A barchan or barkhan dune is an arc-shaped sand ridge, comprising 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 of sand, approximately 30–35 degrees for medium-fine dry sand. The upwind side is packed by the wind, and stands at about 15 degrees. Simple barchan dunes may stretch from meters to a hundred meters or so between the tips of the horns. The word is of Turkic origin, borrowed into English via Russian.
Simple barchan dunes may appear as larger, compound barchan or megabarchan
dunes, which may migrate with the wind. Barchans and megabarchans may
coalesce into ridges that extend for hundreds of kilometers. Dune
collisions and changes in wind direction that spawn new barchans from
the horns of the old govern the size distribution in a given field.