Wind-carried snow will not form drifts unless an obstacle is encountered. Any bump in its path will do, big or little. The obstacle deflects the air flow increasing or decreasing its speed. The obstacle's shape, size and position relative to the prevailing wind determine the shape and location of a snowdrift. Air currents reach their maximum velocity above the ridge crest, then slow down on the downwind side producing a swirling wind pattern or eddy at the ridge crest.
Snow is deposited in a sharply-curved formation or cornice that grows out from the crest.
Under the right conditions, these drifts continue to grow and can extend quite a distance from the crest, seeming to defy gravity. Similar drifts form along road cuts and on flat roofs. As wind rises over a stone fence, it speeds up and forms an eddy on each side. Snowdrifts with sharp-edged cornices form on both sides. As the drifts grow, they alter the wind speed and direction.