How do wind and gravity shape sand dunes?

Sand dunes are mounds or ridges of sand formed by the interaction of wind and gravity.1

Wind acts as the primary transport mechanism, moving sand grains through saltation (jumping) or surface creep. As wind encounters a sand patch, it adjusts its velocity, depositing sand to build the dune. On the windward side, the wind velocity increases, which helps maintain the dune's form.2

Gravity becomes the dominant force on the leeward side. As the dune grows, the leeward slope steepens until the wind separates from the surface, creating a "dead zone" where sand falls. Once this slope reaches the angle of repose—approximately 32° for dry sand—gravity causes excess sand to slip down the face. This process, known as the formation of a slip face, allows the dune to maintain its shape while migrating forward as sand is eroded from the windward side and deposited on the leeward side.2