As the hot air rises, it cools and loses its buoyancy, determining the top of the dust devil. Eventually the devil will exhaust its supply of hot surface air and start to draw cooler air in. This destabilizes the system and the dust devil will disappear remarkably quickly. This can be because it arrives in new territory with no hot air supply or it can simply run out of steam (forward momentum).
Once dust devils start to form, they will continue to do so until the hot surface air and the cooler upper air have mixed to the point where there is no longer a significant temperature gradient. This could be several hours.
We were 4-way training at Marana, Arizona. I had about 150 jumps at the time. We had seen a few dust devils going through the DZ the day before, picking up stuff from the ground (our lunch) and spiralling it up to what seemed a couple of hundred feet up. I was coming in to land from our 8th training jump that day on my Nova. I was flying parallel to the Tarmac runway, which was approximately 75 feet to my right and was on finals at about 50 feet from the ground. I remember looking down and seeing two bushes next to each other but being blown in opposite directions. I thought ‘that’s strange’ and the next thing I knew my canopy had partially collapsed and I was being flung about. The only thing I could think of to do was pump the brakes, which did reinflate some cells. Then I landed very hard on the runway. It was put down to a dust devil but there was no classic column of dust though – maybe because it was centred over the tarmac? I ended up with a few cracked ribs, lots of gravel rash and two fractures to my left radius”
Bob Parr
How big are dust devils?
They are much bigger than you think! If there is a visible part of the dust devil, it’s usually between 2 and 20 feet in diameter, extending upwards to 500-1,000 feet – but can be as large as 30 feet in the center, rising over 3,000 feet. The affected area of turbulent air though is far greater; at least three times the (possibly) visible dust devil. At the top of the chimney the hot air spills out around the sides like a fountain, causing other vortices around the discernible center of swirling dust.
Photo by Mike McGowan
What conditions encourage dust devils?
Certain (jumpable) conditions increase the likelihood and intensity of dust devils:-
1. Flat, barren, desert terrain – flat ground means the whirlwind has more opportunity to suck hot air into the column, fuelling the circulation. Flat surfaces mean the dust devil can travel, increasing in size and ferocity.
2. Different surface terrain, such as grass or tarmac (asphalt) near dry ground, due to the different rates of ground heating.
3. Light or nil wind days – wind may destabilize the vortex, although they can still develop in up to 15pmh winds.
4. Clear skies – the warmth of the sun creates the significant amount of heat needed for them to form.
5. A significant difference in temperature between the surface and the atmosphere
The biggest factor is temperature gradient. If your nighttime low is 65 degrees F (18°C) and your daytime high is 80°F (27°C) you are not likely to see many dust devils. However, if you go from 50°F to 80°F (10°-27°C) they are absolutely guaranteed. That’s because the air next to the surface gets so much hotter than the air above it, generating a lot of power to punch through the thermocline. This will not happen in overcast conditions, nor is it likely near the sea where the water tends to dampen out temperature fluctuations over 24 hours. Humidity is the enemy of dust devils because it reduces temperature swings”
Bryan Burke, UPSA/Skydive Arizona Safety Advisor
Photo by Willy Boeykens
What happens if you fly a canopy into a dust devil?
Your canopy flies most efficiently when the wing is fully pressurized with air flowing smoothly over the top and under the bottom of the canopy, generating lift. A dust devil doesn’t play ball with this happy picture, and the effect varies from a stutter, to a partial collapse, to a fully collapsed canopy, depending on its severity. If this happens near the ground, Mother Nature may slam you into it pretty hard. If you’re higher up, your canopy will re-inflate and re-pressurize once it finds some stable air; such is the nature of a ram-air canopy design.
Note that the pressure inside the cells is stagnant once the ram-air parachute inflates. In other words, there is no continuous flow through the inside of the canopy. If you imagine a circular, whirlwind pattern and then put your canopy entering it at various points, you have a number of possible consequences. Let’s visualize a clockwise rotation. Enter on the left side, and even if your canopy stays flying, you will be flying downwind. On the right side; the opposite. In the middle, you can be suddenly thrust to the left”
Dust devils can throw you into unwanted landing areas. We have had some fairly serious injuries from this. Though more rare, canopy collapse and/or loss of lift seem to cause the most serious injuries. If the turbulence strips lift off the top of the canopy or partially collapses it by moving the stagnation point of the wing onto the top skin, you can either be “dropped” and a flare will do you no good – or, in the case of losing two or three end cells – put into a sudden turn. In the case of being dropped a PLF is about all you can do. If your canopy suddenly goes into an unplanned turn, STEER! If you look where you want to go and steer that way, you might save yourself – somewhat – by keeping your partially collapsed wing level. On the other hand if you lose your three right end cells and enter a sudden right turn, try to square the canopy up with the left toggle. Do not look at where you are going, which just causes you to over-steer even more that way. Look where you want to go”
Bryan Burke
(In)Visibility
Dust devils occur all time when weather conditions are right, but they only become visible when the air picks up loose matter. In 1980 after the eruption of Mount St Helens in Washington State, USA, thousands of square miles were covered in a layer of volcanic ash, in a very fine powder. Literally hundreds of dust devils could be observed, as the whirlwinds picked up the very fine ash.
