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When blizzard winds howl and snow moves horizontally across the earth, I know a beautiful winter-white scene is being created. In the stillness following the storm, sculpted snowdrifts are everywhere: along fencerows, on rooftops, around boulders and across highways. Some drifts are gentle and graceful, shallow and wispy; others are stark and angular, deep and hardpacked. Now stationary in their elegant beauty, snowdrifts were active, moving entities as they formed under the influence of turbulent physical forces.
Snow by itself does not produce drifts; it needs to be carried on the wind. The snow crystal type and wind speed determine when snow begins to move. Light snow crystals, like dendrites, require a lower wind speed; heavier crystals, such as graupel, need a stiffer wind to lift and carry them along. The minimum speed necessary to carry the lightest snow is about 11 mph.
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.
When snow-carrying wind encounters a ridge, it rises and accelerates; the greater the slope, the higher the wind speed. 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. On a grand scale, these drifts can be potentially dangerous if they give way under their own weight (causing an avalanche) or under the weight of an unwary skier or hiker. 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. Eventually the fence is no longer a factor and snow piles near both sides to bury it. The snowdrift becomes smooth and streamlined and is said to be saturated.
A boulder or small clump of vegetation on level ground produces another form of snowdrift. If the boulder is about as wide as it is tall, air flows around it as well as over it. Air blowing around its sides speeds up, scooping out pockets of snow and depositing it on the stone's leeward side. Different sized stones alter the shape and size of the snowdrift formed. A very narrow boulder may cause the wind to scoop out snow completely around it and not leave a leeward drift. On a larger scale, look for scoops around parked cars and house corners, all produced by wind blowing around an object.
Wind force also influences the forming snowdrift. As snow crystals tumble along, the structure of each crystal breaks down losing its individual identity (unlike drifting sand whose grains remain the same). As drifts form, the crystals coalesce, forming a uniform snow mass. The stronger the wind, the greater the force to compact the snow into harder-packed drifts. These can be mined for snow bricks and used to build children's snow forts or winter survival shelters.
By studying snowdrifts, we can learn about wind and its impact upon the land. On a personal level, snowdrifts always bring out the little girl in me. I find it impossible to step over a drift. I must plunge in kicking my way through it! I can't resist the lure of a large drift like the one formed by a snowfence. I must see if it will support my weight so I can walk higher than the fence. The feeling is glorious!
Sure-footed Anita Carpenter climbs the drifts and kicks the snow on winter walks near her Oshkosh home.