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Dust devil

Adrian Thomas

Pilot profile
Expert. Four times British National Champion, most recently 2014. World declared goal record in 2002 (287km with Bruce Goldsmith) British Declared Goal record in 2013 and 2012

An Oxford professor who's studies include Aviation and Evolution. Aerodynamics consultant with Airwave (paraglider and hang glider manufacturer)

1st July 2014
Original post - Paragliding Forum

Nov 2015
Added to the Knowledge Base with Adrian's kind permission

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Dusties are much more common than people imagine and even small ones can be extremely powerful. Having watched people get picked up and slammed when their bunched up glider was plucked off the ground by a dustie, having been picked up from ground handling in the landing field high enough to start thermalling (in the muddiest of mud-island, mid wales), and having been dragged across launch myself in similar circumstances, and having watched gliders, glider bags and instrument cases picked up and thrown by dusties I try to leave my wing in its bag until I am ready to go, and get it out only when I'm ready to launch if dusties are obvious anywhere nearby.

A dust devil sends a pilot flying - On YouTube l PG Forum discussion

People don't give dusties enough respect. Essentially all thermals terminate in a dustie, we just don't usually see them. The power in thermals is immense - and we know that from climbing in them, but at the ground that power has to go somewhere, and dusties are where it goes.

If you have good climbs at your flying site that means there are dusties somewhere, and most good flying sites are positioned where there are good climbs, generally with convergence, and therefore generally with an increased prevalence of dusties.

Dust devils - whirlies if there is no dust - are the stable critical point (a focus) that forms at the contact between a thermal and the ground surface.

A large dust devil l On Vimeo

They form because as the warm air rises away from the ground it draws in surrounding air from all directions, any tiny swirl in that air is magnified as the flow accelerates along the ground, towards the centre of the updraft. If the ground is smooth, there is no wind, and the thermal lasts a long time then the flow at the centre of the updraft will establish a rotation very like a whirlpool, with the core rotating like a solid body (rotational speed proportional to distance from the centre of the core) and then an exponential decay in velocity as you go further out from the core. The rotation is driven by the updraft, so the stronger the updraft, smoother the ground, and longer the thermal lasts, the stronger the dust devil.

Dust devils are most common in convergence zones. Where any difference in direction between the converging flows will cause shearing which drives the rotation in the dustdevils, so they will tend to rotate the same way in the shear layer.

Shear layers and convergence are extremely common on thermic launches, because convergence can provide an abundance of climbs. The most dramatic sites that provide examples of this that I know are Pena Negra launch at Piedrahita, and the main launch at Sopot.

Fast spinning dust devil - On Vimeo

At both of those sites the wind normally blows over the back in the morning, then blows up the front of the hill once the thermals start. In the transition between those two a strong convergence line marches up through launch, with lots of strong dustdevils in it, but convergence lines are often marked by dustdevils wherever they occur. Montalegre often has very visible convergence.

In conditions like that it is a really bad idea to spread your glider out and leave it on launch, or to carry a bunched up glider with your bare hands where they can get tangled in the lines.

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