A hot, windy day settles over Valemount here in the Robson Valley. The summer heat combined with an extended dry period and the sandy nature of our soil quickly drains moisture from much of the soil. As I sit under a tree for some slight respite from the summer sun, I look down the valley and see a dusty column of air arising, spinning its way toward me. As the column whirls past me, dust and sand and other small objects pelt me as I turn to hide my face from the debris. With the sky so clear, I knew immediately what I was seeing: a dust devil. A natural part of everyday weather on Planet Earth, dust devils of one form or another rank second to turbulent eddies as the most common form of natural atmospheric vortex on Earth. An estimated ten dust devils roam the Earth at any given moment.
A vortex (plural: vortices), technically defined, is any circular, closed flow where the medium flows around an axis of rotation. Water eddies and whirlpools are vortices as are tornadoes, waterspouts and dust devils. In all likelihood, more dust devils form on the planetary surface than are actually seen, particularly over surfaces that do not have much dust or other materials to gather into the vortex. Those that form over warm waters, for example, only become visible if they carry water droplets in their swirl.
In their largest, most impressive forms, dust devils can be confused with small tornadoes, their rising shafts of air tossing dust and small objects around the rotating columns. While dust devils show a similar structure to tornadoes, they have one very major difference, and a number of smaller ones. Dust devils develop from the ground up and rarely interact or connect to an overhead cloud of any size. True tornadoes always descend from a large, energetic cumulonimbus cloud.
Dust devils go by many names around the world: whirlwinds, desert whirlwinds, dancing dervishes, desert devils, and sand devils. In California's Death Valley, dust devils may be known as a sand auger or dust whirl. The Navaho people of the American Southwest call them chiindii and believe they are the ghosts or spirits of dead Navajos. Chiindii spinning clockwise are considered to be good spirits while those that spin counterclockwise are bad spirits. Australians call them a willy-willy, whirly-whirly, or a Cockeyed Bob, terms likely derived from Aboriginal words for the entity. In Egypt dust devils are called fasset el 'afreet or ghost's wind. To the Kikuyu of Kenya, they are known as ngoma cia aka, meaning women's devil. In Brazil, a dust devil is called redemoinho derived from moinho de vento (windmill).
A dust devil photographed on Mars by the Spirit rover on Sol 486 (the 486th day of the Martian year). Credit: NASA
We now know that dust devils are not confined to our planet. Orbital observer satellites circling Mars have captured large dust devils on the Martian surface over the years as have Mars landers.
Forming a Dust Devil
In most cases, dust devils arise on hot, sunny days over dry terrain. These conditions produce the forces that cause columns of air to rise from the ground into the atmosphere. Under intense solar heating, the surface temperature soars, as high as 55oC (131oF) or more. The surface heat is then quickly transferred to the overlying layer of air. The resulting hot parcel of becomes much less dense than the surrounding air and rises due to its buoyancy, generating a hot, rapidly rising air column.
As this air column rises, something gives it a little spin, perhaps the breeze aloft, a passing car at ground level, or the influx of cooler air descending to replace the rising parcel. As the hot air at the surface rushes into the developing vortex, the spinning is intensified. If the whirling updraft column rises rapidly, the column may stretch causing the vortex circulation to tighten. As a result, it rotates faster, like a spinning figure skater bringing in her arms. When the invisible winds of this vortex pick up loose surface dust and soil, a visible dust devil is born.
As the air rises, it cools by expansion and in time some will start to descend back through the center of the twisting vortex. Under optimum atmospheric conditions, there will be a balance between the hot air rising along the outer wall of the vortex and the cooler air sinking in the vortex core. As long as a source of hot air remains near the surface of the devil, the circulation can continue, and the devil moves forward, pushed by the wind field in the surrounding air. However, once the warm, unstable air below is depleted or the circulation balance is broken in some way (such as contact with a large obstacle or terrain feature), the dust-devil circulation will collapse and dissipate.
This dust devil spawns a second small dust devil to the right. Credit: NASA/University of Michigan
Because they form from individual rising columns, multiple vortices may skip across the landscape in pairs or in swarms. Because dust devils are relatively small in size and exist rather briefly, they are not influenced by the Coriolis parameter that sends large-scale weather patterns twisting under the influence of the Earth's rotation. Thus, dust devils may rotate either clockwise or counterclockwise, depending on the impact of their initial twist. It is not particularly unusual to see a pair of dust devils, each spinning in a different direction as they march side by side across the landscape.
The majority of dust devils are small, usually less than 1 metre in diameter, and rather short-lived, lasting a couple minutes or less. The longer-lived and larger devils, however, garner the most attention. The largest can expand to 300 metres wide and hundreds of metres high, but most often they are only a few metres in diameter and less than thirty metres tall. Their life spans are typically fifteen minutes, though a well-established devil can persist for an hour or more.
