Classic conditions for dust devils are hot air near the surface of open ground, with considerably cooler air above. If an unstable pocket of hot air rises and penetrates the colder layer above, air at the bottom replenishes it and the process may continue-quickly producing a very tall column of hot air surrounded by cold air.
Earth’s Coriolis effect then imparts counterclockwise rotation to the thin and rising column (in the Southern Hemisphere it would be clockwise) and a dust devil is born. Loose debris, such as fine sand reveals the structure of a dust devil. Particulates can concentrate along the outside of the dust devil, in part due to centrifugal force and in part due to cooler, cleaner air funneling down through the center of the cylinder.
Dust devils vary in size from about ten feet to three hundred feet across, and can reach from 500 feet to 1,000 feet in height, except under special conditions, such as found on flat sandy terrain in, for example, Arizona. Under those circumstances, heights can reach the better part of a mile and winds may reach 60 mph.
Evolution of a Dust Devil
Although dust devils often start out wide and parabolic, as the process continues, the dust devil may become thinner, and due to the principal of conservation of angular momentum, wind speed increases. This is similar to when an ice skater wishes to perform a spin. The feat is accomplished by the pulling in of her arms.
Although the typical lifespan of a dust devil is mere minutes, particularly strong ones in an ideal environment, can last an hour or more. They are especially likely to start at an interface of the ground’s surface, such as that between a sandy field and an asphalt road. Also, heat is most intense during peak hours of sunshine, so one can expect more dust devils to be seen during the middle of the day.
Dust Devil Damage
Ordinarily, dust devils do not produce much damage, and are usually merely a demonstration of the forces of nature to be enjoyed. The more powerful ones, however, are capable of damaging or destroying small buildings, as may be seen in these National Weather Service images of the Coconino County Fairgrounds in Flagstaff, Arizona, in September 2000.
Amazing Video of a Nevada Dust Devil
Watch this amazing video of a Black Rock City, Nevada dust devil, in which the photographer at first films from a distance, and then gradually approaches and enters the devil. It must be seen in its entirety to fully appreciate what even a moderately strong dust devil can be like.
NASA Dust Devil Video & Earthly & Martian Images
NASA offers a 10.6 megabyte QuickTime® movie of a very large dust devil as well as a number of excellent images, including a very special one of a dust devil on the planet Mars. Additional images of Martian dust devils may be seen at the Malin Space Science Systems website.
References and Resources:
Glendale Community College (Maricopa, Arizona) – Dust Devils
NOAA – Dust Devils
NASA – Phantoms From the Sand: Tracking Dust Devils Across Earth and Mars