A sand dune is a semi-permanent accumulation of loose sand that forms in areas where the wind tends to blow in one direction, at velocities high enough to move sand, across a land surface that permits sand to amass in a regular and consistent form. Although usually associated with DESERT, dunes may develop wherever wind and loose sand combine. Most major dune fields, or sand seas (ergs in Arabic), have developed in the Sahara-Middle East desert and in the cold interior deserts of south central Asia. Between 25 and 30 percent of the Earth's desert surface is covered with some form of sand accumulation.

Most sand in dunes is well sorted, meaning that the particles all tend to fall in a narrow size range. Particles less that 0.1 mm (0.004 in) in diameter are less susceptible to wind erosion because they tend to adhere to one another, retain moisture, and form a smooth surface. Sand grains larger than 0.05 mm (0.02 in) in diameter are so heavy that wind velocity is rarely high enough to move them. Wind velocities of at least 16 km/hr (10 mph) are generally required to move sand. Wind tunnel and field experiments show that sand moves in three ways: in suspension as clouds rising as high as 2,500 m/8,200 ft; by saltation --a bounding or leaping motion that accounts for most sand movement; and by creep or surface rolling, which involves only the larger grains. Once movement begins (and if the wind persists), dunes may form wherever the smooth flow of wind and sand is interrupted, such as at shallow dips or small irregularities. The dune, once started, tends to continue to grow. Observations in the central Saharan erg suggest that 40 years are needed to form a dune 3 m (10 ft) high and 100 m (330 ft) long. Once formed, a dune may advance at rates ranging from 15 to 45 m (50 to 150ft) per year.

Dune fields tend to be large, averaging about 188,000 sq km (73,000 sq mi) in area. They all contain combinations of six basic patterns: parallel linear, parallel wavy or crescentic, isolated crescentic (barchan), star or radial, parabolic or U-shaped and sheets or stringers. The parallel groups, called seifs, may grow to lengths of more than 300 km (180 mi). They consist of straight or cuspate ridges that are much longer than they are wide, separated by relatively wide strips of bare surface. They appear to form wherever large-scale parallel winds, such as the trade winds, converge and diverge in a regular pattern. Barchan dunes are small and occur on smooth, hard surfaces where sand supply is limited. Star dunes consist of radiating arms and develop under the influence of multi-directional winds; once formed, they are relatively stationary. Parabolic dunes tend to form under slightly more humid conditions, , where vegetation gains a foothold on the side arms and anchors them while the unstable central portion continues to move downwind. Sand sheets and stringers develop where a large supply of sand is available and fewer directional winds of variable velocity occur.

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