Average dust devil winds swirl with wind speeds of 25-40 km/h (15-25 mph), but strong devils have been clocked with winds of 152 km/h (95 mph). They can amble along the ground at speeds of 25 km/h (15 mph) or race across the terrain as fast as 100 km/h (62 mph). In a typical desert dust devil, two dump-truck loads (around 100 kilograms / 220 lb) of dust, sand, or soil may be carried aloft. Large dust devils can contain enough suspended material to fill half a dozen dump trucks. Though dust, soil, and sand are the most common cargos, any light material, like straw or leaves, can be swirled into its circulation.
While dust devils are small compared to the immense dust storms which blow across the arid regions of the world, their large numbers blow around more dust. Estimates of the amount of material lifted by dust devils may be three times greater than that lofted by duststorms over the course of a year. As a result, dust devils may play an important role in the amount of dust found in the atmosphere and be an influential factor in local climate.
We generally think of dust devils arising over large arid expanses such as deserts or drought-stricken farmland, a common device used in motion pictures to signify intense heat and dry landscapes. However, even a large parking lot can generate enough heat to start a sizable devil spinning upward, and often, these go unnoticed due to the sparsity of dust to make the rising column visible.
But the surface need not be dry land, warm waters can generate devils that are like weak waterspouts, or perhaps fog or steam devils small ascending whirls moving across the waters that are made visible by minute water droplets in the air.
Recent studies indicate that the debris lifted by a dust devil can become electrically charged. Researchers at the University of Michigan detected radio noise emanating from a dust devil and measured an electrical field around it at 100,000 volts per metres. The swirling charged particles also produced a detectable magnetic field.
An impressive dust devil strikes a construction site near Fayette Mall in Lexington, Kentucky in September 2005. Photo courtesy US National Weather Service, Louisville KY
The larger dust devils have the potential to cause damage with their gusty winds and impacts from their swirling material. In Flagstaff, Arizona, a large dust devil struck the Coconino County Fairgrounds on 14 September, 2000. Its winds caused extensive damage to tents, grandstands and booths, and several injuries were reported. Analysis of the damage suggested the devil had wind speeds reaching 120 km/h (75 mph),the equivalent of an EF0 tornado. A suspected dust devil in 21 May 2003, lifted the roof off a two-story auto body shop in Lebanon, Maine and caused it to collapse, killing the owner inside.
Extraterrestrial Dust Devils
As our ability to pick up smaller images on the other planets increased with the advent of orbiting satellites and surface explorers, scientists have found dust devils on a nearby planet: Mars. They first found Martian dust devils when analyzing images from the Viking orbiters in the 1970s. Martian dust devils viewed by orbiting satellites appear much taller and dustier than their Earth-borne counterparts, likely the result of the lower Martian gravity. Images from the Mars Global Surveyor on May 13, 1999, and again two days later, caught dust devils eight kilometres (5 miles) tall cavorting over the dusty Red Planet.
A dust devil on Mars, photographed by Mars Global Surveyor. The long dark streak is formed by a moving swirling column of Martian atmosphere. The dust devil itself (the black spot) is climbing the crater wall.
Dust devils form on Mars in much the same way they do on Earth. They are most frequent on Martian spring or summer days, first appearing about 10 am (local time) as the ground heats and ending about 3 pm when the ground begins to cool. (Mars' solar day is 24 hours 39 minutes long.) Dust devils observed from orbit have been estimated to be as large as 1 to 2 kilometres (0.6-1.25 miles) across their base and towering 8 to 10 km (5-6 miles) high. They have been also observed leaving 15-metre (49-ft) wide tracks on the red Martian surface that extended several kilometres (miles).
The Spirit rover has had several encounters with Martian dust devils. One fortunate encounter on 12 March 2005 apparently cleaned the rover's solar panels of accumulated dust and increased its power production. A similar encounter with the Opportunity rover has ben suspected as the likely cause of power improvement on that probe.
Spirit has photographed dust devils passing its cameras on several occasions, such as this one shown below that passed around local noon on 15 May 2005. The full passage took about 9.5 minutes. The devil moved northeasterly about 1.0 km (0.62 mile) from Spirit's position on the slopes of the Columbia Hills. It crossed the Martian terrain at a speed of about 4.8 metres per second (16 feet per second) and covered a distance of about 1.6 km (1 mile). The width of the dust devil was estimated to be 34 m (112 ft) in diameter.
Spirit Rover views dust devil passing its position, 15 May 2005 Credit: NASA
Learn More From These Relevant Books Chosen by The Weather Doctor