17th April 2006

Dust Devils

Source: Excerpt from The Book " Weather "

Distribution: Arid areas, particularly during hot weather. Height: 0 to 1000 feet high. Cause: Lifting of dust by air-mass rotation combined with updrafts. Associated Weather: Strong local winds, reduced visibility. Hazard Warning: Can cause damage to property. A dust devil is an upward-spiraling, dust-filled vortex of air that may vary in height from only a few feet to over 1000 feet (300 m). Dust devils occur mainly in desert and semi-arid areas, where the ground is dry and high surface temperatures produce strong updrafts. Dust devils resemble mini-tornadoes, but are generally nowhere near as intense or damaging. They normally begin when winds blowing around local terrain features create a rotating air mass in the low or middle levels of the troposphere. This rotation then combines with strong updrafts produced by surface heating of the ground to create a powerful, rising funnel of air. As the air rises, it draws up large quantities of dust. It is this dust that renders the funnel visible. On some occasions, a cumulus cloud will form over the updraft area. This may give the appearance that the dust devil is emanating from the cloud, but this is never the case; it is the rotation in the surrounding air mass that generates the funnel. The presence of the cloud is, however, an indication that the initial updraft was relatively powerful, and the most intense dust devils are often associated with a cumulus cloud. Particularly powerful devils have been known to rip the roof off a house or flip over a car, but normally dust devils pose little threat to life or property.

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Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.