A beautiful phenomenon caught on tape last week by a viewer named Tim Fern near Rice Lake, Wisconsin. A far more common sight in the desert southwest, a dust devil is caused by plenty of surface heating and dust, dirt, or grass that is picked up and makes this rotating column of air visible. Even though these have a tornado like structure, they are pretty harmless usually having winds of 20 or 30 mph. However, these do have enough power to move larger objects like lawn furniture, push over a barbeque, or even knock of limbs and branches from trees so you don't necessarily want to be in one.
A dust devil is a small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons because it is the difference between the hot surface temperature and much cooler air just above the surface that allow these to form. This is why they are common in the southwest with the ground temperature of dirt, pavement, concrete, and other scorching surfaces soaring to 140 degrees or more, but the air just a couple hundred feet up can be 50 or 60 degrees cooler.
Dust devils form when the hot air at the surface rises quickly through a small pocket of cooler, low- pressure air above it. If conditions are just right with calm or very light winds, the air may begin to rotate. As the air rapidly rises, the column of hot air is stretched vertically, thereby moving mass closer to the axis of rotation. 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. These devils typically last just a few minutes or less because it takes perfect conditions to allow them to continue, but some in the southwest U.S. have been known to last an hour or more. I have seen this myself living in southern Arizona for a while. In fact, I have seen more than a dozen of these at one time.
Here is one of the coolest dust devil videos I have seen showing what they are like up close… click here
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