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Dust devil

A dust devil is a strong, well-formed, and relatively long-lived whirlwind, ranging from small (half a meter wide and a few meters tall) to large (over 10 meters wide and over 1000 meters tall). The primary vertical motion is upward. Dust devils are usually harmless, but rare ones can grow large enough to threaten both people and property.

They are comparable to tornadoes in that both are an unusual weather phenomenon of a vertically-oriented rotating column of air. Most tornadoes are associated with a larger parent circulation, the mesocyclone on the back of a supercell thunderstorm. Dust devils form as a swirling updraft under sunny conditions during fair weather, rarely coming close to the intensity of a tornado.

Names

In the southwestern United States, a dust devil is sometimes called a "dancing devil". In Death Valley, California, it may be called a "sand auger" or "dust whirl".

The Australian "willy-willy" or "whirly-whirly" is thought to derive from Yindjibarndi or a neighboring language.

The Navajo refer to them as chiindii, ghosts or spirits of dead Navajos. If a chindi spins clockwise it is said to be a good spirit; if it spins counterclockwise it is said to be a bad spirit.

Egypt has its fasset el 'afreet or "ghost's wind".

Among the Kikuyu of Kenya, the dust devil is known as ngoma cia aka, meaning "women's devil/demon".

In Brazil a dust devil is called "redemoinho" after "moinho de vento" (windmill). In some traditions it contains a dancing Saci.

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 secondary flow in the dust devil 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 upwards and in a circle. As the hot air rises it cools, loses its buoyancy and eventually ceases to rise. As it rises it displaces air which descends outside the core of the vortex. This cool air returning acts as a balance against the spinning hot air outer wall and keeps the system stable.

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.

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" being a near constant. Dusty or sandy conditions will cause particles to become caught up in 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 for sustainability of a dust devil is the extreme difference in temperature between the near surface air and atmosphere. Windy conditions will destabilize the spinning effect of a dust devil.

Intensity and duration

On Earth, most dust devils are very small and weak, often less than 3 feet (0.9 m) in diameter with maximum winds averaging about 45 miles per hour (70 km/h), and they often dissipate less than a minute after forming. On rare occasions, a dust devil can grow very large and intense, sometimes reaching a diameter of up to 300 feet (90 m) with winds in excess of 60 mph (100 km/h), and can last for upwards of 20 minutes before dissipating. One such dust devil struck the Coconino County Fairgrounds in Flagstaff, Arizona on September 14, 2000. Extensive damage occurred to several temporary tents, stands and booths, as well as some permanent fairgrounds structures. In addition, several injuries were reported, but there were no fatalities. Based on the degree of damage left behind, it is estimated that the dust devil produced winds as high as 75 mph (120 km/h), which is equivalent to a moderate-strength EF0 tornado.

On May 19, 2003, a dust devil lifted the roof off a two-story building in Lebanon, Maine, causing it to collapse and kill a man inside.. On June 18, 2008, a dust devil collapsed a shed near Casper, Wyoming, killing a woman.

Electrical activities

Dust devils, even small ones (on Earth) can produce radio noise and electrical fields greater than 10,000 volts per meter. A dust devil picks up small dirt and dust particles. As the particles 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.

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 of dust per second from each square meter (10 lb/s from each acre) of ground it passes over. A large dust devil measuring about 100 meters (330 ft) across at its base can lift about 15 metric tons (17 short 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. When this occurs, they are often called sand pillars.

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. 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.

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. 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.

Related phenomena

A fire whirl or swirl, sometimes called fire devils or fire tornadoes, 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 gases 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 gases which inhibit creation of a stable boundary layer between the rotating/rising gases relative to the surrounding gases.

Hot cinders underneath freshly-deposited ash in recently burned areas may sometimes generate numerous dust devils. The lighter weight and the darker color of the ash may create dust devils that are visible hundreds of feet into the air.

Steam devils are phenomena often observed in the steam rising from power plants.

Notes

References

Dixon, R.M.W.; Moore, Bruce; Ramson, W. S.; Thomas, Mandy (2006). Australian Aboriginal Words in English: Their Origin and Meaning. 2nd ed., South Melbourne: Oxford University Press.