When dry sandy surfaces, unprotected by vegetation, are exposed to winds, the coarser material, the particle size of sand, may be carried along close to the ground, transported by a combination of the rolling and leaping of the individual grains. Where the sand is abundant and the wind is very strong with speeds of more than 15 or 20 mph, a sandstorm results, with the air filled with flying sand up to a height of several feet, having the appearance of 'streaming'.
The impact of the bouncing, jumping grains severely abrades the surface of the ground. This abrasion breaks down clods, destroys stable crusts, and wears down vegetative residues and living vegetation. Thus the sand in a sand storm is a major agent in erosion, both of the soil and of objects, natural and artificial, in its path. It can damage buildings, constructions, machines, crops and even barriers erected to control sand movement. The harsh sand can cause intense pain and even injury to animals and people in its path. The sandstorm continues until the wind abates (it may often last for days) or until some obstacle is encountered, then the sand accumulates into heaps and mounds, known as dunes. Fields, buildings, roads, and even villages have been known to be overwhelmed and obliterated. Sand storms are obviously most common in areas where there is much loose sand on the ground, in particular in deserts, on sandy coasts and near rivers which vary in volume, leaving sand beds exposed during the dry season. Deserts in which sand storms frequently occur include the Sahara, the Gobi, the Mongolian, the Arabian desert, and those of Iran and Turkistan. Wind-blown sands comprise 11% of the Sahara and 30% of the Arabian desert. Several decades ago, the streets of Swakopmund in South-West Africa were swept over by a sandstorm that piled up sand dunes 6 metres high.
In 1997 a sandstorm from the North African desert blanketed the Middle East in a haze of dust so thick that flights could not take off or land, sea ports had to be closed and people had difficulty breathing. The storm, described as the worst ever in Lebanon and the most severe in Jordan in 50 years, reduced visiblity to as little as 200 meters. Israel, Egypt and Syria were also badly affected. Flights in and out of airports at Cairo, Luxor, Aswan and Abu Simbel were suspended.