How do dunes form?
Dune formation involves several physical forces. For dunes to form in sand, three conditions are necessary: the wind must rise to a speed sufficient to lift and move sand grains, the air must be dense enough to push and carry the sand, and the grains must not be too heavy. Dunes form when air containing sand climbs over an obstacle such as a peak or a ridge. Deposition, the process in which the wind drops some of the sand as it moves over the ridge, causes the sand to form a dune. The height and slope of the dune increase until the underlying sand can no longer support more material. (The maximum angle at which the underlying sand can support the piling up of sand is called the angle of repose.) At this point the top of the dune slips in an avalanche or landslide. The landslide makes the top of the dunes flatter, allowing the wind to continue the process by piling more and more sand on top. Depending on the direction of the wind, one side of a dune may collapse faster than the other si
