Why does sand ripple?

Why does sand ripple?

Rick asked me, why does sand ripple.

bq. Wind ripples in the sand show how close the frontiers of science can be. The exact mechanism is not well understood. Sand doesn't really fly in the wind. Grains bounce along in a process called saltation, and bump other grains into the air when they land, called reptation. Somehow millions of these micro-events make visible ripples, a fine example of self-organization in nature. Rivers also make ripples on the bottom. Wind makes waves. Are washboard dirt roads another example? Maybe everything is just fleeting patterns of quantum chaos at an atomic level!

bq. There seem to be two main contenders for explaining wind ripples:

It's the wind.
Wind or water flowing over a flat surface "trips over itself" because of friction with the bottom that slows it down. This creates rolling vortices of tubulence, an eddy effect that in turn produces ripples.

It's the sand.
Wind ripples result from just the saltation and reptation of individual sand grains.

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