On sloping sand surfaces, the downwind perpendicular to ripple crests is not, as commonly believed, an unbiased indicator of current direction, for the ripple-forming creep load is deflected downgradient as a function of the surface gradient, the orientation of the surface relative to the wind, and the friction angle of the sand. The force required to initiate motion on sloping surfaces is likewise dependent upon these parameters. The mean direction of sand movement is also deflected downgradient from the applied fluid force, but by a lesser amount than the ripples.

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