DUST DEVIL

A 'dust devil' is a rotating updraft, ranging from small (half a meter wide and a few meters tall) to large (over 10 meters wide and over 1000 meters tall). In the southwestern United States, dust devils can be known as 'dancing devils'. Navajo refer to them as chiindii, a ghost or spirit of a Navajo. If a chiindii spins clockwise it is said to be a good spirit; if it spins counterclockwise it is said to be a bad spirit. In Death Valley, California, they may be called a 'sand auger' or 'dust whirl'.
In Australia, they are commonly known as a 'willy willy', from an Aboriginal word. ^[1]
In Egypt, they are usually called "Fasset El 'Afreet" or the ghost's wind.
Dust devils are usually harmless, but rare ones can grow in size to threaten both people and property. They are comparable to tornadoes in that both are an unusual weather phenomenon of swirling air vortices. Tornadoes form as an updraft attached to a wall cloud at the back of a thunderstorm. Dust devils form as an updraft under sunny conditions during clear to fair weather, rarely coming close to the intensity of a tornado.

Contents

Formation

Electrical activities

Martian dust devils

Related phenomena

References

External links

Formation

Dust devils form when hot air near the surface rises quickly through a small pocket of cooler low pressure air above it. If conditions are just right, the air may begin to rotate. As the air rises suddenly, the column of hot air is stretched vertically causing intensification of the spinning effect by the scientific principle conservation of angular momentum. The spinning effect causes other hot air to speed horizontally inward to the bottom of the newly-forming vortex. As more hot air rushes in toward the developing vortex to replace the air that is rising, the spinning effect becomes further intensified and self-sustaining. A dust devil, fully formed, is a funnel-like chimney through which hot air moves both upwardly and circularly. Eventually the hot air will cool and descend back through the center of the vortex. This cool air returning acts as a balance against the spinning hot air outer wall and keeps the system stable.^[2]
The spinning effect, along with surface friction, usually will produce a forward momentum. The dust devil is able to sustain itself longer by moving over nearby sources of hot surface air. Dust particles sucked into the air will create drag and act to slow the system down.
As available extreme hot air near the surface is channeled up the dust devil, eventually surrounding cooler air will be sucked in. Once this occurs, the effect is dramatic and the dust devil dissipates in seconds. Usually this occurs when a dust devil isn't moving fast enough (depletion) or begins to enter a terrain where the surface temperatures are cooler, causing unbalance.
Certain conditions increase the likelihood of dust devil formation.

★ 'Flat barren terrain, desert or asphalt:' Flat conditions increase the likelihood of the hot air "fuel" to be a near constant. Dusty or sandy conditions will cause particles to become caught up into the vortex, making the dust devil easily visible.

★ 'Clear skies or lightly cloudy conditions.' The surface needs to absorb significant amounts of solar energy from the Sun to heat the air near the surface and create ideal dust devil conditions.

★ 'Light or no wind and cool atmospheric temperature.' The underlying factor and sustainability of a dust devil is the extreme difference in temperature between the near surface air and atmosphere. Wind conditions will destabilize the spinning effect of a dust devil.

Electrical activities

Dust devils, even small ones (on Earth) can produce radio noise and electrical fields greater than 10,000 volts per meter.^[3] A dust devil picks up small dirt and dust particles. As the particle whirl around they bump and scrape into each other and become electrically charged. The whirling charged particles also create a magnetic field that fluctuates between 3 and 30 times each second.^[3]
These electrical fields assist the vortices in lifting materials off the ground and into the atmosphere. Field experiments indicate that a dust devil can lift 1 gram (0.03 oz) of dust per second from each square meter (10.8 ft²) of ground it passes over. A large dust devil measuring about 100 meters (328 ft) across at its base can lift about 15 metric tons (16.5 tons) of dust into the air in 30 minutes. Giant dust storms that sweep across the world's deserts contribute eight percent of the mineral dust in the atmosphere each year during the handful of storms that occur. In comparison, the significantly smaller dust devils that twist across the deserts during the summer lift about three times as much dust, thus having a greater combined impact on the dust content of the atmosphere.^[5]

Martian dust devils

Dust devils also occur on Mars, and were first photographed by the Viking orbiters in the 1970s. In 1997, the Mars Pathfinder lander detected a dust devil passing over it. ^[6][7] Martian dust devils can be up to fifty times as wide and ten times as high as terrestrial dust devils, and large ones may pose a threat to terrestrial technology sent to Mars.^[8]
Mission members monitoring the Spirit rover on Mars reported March 12, 2005 that a lucky encounter with a dust devil has cleaned the solar panels of that robot. Power levels dramatically increased and daily science work was anticipated to be expanded. ^[9] A similar phenomenon (solar panels mysteriously cleaned of accumulated dust) had previously been observed with the Opportunity rover, and dust devils had also been suspected as the cause. ^[10]
A large-scale cyclone has also been observed on Mars.

Related phenomena

A Fire whirl or swirl , sometimes called fire devils, can be seen during intense fires in combustible building structures or more commonly in forest or bush fires. A fire whirl is a vortex-shaped formation of burning gasses being released from the combustible material. The genesis of the vortex is probably similar to that of a dust devil. But, as distinct from the dust devil, it is improbable that the height reached by the fire gas vortex is greater than the visible height of the vortical flames because of turbulence in the surrounding gasses which inhibit creation of a stable boundary layer between the rotating/rising gases relative to the surrounding gasses.^[11]
Steam devils are phenomena often observed in the steam rising from power plants.^[12]

References

1. The Cambridge Australian English Style Guide, , Pam, Peters, Cambridge University Press, 1995, ISBN 0-521-43401-7
2. National Audubon Society Field Guide to North American Weather, Ludlum, David M., , , Knopf, 1997, ISBN 0-679-40851-7
3.
4.
5.
6. Dust Devil Vortices at the Ares Vallis MPF Landing Site
7. Martian Dust Devils Caught
8. Studying Earth Dust Devils For Possible Mars Mission
9. Spirit Gets A Dust Devil Once-Over
10. Did You Know?
11. WILDFIRE MODELING, IR OBSERVATIONS AND ANALYSIS
12. Lyons, Walter A. The Handy Weather Answer Book. Detroit, MI: Visible Ink Press, 1997.

External links

★ Australian Dust Devil Photos

★ ''Dancing with the Devils'' Video

★ Dust Devil Imaged by ''Spirit'' Rover on Mars

★ Japanese Dust Devil Video

★ Matador Dust Devil Project

★ Page with many movies of Martian dust devils as seen by Spirit, with enhanced images as well as ratings.

★ The Bibliography of Aeolian Research