‘Dust Devil’ the Amazing Power of Nature…
When hot air near the earth’s surface rises quickly through a small pocket of cooler, low-pressure air above it, the air may begin to rotate if it meets specific ambient environmental conditions. This is a favorable setting to initiate a strong, wellformed, and relatively long-lived whirlwind called ‘Dust Devil’. It is ranging from just about half a meter wide and a few metres tall to more than 10 metres wide and more than 1000 metres tall. The primary vertical motion of this whirlwind is upward. Dust devils are usually harmless, but can on rare occasions grow large enough to pose a threat to both people and property. They are like tornadoes in that both are a weather phenomenon of a vertically oriented rotating column of wind. Most tornadoes are associated with a larger parent circulation, the mesocyclone on the back of a super-cell thunderstorm. Dust devils form as a swirling updraft under sunny conditions during fair weather, rarely coming close to the intensity of a tornado.
Formation of ‘Dust Devil’ is described as a weather phenome- non. As its air rapidly rises, the column of hot air is stretched vertically, thereby moving mass closer to the axis of rotation, which causes intensification of the spinning effect by 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. 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.
As available extreme hot air near the surface is channeled up the dust devil, eventually surrounding cooler air will be sucked in. Once this occurs, the effect is dramatic, and the dust devil dissipates in seconds. Usually this occurs when the dust devil is not moving fast enough or begins to enter a terrain where the surface temperatures are cooler, causing instability.
The probability of ‘Dust Devil’ formation is favored by flat barren terrain, desert or tarmac. The flat conditions increase the likelihood of the hot-air “fuel” being a near constant. Dusty or sandy conditions will cause particles to become caught up in the vortex, making the dust devil easily visible. Also, clear skies or lightly cloudy conditions increase the potential of initiating ‘Dust Devils’. The surface needs to absorb significant amounts of solar energy to heat the air near the surface and create ideal dust devil conditions. Light or no wind and cool atmospheric temperature likely to trigger 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.
Most dust devils on earth are very small and weak, often less than 3 feet in diameter with maximum winds averaging about 45 miles per hour, and they often dissipate less than a minute after forming. On rare occasions, a dust devil can grow very large and intense, sometimes reaching a diameter of up to 300 feet with winds in excess of 60 mph and can last for upwards of 20 minutes before dissipating.
Although the Dust Devils are usually harmless larger ones can pose a threat to both people and property.
One such dust devil struck the Coconino County Fairgrounds in Flagstaff, Arizona, on September 14, 2000, causing extensive damage to several temporary tents, stands and booths, as well as some permanent fairgrounds structures. Several injuries were reported, but there were no fatalities. Based on the degree of damage left behind, it is estimated that the dust devil produced winds as high as 75 mph, which is equivalent to an EF0 tornado. On May 19, 2003, a dust devil lifted the roof off a two-story building in Lebanon, Maine, causing it to collapse and kill a man inside. In East El Paso, Texas in 2010, three children in an inflatable jump house were picked up by a dust devil and lifted over 10 feet, traveling over a fence and landing in a backyard three houses away. On December 31, 2014, during the setup for a cook-off in the Rose Bowl Fan Fest area, a dust devil was caught on video. No one was seriously injured.
Dust devils were blamed for about hundred aircraft accidents in past years. While many incidents were simple mechanical problems, a few have been fatal inflight incidents. Dust devils are, however, considered major hazards among skydivers as they can cause a canopy to collapse with little to no warning, at altitudes considered too low to cutaway, and contribute to serious injury or death of parachutists. Dust Devils also have health concerns among some areas where there are some endemic health issues occur. For example the ‘Valley Fever’ is endemic in some arid areas, such as the southwestern United States, California, northwestern Mexico, Central America, and South America. “Cocci” fungus grows naturally in alkaline soil and the fungus spores, that lie dormant and can be picked up by Dust Devils, are blown around. When a person breathes this it causes fungal pneumonia. While usually not a serious threat it can be dangerous to some people.
Dust Devils show electrical activities too. Even small ones on Earth, can produce radio noise and electrical fields greater than 10,000 volts per metre. A dust devil picks up small dirt and dust particles. As the particles whirl around, they bump and scrape into each other and become electrically charged. The whirling charged particles also create a magnetic field that fluctuates between 3 and 30 times each second.
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 metre of ground it passes over. A large dust devil measuring about 100 metres across at its base can lift about 15 metric tonnes of dust into the air in 30 minutes. Giant dust storms that sweep across the world’s deserts contribute 8% of the mineral dust in the atmosphere each year during the handful of storms that occur. In comparison, the significantly smaller dust devils that twist across the deserts during the summer lift about three times as much dust, thus having a greater combined impact on the dust content of the atmosphere. When this occurs, they are often called sand pillars.
- Kusala Madhushani Premaratne