These waves or ripples in the sand are caused by the saltation of the sand grains when blown by the wind. The distance between the ridges is dependent on the average size of the sand grains and the average speed of the wind. Saltation occurs when the wind picks up the sand and starts it bouncing over the surface. You can see a video showing how this works by clicking here. The same process is at work with automobile tires and dirt roads that causes washboarding.