Surfing on Snow

In much of Alaska we've felt the last of temperatures warm enough to change snow into water, at least for 1994. But many of us melt snow all the time, using nothing but a bit of muscle and our cross-country skis.

According to Samuel Colbeck, a geophysicist with the Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire, gliding on skis is actually like surfing on a microscopic layer of water. It's hard to picture surfing when your hair is covered with frost, but the rubbing of ski on snow makes it possible.

Although friction is one of the major forces interfering with a skier's glide, it also allows a skier to move efficiently because it creates heat. As a skier slides over snow, weight and friction combine to melt the surface of the ski trail, a process that's easier to understand close up.

Although the waxed bottom of a ski and well-packed snow look flat, on a microscopic level they're both as bumpy as the Alaska Range. Each surface contains miniature peaks and valleys. When the peaks of snow crystals rub against wax-coated polyethylene ski base, the temperature at the interface rises above 32 degrees Fahrenheit. A tiny portion at the tip of each snow crystal melts, and the ski base glides on a film of water about one micron (one-millionth of a meter) thick.

Because the bottom of a waxed ski and the surface of ski trails are both grainy, a surprisingly small percentage of ski base is ever in contact with snow on an established trail. In dry snow at 15 degrees, for example, researchers found that only one percent of the ski-base area was in contact with the snow surface. Skiing over freshly-fallen snow is a different story. In fresh snow, or in snow close to the melting point, about 80 percent of the ski base is in contact with the trail because water forces much of the air out of the snow pack.

In warmer weather, a skier loses energy by plowing snow away, compressing the snow and shearing bonds between snow crystals. Fresh snow also has a finer grain structure, which makes for more friction-producing contact points with the ski. In older snow, the crystals fuse together over time, forming a course-grained snow pack that causes less friction and allows skiers to fly.

Colbeck said an average skier generates about 200 watts of heat on each ski while gliding. He likened that to the heat given off by two 100-watt light bulbs, spread out over the area where the ski base touches the snow. A competitive ski racer has even more melting power, generating about 300 watts per ski. Evidence of a skier's work is often seen as shiny ski tracks, which are actually the collective reflection of polished snow crystals.

Skiing in really frigid temperatures can reduce the release of heat, and the chance for a good glide. Colbeck said that although a ski creates its own water to glide on, the friction process loses heat to ski base, snow crystal and air. As the ambient temperature drops and gets farther away from 32 degrees, a skier works harder to overcome heat loss, paying a higher energy price for the same glide that seemed effortless near the freezing point.

For those cabin feverish enough to ski at temperatures colder than 10 below, a suggestion: why not avoid frostbite and enjoy the same sensation by trucking a few tons of sand into your living room? Although Colbeck believes a water film still occurs at 10 below, he said it's too thin to function as a good lubricant. Tests performed on ski glide at those temperatures confirmed that skiing on sand would be just as fast. Or, in this case, slow.

As for me, I think I'll stick to temperatures more suited for water skiing.