A crescent-shaped dune with limbs downwind.

A sand dune that is crescent-shaped in map view. Barchan dunes form in areas of limited sand supply. They move across the desert floor with their gently sloping convex sides facing upwind and their steeply sloping concave sides facing downwind.

Desert crescentic sand dune.

An isolated, crescent-shaped sand dune. The horns of the crescent point downwind, the direction of migration. From a Turkish word for sand hill, although detailed etymology is unclear.

A moving, isolated, crescent shaped dune. The convex surface points toward the wind.

A crescent-shaped eolian sand dune that moves across a flat surface with its convex face upwind and its concave slip face downwind.

(Also spelled barchane, barkhan.) A crescent-shaped dune or drift of windblown sand or snow; the arms of the crescent point downward. Conditions under which barchans form are a moderate supply of material (sand or snow), and winds of almost constant direction and of moderate speeds.

A barchan dune is an arc-shaped sand ridge, comprised of well-sorted sand. This type of dune possesses two "horns" that face downwind, with the slip face (the downwind slope) at the angle of repose, or approximately 32 degrees. The upwind side is packed by the wind, and stands at about 15 degrees.