How Sand Dunes Form
April 22, 2016
The photo above shows a sand dune taken near the town of M'Hamid in southern Morocco, not far from the Erg Chegaga dune fields. Note the grains of sand blowing upslope on the sunlit side of this dune formation. These grains are being deposited on the lee side (in this case the shadowed side) of the dune. In the arid conditions found in the western Saharan Desert, it only takes a wind of about 10 mph (16 kmph) to set grains of sand in motion. Of course, storms or strong winds will lift the grains completely. But even in fair conditions, if the wind is blowing, sand is transported by saltation; individual grains bouncing up the windward slope and then sliding down the lee side or slip face. Though winds may blow in different directions from day-to-day, the prevailing direction of the wind is the direction in which the dune migrates. During the winter season, the prevailing winds blow from the west-southwest here. Photo taken on February 21, 2016.