How do crescent-shaped barchan dunes form?
A barchan is a crescent-shaped dune viewed from above. It commonly develops with limited sand supply, sparse vegetation and a dominant wind direction.
The two dune slopes
Wind carries grains up the gentle windward slope; grains crossing the crest slide down the steeper lee slope. The two horns often move faster and generally point downwind. Position and shape can change over time. Local wind shifts, moisture and obstacles distort the ideal crescent; not all dunes are barchans.
Consequences of movement
A migrating dune can cover roads or fields. Its speed depends on sand volume and wind conditions, and can be measured by comparing dated satellite images. A single photograph cannot establish annual speed. Reading horn direction also requires attention to terrain and changing winds. Migration is successive transport and deposition of grains, not a whole rigid mound sliding at once.