About 40% of the world is comprised of dry lands. These are hyper-arid, semi-arid or dry sub-humid areas which are characterized by low rainfall and can become subject of cyclical droughts and due to dryness, the dirt in these areas becomes loose and can be easily picked up by wind (Drynet, 2009). The blowing wind picks up the grains of sand and carries them to distances. While larger grains of sand (0.6-1 mm) fall back down within few hours, smaller grains (<0.6mm), classified as dust, hover in the air and can be blown for thousands of miles.
A dust storm is a strong wind that transports fine particles like clay, silt and other finer materials over a long distance. During the storm, these particles swirl around in the air. A dust storm can spread over hundreds of miles and can reach a height of over 10000 feet. The winds in the storm can reach up to 40 km/hour. Though wind speeds are not scary, the presence of dust particles makes it very difficult (BBC News, 2009).

How the Dust Becomes Airborne

In several field observations and wind tunnel laboratory research, efforts have been made to understand the physical process through which the dust particles become airborne. The dry land where dust storm starts have characteristic land properties where the surface is made up of separate particles which are held in place by their own weight. At low wind speed, there is no or minimum interaction between them. However, increasing wind speeds begin to add energy to the particles and after a threshold wind speed, these particles will begin to vibrate. Further, increase in the wind speed ejects these particles into the air. When these particles fall back on the ground, their impact ejects surrounding particles into the air resulting in a chain reaction. Once ejected, these particles are carried by wind in one of the three transport modes, which depend on their size, shape and density (United Nations, 2004)...

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