“And so the hours dragged by until the sun stood dead above our heads, a huge white ball in the noon sky, beating, blazing down, and then it happened – suddenly, a whirlwind! Twisting a great duststorm up from the earth, a black plague of the heavens filling the plain, ripping the leaves off every tree in sight, choking the air and sky. We squinted hard and took our whipping from the gods.”

Thus speaks a sentry in the play 'Antigone', written by Sophocles in Greece around 450 BCE, capturing a scene that many dust devil researchers two and a half millennia later will doubtless appreciate. As long as people have been writing anything, they've been writing about dust devils.

Dust devils are among the most prominent dynamic phenomena observed on the surface of Mars, and are common in dusty regions of Earth. Dust devils are whirlwinds or vortices made visible by lofting dust from the surface. They are caused by strong heating of the ground, which forms a layer of hot, buoyant air when the ambient wind is not too strong. It's usually sunlight that heats the ground, but some related features can form in forest fires or even on fresh, hot volcanic ash plains. The hot air rises in plumes, making a converging flow near the ground. This convergence intensifies any rotation in the ambient wind to form a vortex, much like the vortex that forms in a draining sink.

Dust devils have some features in common with tornadoes, although these are powered by storm systems aloft rather than direct solar heating; their funnels also descend from the cloud layer rather than rising from the ground. Hurricanes, on the other hand, are driven by the warm (ocean) surface more analogously to dust devils, but use moist air (in effect, steam) rather than dry air as their working fluid. Hurricanes are also large enough that the Earth's rotation determines their direction of spin, whereas the rotation of dust devils has a random direction.

Dust devils are usually just an interesting desert curiosity, and sometimes a nuisance at soccer games or picnics. But occasionally, they can be deadly. In the process recently of researching a historical review of dust devils I found a number of instances where people have been killed by dust devils, due to light buildings collapsing or to vehicle, aircraft or parachute accidents. Damage to light structures, fires caused by items lofted onto power lines, and injuries (including children with broken limbs when the 'bouncy castle' they were playing on was lifted across a busy street) are not uncommon. By deaths and dollars overall, dust devils are between a hundred and a thousand times less dangerous than tornadoes, but this is little consolation to those involved.

On Mars, dust devils may be larger, but are probably not terribly dangerous. Indeed, they can actually be useful. Dennis Reiss and I showed last year that the "clearing events" that swept dust off the solar panels of the Spirit rover and rejuvenated its power supply occurred with a frequency consistent with that of encountering vortices of an intensity strong enough to lift dust in Martian conditions (known from laboratory tests). The clearing events also coincided with the onset of dust devil season.

Dust devil research is at an exciting stage, moving from "trophy hunting" and "stamp collecting" to more systematic and synthetic investigations. By "trophy hunting" I mean isolated and sometimes poorly-documented meteorological measurements collected by adventurous chasing of dust devils in pickup trucks – jolly good fun, of course, and providing important example data on the basic properties of vortices, but susceptible to vehicle noise and selection biases (people like to chase the biggest, slowest devils). "Stamp collecting" refers to surveys of dust devil activity, either by thirsty enterprise of sitting out in the heat and watching for weeks with binoculars, or trawling through reams of spacecraft images. These efforts make good student projects! But in many cases, the statistics have been collected without much direction in their analysis.

In recent years, however, the growing body of information from these studies, together with some more elaborate analysis, and the insights from ever-improving numerical simulations ("Large-Eddy Simulations" or LES -- like weather-forecasting models, but at a much smaller scale) have started to link the various surveys and in-situ measurements into a more coherent picture. Much of this work was presented at a week-long dedicated workshop in February 2015 at the International Space Science Institute in Bern, Switzerland, and the proceedings of that workshop are being written up into a formidable book.