In unimodal wind regimes (where wind primarily blows from one direction) and where this is little or no vegetation present, barchan dunes form. Winds racing around the edges create 'horns' in the direction of the dune movement. On the slip face, sand avalanches occur when sand piles up beyond the angle of repose, which is about 34 degrees. Compare barchan dunes with parabolic dunes.