Friction with the ground slows air down near ground level, so air 500 feet above the ground will, on average, be faster-moving than ground-level air, regardless

Friction with the ground slows air down near ground level, so air 500 feet above the ground will, on average, be faster-moving than ground-level air, regardless of elevation. See http://en.wikipedia.org/wiki/Wind_gradient http://en.wikipedia.org/wiki/Wind_gradient