How wind draws a dune

Wind exerts a force on loose sand. When it overcomes a grain’s weight and the forces holding it to its neighbours, the grain can lift into a short hop. Geologists call this saltation. On landing, it strikes the surface and can set other grains moving. Repeated hops carry sand across exposed ground, while larger grains may roll or creep along the surface after being struck. A small impact becomes part of a continuing exchange.

Impacts and differences in grain size help organize sand into ripples. Their regular lines cover a slope at a much finer scale than the dune itself. Surface moisture, vegetation and crusts alter the supply of loose grains available to the wind. The neat pattern is therefore an active surface form: fresh movement can sharpen it, interrupt it or cover it with a different arrangement.

On a simple dune, sand travels up a windward slope and accumulates beyond the crest. As the sheltered face steepens, thin layers slide down under gravity. This steep lee slope is the slip face. Transport up one side and deposition on the other gradually renew the dune. Its outline can remain familiar while individual grains take new positions, much as a wave can hold a shape while water moves through it.

Wind direction and the supply of sand help determine the larger pattern. A dominant wind can favour crescentic ridges; winds arriving from several directions can build more complex peaks and arms. Vegetation can hold parts of a dune in place. Mingsha’s overlapping slopes belong to this wider family of wind-shaped landforms, with local topography guiding the arrangement around the spring. The large ridge and the smallest ripple share the same basic ingredients: movable grains, air and gravity.