A dust devil in Arizona – By NASA (NASA web page & source file) [Public domain], via Wikimedia Commons
A dust devil is a strong relatively long-lived whirlwind, ranging from half a meter wide and a few meters tall to more than 10 meters wide and more than 1000 meters tall. Dust devils are usually harmless, but can on rare occasions grow large enough to pose a threat to both people and property. Dust devils form when hot air near the surface rises quickly through a small pocket of cooler, low-pressure air above it. A fully formed dust devil is a funnel-like chimney through which hot air moves, both upwards and in a circle. As the hot air rises, it cools, loses its buoyancy and eventually ceases to rise.
Dust devil on Mars, photographed by the Mars rover Spirit.
By NASA – http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20050527a.html
Transferred from English Wikipedia, original upload 3 June 2005 by Plateau99 as Image:Marsdustdevil2.gif, Public Domain, Link
Serpent Dust Devil on Mars – By NASA/JPL-Caltech/Univ. of Arizona – http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA15116 file, Public Domain, Link