A wave made of sand
How Does a Sand Dune Move?
A dune travels across the desert without any single grain going all the way. The shape moves; the sand does not.
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Step 1 of 5
Wind bounces sand rather than lifting it.
Sand is too heavy to be carried far. Instead grains hop along the ground, and each landing knocks others up, so the whole surface creeps downwind.
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Step 2 of 5
Any obstacle starts a dune.
A rock or a bush slows the wind behind it, so sand drops there. The pile becomes its own obstacle and gathers more, and a dune grows from almost nothing.
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Step 3 of 5
The two sides are completely different.
The windward side is a long gentle ramp that grains roll up. The far side is a steep face where they tip over and slide, always at about the same angle.
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Step 4 of 5
So the shape travels and the sand does not.
Grains are removed from the back and dropped at the front over and over. The dune arrives somewhere new made of sand that was never at the front before.
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Step 5 of 5
Wind direction decides the shape.
Steady wind from one side makes crescents. Wind from two directions makes long ridges. Wind from everywhere makes tall star-shaped dunes that hardly move at all.
Dunes hum. Sand sliding down a big slip face can set the whole face vibrating, low enough to feel and loud enough to hear from far off.
The short version
Wind rolls sand up a dune's gentle slope and tips it down the steep one, so the shape travels while the grains only ever move a little.
Try it yourself
Blow gently across a saucer of sand. The pile that builds up has a gentle side and a steep one, at once.