A large 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 a 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.

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

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.

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.

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.

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