You see them mostly on hot, dry days, weaving along the edges of fallow fields: tall, snaking columns of whirling dust. They’re dust devils, and they’re coming under increasing study: not so much because of their effects on Earth but because they’ve also been seen on Mars, where they may threaten future landers.
Even on Earth, they’re not entirely harmless. Their effects range from the annoying to the destructive (the strongest dust devils can damage small structures) to the deadly (light aircraft, helicopters and blimps are all vulnerable).
Atmospheric scientist Nilton Renno, now at the University of Michigan at Ann Arbor and formerly an assistant professor at the University of Arizona, began to study dust devils after a friend was killed by one: the friend was sitting next to his sailplane when a dust devil swept over it, lifted it up in the air, and dropped it on him. As detailed in an article in the July issue of Discover Magazine, he developed the first good theory of how dust devils develop.
As the sun heats the ground, it also heats the air just above the ground. Because the ground is uneven, the air doesn’t heat up equally. The result is “bubbles” of hot air that rise through the cooler air above them, leaving behind an area of low pressure, into which air is drawn from the sides. The result is a column of rising air that may begin to spin because of shifting winds or just because of the uneven way air rushes into its base. As the column gets taller and thinner, it spins faster, just like figure skaters spin faster when they draw in their arms.
Another way in which ground heating leads to dust devils occurs where warmer ground is next to cooer ground: i.e., between an irrigated and a fallow field. The cooler, denser air flows over the warmer field; the rising warmer air rushes over the cool field to take its place. As the two masses of air rub over each other, they create a rolling tube. A strong updraft can lift the center, creating an arched, spinning tube of air. Friction with the surrounding air stops the spinning at the center of the arch, pinching off the two ends, which form a pair of matched dust devils, spinning in opposite directions. They may revolve around each other, or one may die off.
Dust devils range from 2.5 centimeters in diameter to 300 metres in diameter, can tower as much as 300 metres into the air, and can move cross-country at speeds ranging from 25 kilometres per hour to 100 kilometres per hour. Wind speeds are typically 25 to 40 kilometres per hour, but speeds of 152 kilometers per hour have been recorded. They’re usually short-lived: they need a steady supply of warm air. Once a dust devil moves over cooler land, it will quickly dissipate.
The pressure at the core of the dust devil is as little as one-hundredth of that outside. In essence, a dust devil is a giant vacuum cleaner, which sucks up anything loose it passes over–which means you can see not just dust devils, but trash devils, snow devils, water devils, and, around fires, even flame devils. A large dust devil can lift thousands of kilograms of dust. (Heavier, coarser particles, such as sand, get picked up but are then thrown out.)
Dust devils also generate powerful electrical fields. All those swirling particles rubbing against each other exchange electrons. Dust particles gain electrons, getting a negative charge, while heavier particles such as sand lose electrons, becoming positively charged. Since the sand is flung out, the dust devil ends up with a powerful negative charge overall–roughly 10,000 volts per meter.
No need to worry about being electrocuted by a dust devil passing over you, though: there’s almost no current involved. Nor do you have to worry about lightning bolts. On Earth, it takes an electric field of 3 million volts per meter to generate a lightning discharge.
On Mars, on the other hand, the thin atmosphere requires only 20,000 volts per meter to generate lightning, something Mars’s larger dust devils can quite probably manage, posing another threat to spacecraft.
There’s not much you can do about a dust devil but get out of its way: we can’t predict where one will form or how strong it will be, and can’t stop it once it’s up and spinning. But the more we learn about dust devils on Earth, the better we’ll be able to protect future spacecraft–and, someday, explorers–on Mars.
