A barchan is a type of sand dune characterized by its crescent shape and horns that point downwind. These dunes typically form in arid regions where there is a limited supply of sand and consistent wind direction. Barchans can vary in size, often reaching heights of several meters and lengths of up to hundreds of meters. Barchans are formed through the process of wind erosion and deposition. As wind blows sand, it accumulates on the upwind side, creating a steep slope. The downwind side is more gently sloped, allowing the dune to migrate over time. This movement can be influenced by factors such as wind speed and sand availability.