Everybody’s familiar with generic dust devils, especially here in the desert, though it seems few really know much about them. To most they’re just a fleeting visual oddity or at worst an enlightening nuisance, until you’re three seconds from touchdown in a glider. After landing through more jacks-in-the-box than I remember at this point, my commonplace familiarity has weathered to a crusty alloy of eager fascination and wary dread.
Our atmosphere is populated with spinning phenomena of all kinds and sizes, from those that toss houses around to the minuscule jets erupting from a stick of firewood. All share the same helical structure, despite extreme contrast in their origins, media, and cargo.
First, let’s exclude hurricanes and tornadoes as classes by themselves, in terms of size, power, and physical genesis. Even we don’t need that much energy for a good time! And may as well dismiss the entire range of ordinary rotor phenomena too, if for no other reason than their axis is principally horizontal, not vertical, and our aim is to climb away from that axis rather than along it.
Scaling down from there, whirlwinds of one kind or an other occur naturally in nearly every physical environment, many benign, some horrifying. Wing vortices, smoke rings, the wakes of falling maple seeds… Anyone who’s ever hitchhiked knows about those gritty swirls that kick up on roadsides after a freight truck roars by. They’re all about wind too (albeit highly localized), even on a calm day! And then we have a gazillion unique moments that can only speak for themselves, like the magical one linked here:
What’s called fire tornadoes are hellish gyres that erupt from within already raging flame. Enough to make you wish for the seeming opposite, a waterspout! Other variants consist of driven snow, sometimes called spindrift. The video linked here illustrates a wobbly mix of icy wind, thermal convection, and even rotor:
Now about our impetuous friends, the devils: objects of any kind flying in a circle (except helicopters and drones) betray certain rising air. Visible dust and litter usually falls away by a couple thousand feet up, but I’ve circled with a chunk of sod at 3000 AGL and flown through pepper storms of sand lofting into cloud base more than two miles above a lakebed!
Devils normally spin counterclockwise in the northern hemisphere due to the weak yet ever present Coriolis effect, but topography and localized wind can exert greater influence in either direction. Notice how eddies at the shore of a river tend to swirl away from the main current toward land? That’s due to drag of course, and the same thing happens in air. High ground pulls it into whorls that spiral off from stronger flow, like behind a freight truck only different. (The reason we care is, turning against any rotation allows smaller circles in stronger lift with flatter bank, and therefore slower airspeed for a faster climb.) Then as rotation slows and widens with height, its direction gradually ceases to matter. Final authority: if there are birds nearby, always presume unerring instinct and follow their lead.
In desert country where devils are most common, they typically match the color of ground below and may be nearly invisible from high overhead. But bright sun can help you spot the shadows of dust columns, faint gray ovals moving downwind. Those are more discernible when the sun is high (the very time that devils are most numerous), as light is then falling further through the column, concentrating and darkening its subtle shadow. Also, diffused light coming the other way will make minute terrain features appear blurry. Watch closely any small area that seems less sharply visible, to find if that blurriness migrates across the surface. Blurry spots creeping along beside faint shadows are the blue sky equivalent of a sure thing!
Dust devils can provide crucial detail about surrounding surface conditions as well. Two devils less than a mile apart imply a line of lift between them, and three in a row nearly confirm it. That line might drift sideways at approximately a walking pace, while devils tend to slide along it in a geometric sum of two separate air masses’ movements. Or a line may hold relatively static as the devils follow it, straight downwind. Maybe both, depending where you look. Over open country under a cloudless sky, such information might be all you have – and all you need – to inform your next decision…
Finally, a couple more links to devilish video. The first is a classic biggie in the Valley of the Sun (Phoenix), and excellent as the photographer’s effort was, it’s good we were on different roads while she was filming:
This last link is to Burning Man in the Black Rock Desert of Nevada, one moving picture worth several thousand words!