How do sand dunes move across deserts?

Sand dunes move because wind transports grains by saltation, where particles bounce and hop, and by surface creep, where larger grains roll along the ground. On many dunes, sand climbs the windward slope and avalanches down the slip face, which is usually around the angle of repose of about 30 to 34 degrees. Over time, this repeated movement makes the whole dune migrate, sometimes by several meters a year. The direction and speed depend on wind strength, sand supply, and the presence of vegetation or moisture. Different dune shapes, such as barchans and star dunes, form under different wind patterns.