Mystery of the sands unmasked

One of the delights of Google Earth is to commit a little Thesigery in the comfort of your front room and traverse the Sahara, the Empty Quarter of Arabia, the Namib or the Gobi. Not only are there dunes on gargantuan scales, but zooming-in from 30 m Landsat to 65 cm Quickbird images on Google Earth reveals a dune hierarchy down to largish ripples. And not all dunes are classic in shape. In the same issue of Nature as a retrospective review of Ralph Bagnold’s classic The Physics of Blown Sand and Desert Dunes, French, Algerian and US workers give a clue to the fundamental controls over dunes systems, that was not available to early researchers (Andreotti, B. et al 2009. Giant aeolian dune size determined by the average depth of the atmospheric boundary layer. Nature, v. 457, p. 1120-1123). They conclude that the general dynamics are analogous to those in flowing water; i.e. like a river, the wind has a capping surface that is the thermal inversion in the atmosphere marked by the tropopause. Flow that is  physically bounded involves a series of resonances (as in a flute), which help to explain the tiered nature of dune systems and also their maximum size in a particular area of desert. Together with seasonal shifts in wind direction, fluctuations in the ‘depth’ of the wind combine together to produce the hypnotically addictive disorganised order that makes big sand deserts so attractive, despite their dangers.

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