How a Dune Moves

[A] A sand dune is not a heap that happens to be there. It is a moving object with a stable shape, and the shape is a consequence of how sand travels. [B] Grains do not float in the wind; they hop, landing hard enough to throw up further grains, so that transport happens in a thin layer close to the ground.

The wind pushes grains up the gentle windward slope until they reach the crest. [C] Beyond the crest the wind separates from the surface and slows, so the grains it was carrying fall out and pile up until the slope behind becomes too steep to hold them. It then collapses, at an angle that is remarkably consistent, close to thirty-four degrees in dry sand. [D] Grain by grain, the windward side is stripped and the leeward side is rebuilt, and the dune advances downwind while keeping the same profile. A dune photographed a century apart looks identical and is in a different place, having travelled perhaps two hundred metres in the interval.