Ripples cover most dry, bare, sandy surfaces in deserts. They are absent only under certain conditions: where the sand is very coarse, where the drag velocity is high (and thus sand can be readily entrained), where sand is dropped into hollows because of low wind velocities, and on active dune slipfaces. Most ripples are sinuous, with wavelengths ranging from centimeters to tens of meters; ripples of fine sand are less curved than those formed from coarse sand. Stronger winds also result in greater sinuosity. Ripple heights vary from <1 cm to as much as 30 cm. Most ripples develop quickly (from minutes to hours) and travel rapidly (around 5 m/day in moderate breezes); they also tend to be short-lived. Most ripples are transverse to wind flow, but they can form oblique to the wind on sloping surfaces where gravity exerts a force on the sand grains.
Ripple crests are composed of coarse-grained sands, with fine-grained sands coming to rest in their troughs. As with dunes, formation of ripples is not fully understood, and even the most acceptable models do not account for the ridge-coarse sand, trough-fine sand associations. Hypotheses for ripple formation require that saltation and reptation take place. Saltation refers to the high-energy, long-leaping, rebounding motion of wind-disturbed sand grains. Reptation, conversely, is the much less vigorous low-hop motion of sand grains physically dislodged by saltating particles. Each saltating grain is believed to trigger about 10 reptating ones, and thus reptation accounts for a high proportion of transported sands. The strongest current hypotheses for ripple formation are the following:
Deserts of the World
This page last modified on October 7, 1998.