Dust Devils
A dust-devil is a whirlwind into which dust and debris gets caught up, making it visible. Dust devils form through different mechanism.
One method for dust devils to form is by strong surface heating, typically under clear skies and light winds, when the sun can warm the air near the ground to temperatures well above those just above the surface layer. Once the ground heats up enough, a localised pocket of air will quickly rise through the cooler air above it. The sudden upward rush of hot air causes 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 is intensified.
Another method for dust devils to form is through sunlight providing strong heating of the surface, and when winds are generally light. The heated land surface can start to produce convective rolls of air (as in the diagram above). Some of these rolls can get tilted upright, producing a dust devil.
The dust devil, once formed, is a funnel-like chimney through which hot air moves both upwardly and circularly. If a steady supply of warm unstable air is available for the dust devil, it will continue to move across the ground. They are usually only 10 to 50 feet in diameter, and generally not extending more than 100 feet into the air. However, once the warm unstable air is depleted or the balance is broken in some other way, the dust devil will break down and dissipate.
Dust Storms
Intense dust storms reduce visibility to near zero in and near source regions with visibility improving away from the source. On the edges of blowing dust and within downstream, visibility is reduced to 800 - 4,800 meters. Beyond that, visibility quickly returns to 3,200 - 8,000 meters. Visibility may remain at 5,000 - 9,000 meters in dust haze for days after a dust storm. Dust settles when winds drop below the speed necessary to carry the particles, but some level of dust haze will persist for longer periods of time.
From an operational point of view, it is also important to recognise that air-to-ground or slant visibility in dust storms is generally reduced compared to reported horizontal surface visibility. Therefore, it may not be possible to pick out an airfield from above, even when reported horizontal surface visibility is three nautical miles or more.
In order to have a dust storm, we need to get dust up in the air. This section briefly summarises the physical processes that make that happen.
Depending on the wind speed and the size of the particles, sediment can move in three different ways: creep, saltation, and suspension.
- Creep: is the movement of particles along the ground by rolling and sliding. Large particles and/or light winds favour creep.
- Saltation: moves small particles forward through a series of jumps or skips, like a game of leap-frog. The particles are lifted into the air, drifting approximately four times farther downwind than the height they attain above ground. As it returns to earth, a saltation particle then either bounces or hits another particle. If another particle is struck, that particle will jump up and forward to continue the saltation process.
- Suspension: occurs whenever surface sediment materials are lifted into the air and held aloft by winds. If the particles are sufficiently small and the upward air currents strong enough to support the weight of the individual grains, they will remain aloft. Since larger particles settle more quickly, it is only the smaller particles that remain suspended. Likewise, as wind speed increases, progressively larger particles are suspended. Under strong wind conditions, suspended dust particles may be lifted thousands of meters upward and thousands of kilometres downwind, held in suspension by turbulent eddies and updrafts.
We have already seen that unstable conditions favour the lofting of dust and the formation of dust storms. Stability also has a strong influence on how dust disperses. This graphic depicts the dispersion of dust plumes generated under stable and unstable conditions. For dispersion in both horizontal and vertical directions, plumes disperse more in an unstable environment. However, the effect is significantly more pronounced for vertical dispersion. When the atmosphere is stable, the dust remains relatively concentrated vertically, compared to dispersion under unstable conditions. Under neutral conditions, the plume will spread roughly equally in horizontal and vertical directions.
Where does the dust go? Remember that dust storms are typically several thousand feet high and that they can frequently extend up to 15,000 feet. Also recall that wind shear contributes to turbulence needed to loft dust. The winds aloft carry dust in a different direction than surface winds.
Most dust particles are hygroscopic, or water-loving. In fact, small particles usually form the nucleus of precipitation. Because of this affinity to moisture, any precipitation will very effectively remove dust from the troposphere.