What you don’t see can kill you”
Dust devils can start out visible (picking up dust and other debris) and then ‘disappear’ as they enter asphalt or grass (picking up relatively little debris) but they may still be there. Reading countless first-hand descriptions like the ones here, in the course of researching this article, the most dangerous dust devils seem to be the ones with no dust. The towering chimney of dirt acts like a warning of the area to avoid. If there is no debris on the ground to be swept up into the whirlwind, they are more difficult to spot.
I was on a big way jump at the Powerplay Event at Skydive Perris in the summertime several years ago. I was flying a PD Storm 120, turning onto my final leg at 300 feet to land north to south on the grass LZ. I suspect the dust devil approached from the south masked by the grass, as I didn’t see it until the canopy in front of me buckled first. Then a few seconds later at about 200 feet, my canopy partially collapsed and got spat out the side of the dust devil over the runway. I didn’t have enough altitude to let the canopy naturally recover and I was in the way of potential incoming aircraft traffic. So I used brakes to get the wing back over my head and executed a deep emergency flat turn away from the tarmac and back over the LZ, until I was facing the landing direction again. I had enough altitude to execute a landing from a toggle surge (tandem style). I’m lucky I was only caught up in the dust devil for several seconds, and still had enough altitude to take some evasive action”
Maxine Tate, Flight-1 Instructor
Photo by Willy Boeykens
As a Skydiver what do I need to know?
- Be aware of the conditions that make dust devils more likely and be on your guard. Look out for them on the ground and be vigilant under canopy. Scan the ground for the tell-tale dusty columns. Especially check upwind and in your intended flight path. Alter your course to avoid them.
- Make choices – choose if you want to jump in these conditions, and monitor it as the day goes on. If in doubt ask a local. Late morning and early afternoon are peak times. Areas with different adjacent surfaces, such as grass/desert or tarmac/grass, are prime spots; you can choose to avoid landing near such areas.
- The dust devils will normally travel with the wind. This lets you plot their course and plan a non-intercept flight plan.
- Give them a far wider berth than seems necessary. The disturbed air extends three to five times around the visible chimney of dust.
- Be most vigilant on finals. Near the ground is the most perilous. Higher up you have a better chance of your canopy re-inflating.
- A PLF may save broken bones if you are slammed towards the ground,
- Don’t be so caught up looking for the invisible enemy that you forget to check your airspace for other canopies.
- The biggest danger is from the invisible dust devils (without particles). Check for windsocks and flags in the landing area that are pointing in different direction, or madly changing. Other warning signs are smoke going at different angles; adjacent shrubs where the foliage is blowing in opposite directions, other canopies being affected by strange turbulence.
- Going on a canopy course should provide you with knowledge of the best inputs in such situations.
- If you do get caught in one, keep calm and carry on flying (as the Brits would say).
I had a friend die in Ohio a long time ago. His canopy went completely to shit at about 40 feet. I’m sure he hit a “dust devil” but in Ohio there’s no dust so they just call it unexpected turbulence. Jumping in Eloy, Perris, Elsinore or other desert-like areas has its advantages and disadvantages regarding dust devils. They are more common because of the conditions, but we can almost always clearly see and avoid them. In other areas they are harder to recognize. Windsocks and flags on the ground are always good indicators. In Ohio I also sometimes saw cut grass flying around. Also being aware when other canopies in front of you hit turbulence and moving off the direct downwind path of those canopies“
Over the last 28 years I’ve made over 22,000 jumps in the US desert southwest. Dust devils can pop up anytime. They are simply another obstacle to be expecting, looking for and avoiding.
– The first thing I do after getting under canopy is check my airspace to be sure there is no one so close that I need to avoid them.
– When I know I have plenty of space I look to see which way the wind is going, decide on where I’m going to land and plan out my pattern.
– Then I immediately look everywhere around me to again check for traffic, and to anticipate all the other canopies I can expect to be flying a similar pattern to a landing area close to my intended landing area.
– Then I check for dust devils. Every jump. If I see any of the indicators you mentioned which are warning signs of dust devils, then at a high altitude I alter my pattern and landing area to avoid them“
I have made thousands of jumps while there were dust devils in the area. I never let one of them hit me”
Dan BC, World Champion, Skydive Perris DZ Manager
Article by Lesley Gale, compiled from many sources, thank you all
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To watch turbulent air patterns, see related article, What You Don’t See Can Kill You
