A measure of the symmetry of a ripple form, expressed by the ratio of ripple wavelength to ripple height. Flowing-water ripples (current ripples) have an index of 8–20 and are asymmetric, with a steeper face downstream (lee) and a gentle upstream-facing (stoss) side. The ripple index of wind ripples is 30–70; the structures are much flatter, reaching heights of 1 cm only. Wave-formed ripples (oscillation ripples) have a ripple index of 4–16 and have a more symmetrical profile form.