How thick is safe ice?

Thickness is only one factor contributing to safe ice. Ice over swiftly moving rivers or streams may be unsafe no matter how thick it is. The current wears away the underside of the ice which may give way at any time.

Larger or moving bodies of water freeze more slowly than still pools or shallow ponds. Temperature is then no guarantee of safety.

The age of ice also affects its safety and appearance.

Solid mid-winter ice: The safest ice

Opaque and white, mid-winter ice is the product of many days of below freezing temperatures. Twelve inches of this ice can support automobiles. We haven't had these conditions in the Boston/Cambridge areas for years so be careful that you don't confuse solid ice with rotten ice.

Black ice: new or young ice

This is the first ice of the season or ice which has frozen over or previously thawed ice. It may crack when you skate on it, but, if evenly thick, two inches of this tough elastic ice will safely support small numbers of scattered groups of skaters.

Beware: If ice is not evenly thick, two or four inches may not be safe. Frequently the ice is thinner at the center of a pond or lake than at the edges. With just few inches of new ice, it's best to stay around the edges of a pond until more cold days age the ice.

Old, or "rotten" ice

Ice that has begin to thaw is "rotten" and is especially dangerous in late winter up North and a constant danger around Boston. The ice may be solid on the surface, but unfrozen water may channel the ice's underside. This ice can break easily under the weight of a single skater.

Your safest bet for skating outdoors in Greater Boston: Skate in shallow bodies of water no more than 2-3 feet deep at the center. If you fall in, you'll just get wet.

Ice is not slippery

Ice, like glass, is not slippery. We actually slip on a layer of water on top of the ice.

An ice skater's weight when concentrated upon the thin blade causes more ice to melt than a walker's boot. The surface water with its low resistance allows the skate to glide. Once the pressure is removed the ice refreezes.

If you attempt to walk on ice skates, lifting your forward foot, leaving your weight on the rear foot, the ice under your rear foot will melt; your foot will slide out from under you and down you'll go.

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