The Carlisle Mosquito Online

Friday, December 15, 2000


Skating on thick ice

When is ice black and when is it white? Benfield first explained this in the Carlisle Gazette of February 8, 1980.

Pure ice is a beautiful, highly transparent substance. Because of its transparency, skaters talk about black ice, which contains so few air bubbles that sunlight can easily shine through it with little reflection. It is usual for black ice to occur, if at all, early in the season before snow comes.

After snow falls on a frozen pond, the ice which forms from it will contain air spaces which reflect enough sunlight to make the ice look white, not black. This winter there has been so very little snow that the ice which covers our ponds is still rather transparent. It is fun to look through black ice and see fish and other aquatic life in the water underneath. A friend told me she was skating the other day and suddenly noticed something below looking like a bunch of carrots. The carrots moved and she realized she was looking at goldfish, swimming close together (to keep warm?).

Where transparent pond ice has vertical cracks, one can of course get an idea of its thickness. But as with a slab of glass, the ice is thicker than it looks. As for all transparent substances, ice has an index of refraction, which is a measure of the bending of light rays in transmission. This index also tells how much light waves are slowed down while traveling through the substance (which is directly related to the bending effect), and how much thicker the material actually is than its seems to be.

Compared with most transparent materials, ice has a small index of refraction, 1.3. If you are skating on a pond where the ice looks as though it is six inches thick, it is actually 1.3 times that thick, about eight inches.

Among ice's unusual properties is its very large coefficient of expansion. This means that if you cool a piece it contracts a good deal more than most solids would. Therefore, if the outdoor temperature drops after ice has formed on a pond, it will tend to crack, sometimes quite noisily.

Where these cracks are very thin, one sometimes sees vivid prismatic colors. These colors arise because of the wave nature of light; vivid colors are associated with other thin films, as with oil on water.

Ice has another remarkable characteristic: under pressure its melting point drops. That is, if you take a piece of ice which is at 32°F (0°C) and maintain it at that temperature while subjecting it to pressure, it will melt. Remove the pressure and it will solidify again. It is this physical fact which makes skating and skiing possible.

A skater's weight, supported by only a square inch or less of the area of the steel blade in contact with the ice, produces such a high local pressure on the ice that is momentarily melts, providing a lubricating film of water. This same effect occurs under a ski where the little points of snow flakes are briefly subjected to an enormous increase of pressure, turning them into lubricating water.

Almost 50 years ago, when descending one day from Mt. Washington on skis in arctic weather, the temperature was so low that my weight was not enough to produce this pressure-melting (I wasn't so fat then). As a result, instead of skiing normally, I had to push myself downhill with my ski poles, as though skiing on sand. This is the only time in my life that I have skied in such cold conditions.

In this same connection, I remember hearing a story about a visit to New York by a man from Winnipeg in central Canada, where winter temperatures of -30°F and colder are quite normal. Being fond of skating and good at it, he brought his skates with him and tried them out in Central Park or somewhere. He promptly fell on his ear! His skate blades were too thin; they were meant for very cold Winnipeg ice. In New York they produced enough melting under his weight to sink too far into the ice.

If a lightweight New Yorker were to try skating on Winnipeg ice, I suppose he might have the same kind of trouble I had on Mt. Washington.

A.E. (Ben) Benfield is a longtime Carlisle resident who lives on West Street. He wrote a column for the Carlisle Gazette and then for the Carlisle Mosquito when the two papers merged in 1983. Benfield was also chairman of the Carlisle Conservation Commission in the '60s and '70s when large pieces of conservation land were purchased by the town.

2000 The Carlisle Mosquito