Dust devil

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For other uses of this phrase, see Dust devil (disambiguation).

Dust devil in Johnsonville, South Carolina

A dust devil or whirlwind 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. In Death Valley, California, they may be called a sand auger or dust whirl.^[1]

In Australia, they are commonly known as a willy willy, from an Aboriginal word. ^[2]

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 a downdraft from a thunderhead 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.

1 Formation
2 Intensity
3 Electrical activities
4 Martian dust devils
5 Related phenomena
6 References
7 External links

[edit] Formation

Dust devils form when extremely 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 will cause 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.^[3]

The spinning effect, along with surface friction, usually will produce a forward momentum (known in physics as momentum). By moving, 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.^[4]

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.

[edit] Intensity

Most dust devils, forming in hot dry weather, hardly reach more than a few meters in height. While dust devils can form almost anywhere, dust devils are easily seen over dusty fields or sandy desert, where dust particles make them visible.

The phenomenon that forms dust devils can even develop over dry pavement, such as asphalt. It is observable when it makes contact with a person, or as litter and leaves blowing circularly on pavement.

Typical diameters of dust devils range from 10 to 300 feet, with an average height of approximately 500 to 1000 feet. Dust devils typically last only a few minutes before dissipating, however they can sometimes last for tens of minutes, under optimal conditions.^[5]

Certain dust devils can reach near the intensity of an F0 Tornado, possibly causing damage to people and property. For instance, a dust devil in Trenton, North Dakota on May 7, 2006 caused moderate injuries to a 4-year old girl. The girl and trampoline she was jumping on reportedly lifted 25 feet (approximately 8 metres) into the air.^[6]

[edit] Electrical activities

It was recently found that even small dust devils (on Earth) can produce radio noise and electrical fields greater than 10,000 volts per meter.^[7]

[edit] Martian dust devils

A dust devil on Mars, photographed by Mars Global Surveyor. The long dark streak is formed by a moving swirling column of Martian atmosphere. The dust devil itself (the black spot) is climbing the crater wall. The streaks on the right are sand dunes on the crater floor.

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. ^[8]^[9] 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.^[10]

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. ^[11] 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. ^[12]

A large-scale cyclone has also been observed on Mars.

Dust devil on Mars, photographed by the Mars rover Spirit. The counter in the bottom-left corner indicates time in seconds after the 1st photo was taken in the sequence. Three other dust devils appear in the background.

[edit] Related phenomena

Fire devils are whirls of fire that can occur over large fires and cause fire damage. Fire devils can also be caused by wildfires during severe thunderstorms.^{[citation needed]}