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While dust storms and black blizzards are the ultimate expression of blowing dust, dust borne on the wind is with us almost all the time. Aeolian or Eolian transportation (after Aeolus, a Greek god of the wind) is the term used to describe particles transported by the wind. Dust becomes airborne when winds traversing arid land with little vegetation cover pick up small particles such as sand, dust, soil and other debris and send them skyward. Aeolian transport may occur through suspension, saltation, or creep of the particles.
To see the processes by which dust becomes airborne, we must get down on our knees and take a closer look at the surface of the earth where the wind interacts with the particles.
The smallest dust particles are held in the atmosphere through suspension. Suspension occurs when surface materials are lifted into the air and upward air currents are strong enough to support the weight of the particles and hold them aloft indefinitely. Typical wind speeds near the Earth's surface will suspend particles with diameters less than 0.2 millimetres (8/1000 inch). Severe windstorms can hold large particles caught in turbulent eddies aloft for some time and push them to high altitudes, which enhances their travel distance. Under strong wind conditions, suspended dust particles may be lifted thousands of metres upward and thousands of kilometres downwind, held in suspension by turbulent eddies and updrafts.
Saltation (Latin for leaping) moves small particles forward through a series of jumps or skips, like a game of leap-tag. Saltation under typical wind speeds normally lifts particles no larger than one centimetre (0.4 inch) in diameter above the ground about a metre or two (3-7 ft). These fine soil particles are lifted into the air, drifting across the surface approximately four times further downwind than the height they attain above ground. But these dust particles are only suspended briefly as they are too heavy to remain airborne long. A saltating particle may then hit another particle as it returns to earth that will jump up and forward to continue the saltation process. Saltation is more or less continuous process at high wind speeds. From a distance, a field of saltating particles appear as if they are constantly suspended, creating a fuzzy layer next to the ground.
When saltating particles hit larger particles too heavy to hop, they nudge the larger grains (up to six times larger than saltating particles) slightly and slowly forward, a sliding and rolling movement known as creep. Creep usually requires wind speed exceeding 16 km/h (10 mph).
Aeolian transport is an important process for soil erosion, dune formation and alteration, and redeposition of soil particles. The great deposits of windblown soil around the world are known as loess, much of which originated from the debris left by glacial action during the last ice age. The thickest known loess deposit -- 335 metres or 1075 ft -- is on the Loess Plateau in China. Maximum accumulations of loess range generally from 20 to 30 metres (65-100 ft) thick in Europe and the Americas.
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