How do barchan dunes form?

Barchan dunes are crescent-shaped sand dunes that form in areas where the wind blows consistently from one direction and sand supply is limited.12

The formation of these dunes is driven by spatial variations in wind velocity, which are thought to be organized into long spiral vortices parallel to the direction of flow. As wind moves across a desert surface, it transports sand through saltation (bouncing) and creep. When this wind encounters a patch of sand, it adjusts its velocity, depositing sand to build a dune.23

A barchan dune is characterized by:
* Asymmetry: A gentle windward slope and a much steeper leeward slope, known as the slip face.1
* Crescent Shape: The dune is convex toward the wind, with its "horns" pointing downwind, marking the lateral advancement of the sand.1
* Equilibrium: Once the slip face reaches the angle of repose (about 32°), the dune maintains its shape and migrates forward as a whole, with sand being eroded from the windward side and deposited on the leeward side.13