WHAT ARE DUST DEVILS?
A dust devil is a strong, well-formed, and relatively long-lived whirlwind, ranging from half a meter wide and a few meters tall to over 10 meters wide and over 1000 meters tall. They resemble a funnel type chimney through which hot air moves both upward and downward.
Dust Devils are common around the world and known by different names in different locations. People in the southwestern United States refer to them as “dancing devil” or a “sun devil”. In Death Valley, California, it takes the name “sand auger” or a “dust whirl”. People in the Australian outback call them “willy-willy”, whereas people in most parts of Arabia and the Middle East refer to them as “djin” or devils.
WHERE DO THEY FORM?
Dust devils form in flat areas with strong surface heating, usually at the interface between different surface types. Flat conditions provide constant hot air that acts as the “fuel” for these dust devils. They can form on any type of surface, be it dust or asphalt. However, only the presence of dust makes this phenomenon visible to the human eye.
Dust devils form most frequently in desert areas where the sun beats down on the poorly conducting sand surface. It can however, also form even on irrigated fields, dirt roads, and even parking lots where nearby large buildings cause irregular air currents. The presence of hot cinders in recently burned areas is an ideal condition for dust devils to form.
The conditions for the formation of Dust Devils ripen when the skies are clear and there is little or no wind. The surface needs to absorb significant amounts of solar energy to heat the air near the surface and create conditions ideal for the formation of dust devils.
The underlying factor for sustainability of a dust devil is the extreme difference in temperature between the near-surface air and the atmosphere. Windy conditions will destabilize the spinning effect of a dust devil.
HOW THEY FORM?
When the sky is clear and the winds are light, the surface of the earth warms the air near the surface to temperatures well above the temperatures that prevail slightly above the ground. The density of air tends to decrease when temperatures increase and the less dense tend to rise.
When surface heating intensify in a localized pocket, and air there would suddenly become considerably warmer and try to rise, when it immediately encounters the relatively cooler air above. This cool and stable air tries to resist the warm air, just like the surface tension of water. However, almost any sort of irregularity, be it a minor gust of wind, the nearby motion of a car or even a passage of an animal leads to a breach of this stability and a bubble of hot surface air shooting upward, initiating the dust devil. The place on the surface that is hotter than the rest will be from where such a bubble shoots up first.
The sudden up rush of hot air causes air to speed horizontally inward to the bottom of the newly forming funnel. One of the rules of the physics of moving air is that its vorticity is preserved. This law of nature leads to a large speed-up in the circular motion of air spiraling inward to the bottom of the new funnel.
If this rapid rush of hot air continues to stretch in the vertical direction, it will increase its rotational speed. This phenomenon is the conservation of angular momentum, similar to the increased spinning of an ice skater as they bring their arms in toward their bodies. As more hot air rushes in from the surface to the bottom of the newly formed vortex, to replace the air that rises, the spinning effect intensifies and becomes self-sustaining.
When the dust devils thus form, it sucks up small dirt and dust particles. When these particles whirl around inside the vortex, they bump and scrape into each other and become electrically charged. The whirling charged particles create a magnetic field that fluctuates between 3 and 30 times each second, and 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 of ground it passes over. When the dust devil begins to move across the ground, they pick up more and more dust, which gives the dust devils its visible appearance.
The hot rising air eventually mixes with the cooler air above and looses its buoyancy. This air now descends through the center of the vortex. Under optimal conditions, a balance between the hot air rising along the outer wall of the vortex and the cooler air sinking in the vortex occurs. When this occurs, the vortex begins to move across the ground. By thus moving over nearby sources of hot surface air, the dust devil sustains itself for a longer period.
When all available extreme hot air near the surface channels up the dust devil, eventually the cooler air gets sucked in. Once this occurs, the dust devil dissipates in seconds. This happens when the dust devil is not moving fast enough or begins to enter a terrain where the surface temperatures are cooler, causing unbalance.
INTENSITY
Dust Devils are comparable to tornadoes in that both are a weather phenomenon of a vertically oriented rotating column of air. However, while 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.
Most dust devils are very small and weak, often less than 3 feet (0.9 meters) in diameter, with winds averaging about 45 miles per hour (70 km/h), and dissipating within a minute after forming. However occasionally dust devil do 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.
Dust devils tend to form in groups. Large dust devils sometimes have little, shorter-lived devils traveling along behind it.
Dust devils also occur on Mars. 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.
IMPACT
Small dust devils are usually harmless, but big ones may cause devastation to people and property. Big Dust devils uprooting roofs of sheds is a common occurrence in Arizona, USA. One of the biggest dust devils recorded here was on September 14, 2000 at Coconino County Fairgrounds in Flagstaff, Arizona, This dust devil produced as high as 75 mph (120 km/h), which is equivalent to a moderate-strength tornado, and caused extensive damage to property. There were no casualties though.
REFERENCE:
http://www.wrh.noaa.gov/fgz/science/dustdvl.php?wfo=fgz
http://www.gi.alaska.edu/ScienceForum/ASF2/227.html
http://amsglossary.allenpress.com/glossary/search?id=dust-devil1.
http://www.berkeley.edu/news/media/releases/2002/05/29_dust.html
Lyons, Walter A. The Handy Weather Answer Book. Detroit, MI: Visible Ink Press, 1997.
