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.

Types of Sand Dunes

In places where tons of sand accumulate, seas of sand bury the landscape. Wind shapes the sand into dunes that exist in a variety of shapes and sizes, depending on the character of the wind and the sand supply.

When the wind blows steadily in one direction, crescent-shape dunes called barchan dunes form, with each tip of the dune pointing in the direction of the wind. (Barchan dunes are also known as crescentic dunes.)

Constant shifting of wind in similar conditions will form a group of crescent-shaped dunes pointing in different directions. These dunes will overlap each other and create a constantly changing star dune.

When enough sand accumulates for the ground to be completely buried and moderate winds are blowing, simple, wave-like dunes called transverse dunes are formed. The crests of transverse dunes lie perpendicular to the wind direction.

Powerful winds may break through transverse dunes and change them into parabolic dunes. Usually, they are much larger than barchan dunes (because there is more sand), and their ends point against the direction of the wind.

If there is abundant sand and a strong, steady wind, the sand streams into longitudinal dunes (also called seif dunes, after the Arabic word for "sword"), whose axis lies parallel to the wind direction. The Rub al Khali, an erg in the Arabian Peninsula, contains seif dunes that stretch for nearly 200 km and reach height of over 300 km.

Reversing dunes refer to dunes that occur in places where winds periodically reverse direction. They can exist as varieties of any of the above types of dunes.

On a somewhat unrelated note, Great Sand Dunes National Park of Colorado was just recently (September 13, 2004) made the 58th national park of the United States, the first to be created under the Bush Administration. There are still legal fights over the land involving rights to subsurface water bought by private companies.
Marshak, Steven. Earth: Portrait of a Planet
http://pubs.usgs.gov/gip/deserts/dunes/
Great Sand Dunes National Park:
http://www.denverpost.com/Stories/0,1413,36~154~2406748,00.html