The wet world where ski meets snow

If you've ever seen a man with electrical wires coming from his skis, you probably passed Sam Colbeck. Colbeck, a retired geophysicist at the Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire, studied the physics of skis and ice skates.

People sometimes think pressure is the force that propels skis and skates, but Colbeck said that's not so. To demonstrate the real driver of the system, Colbeck rubbed his hands together. The warmth of friction he felt is the same warmth that enables the movement of skis, snowboards, dog sleds, and ice skates.

When skis or steel blades move against ice crystals, they melt a layer of water that's incredibly thin, about one-millionth of a meter. That diminutive pool allows a skier or skater to glide like a surfer. Colbeck said a 100-pound downhill racer going about 60 miles per hour generates the same heat as if he had three 100-watt light bulbs under each ski. The shiny tracks he leaves behind are the collective reflection of polished snow crystals.

As anyone who has tried to ski at 20 below can testify, the friction system doesn't work well at cold temperatures because the heat of friction can't warm the snow to above the freezing point. Cold snow is also hard snow, Colbeck said, which is why skis need glide wax.

In cold weather, hard waxes make the ski base more resistant to sharp ice crystals. Warmer snow, especially that near the freezing point, presents a different challenge. In warm snow, a skier generates so much meltwater that it sucks at ski bottoms the way a glass of ice water sticks to a glass table. The solution is a softer wax that repels water. When the snow temperature is close to 32 degrees Fahrenheit, fluorocarbon waxes work best. Colbeck tested fluorocarbon waxes versus regular paraffin waxes and found that meltwater tends to ball up on a fluorocarbon-waxed surface, reducing the contact area between ski base and snow. Though fluorocarbon waxes work, they don't come cheap; they cost more per ounce than silver.

Today's ski bases are made of high-density polyethylene, an elastic material that doesn't lose heat to the air quickly, a major reason sleds with metal runners groan at low temperatures. The plastic bases often come in black or white. Colbeck thinks a black base is a better choice for the north. Skis with black bases are able to absorb energy from the sun because photons in the snowpack bounce up to warm the ski from underneath. Bobsledders know that a warmer blade causes more meltwater; they heat their runners with a blowtorch before pushing off. Speed skaters who use “clapper” skates do quite well, which Colbeck said is probably due to their ability to keep the blade of their skate on the ice longer, where friction makes it stay warmer than in the open air.

For all the interest in gliding faster on snow and ice, Colbeck said there's been a remarkable lack of interest by ski companies in the physical processes that occur where ski meets snow. Most of the industry advances have been like genetic mutations, he said, where a company happens to select a new base material or color, it works, and everyone copies them.

“The geometry of ski bases is not well understood,” he said. “It's a science with a long way to go.”