Ever notice how sand dunes in a windy
desert appear to be somewhat regularly spaced? Wonder how that happens? Its a result of something known as the
Helmholtz-Kelvin instability, a phenomenon that occurs when two liquids in contact which each other cannot mix.
In the case of a desert, the particles of sand are so small that they can, in some respects, be treated as a liquid. Imagine if the desert was perfectly flat. Then the wind would move smoothly across the desert surface, picking up a particle here, dropping a particle there. This is described as laminar flow. However, if a few sand particles got dumped as a group in one location, then the laminar flow would be disrupted. The wind would have to slow down to go over the bump and speed up to pass over the hollow where the sand particles came from. Slowing the wind down would cause more sand to be deposited there and speeding it up would take away sand from the hollow. As a result, you get amplification of both the pile and the dip in the desert surface.
What determines the spacing of these bumps? The answer is actually quite simple - twice the average distance a sand particle is carried in the wind. The sand, on average, can travel from one pit to one bump, creating a periodic surface of pits and bumps that are closer in slower winds and further apart in faster winds.
The same phenomenon results in the flapping of flag, ripples forming on the surface of a pond and the striped pattern of clouds observed when moist heaver air blows against cooler, drier air.