whatA desert seen from above, drawn as crest lines only. From flat scattered sand, ripples appear, the ripples swallow one another into fewer and larger dunes, and the whole field creeps downwind while you watch. Click anywhere to set a new wind direction and the pattern tears itself down and rebuilds across it.

whyNobody shapes a dune. Wind moves single sand grains, yet what appears has a wavelength, a direction and a drift that belong to the field as a whole. Werner's model showed in 1995 that three rules about one slab of sand are enough, and the same physics writes ripples on Mars.

howThe ground is a grid of stacked sand slabs. Over and over, one random slab is picked up, unless it lies in the wind shadow of taller sand upwind, and carried a few cells downwind; it lands for good in shadow, easily on sand, reluctantly on bare ground, otherwise it is carried on. Any slope steeper than the angle of repose avalanches. That is all. Crests are drawn where the height peaks against the wind, heavier the taller the dune; the arrow at top right is the wind. The readout counts crests, falling as ripples merge, mean spacing, growing, and the measured downwind drift. One integer seeds every random decision.