Sand dreams and spontaneous order

One of my dreams is to go to Niger. In the meantime I will have to be satisfied reading the excellent Sahara: A Natural History, by Marq de Villiers and Sheila Hirtle. No student of spontaneous orders can ignore the following passage:

Obviously [sic], dunes are formed because jumping sand bounces more easily off hard surfaces than off soft ones, so that more sand can be moved over a pebbly desert surface than over a smooth or soft one. Even a slight hollow, though, or a rock, will reduce the amount of sand that the wind can carry, and a small sand patch begins. Very quickly, this patch will trap more sand. When the patch is big enough, it begins to change the wind velocity about it. The winds decrease near the surface and deposit more sand on the patch. Quickly, the dune is built up.

If the conditions are right, the dune will grow rapidly: In days it will be taller than a man, and in just a few weeksit can reach sixty-five or one hundred feet, and keep growing. Over time, the windward slope eventually adjusts itself, and the wind velocity close to the sand increases to compensate for the drag imposed by the sandy surface. The smooth leeward slope steepens until the wind can’t be deflected down sharply enough to follow it, leaving a “dead zone” into which the sand falls. When this so-called dispositional slope reaches the natural slope angel of dry sand (about thirty-two degrees), the added sand cascades down the slope, now called the “slip face.” The dune has stopped growing — there is no gain or loss of sand — though it continues to move forward as a whole, slowly, ponderously, relentlessly.

It turns out that driving on sand is an art, and no one can avoid getting stuck in the long run. Fortunately (to my surprise) most of the surface of the Sahara is rock rather than sand.

Here is a lecture on spontaneous order and sand. Here is advice on how to build a better sand castle.

Every now and then someone is inclined to think that this kind of analysis holds the key to either business cycles or stock market crashes. It has yet to come through, but in the meantime I will keep dreaming of Niger.


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