Is that ice safe?

Posted: Friday, December 01, 2000

When judging the safety of ice, it is better to err on the side of caution, because there may not be a chance to make a mistake a second time.

Central Emergency Services in Soldotna offers the following general guidelines regarding ice safety:

You cannot gauge the strength of ice by its appearance. Ambient air temperature, the thickness of the ice, presence of an insulating layer of snow, and the depth of the water under the ice all affect its strength. So does the size of the body of water, its chemical composition and the weight distribution of the ice on the water.

Generally speaking, new ice is stronger than old ice. Ice formed by direct freezing of water is stronger than ice resulting from melting snow, refrozen ice or ice formed from water that surged through cracks and refroze on the surface. A few inches of new ice may be strong enough to support a person, while a foot or more of old, rotten ice may not.

Ice may vary greatly in thickness. Ice that is several inches thick in one spot may be only an inch thick nearby.

Right after freeze-up, ice in the middle of a lake is thinner than ice along the shoreline.

River ice thickness in any given spot varies throughout the winter depending on the strength of the current beneath it. Remember that thawing upstream of a hard freeze may move the river's channel around under the ice during the course of a winter.

River ice is generally 15 to 20 percent weaker than lake ice. Adjust your expectations accordingly.

Ice thickness is not always the most accurate measure of its strength. Clear ice is strongest. Newly formed ice is stronger than older ice. Ice formed by freezing slush or overflow is weaker than clear ice.

Snow insulates ice and inhibits its formation. Plus the weight of the snow can decrease the weight-bearing capacity of ice.

Continuous travel over the same path will weaken thin ice.

Driving fast over thin ice can create a wave effect in the ice similar to a boat wake, which may crack ice ahead of the vehicle.

Sudden application of brakes increases the weight pressure of a vehicle on ice -- especially toward the front of the vehicle.

Ice that is booming or cracking on cold days does not necessarily mean the ice is dangerous. It is simply expanding and changing shape with changes in temperature.

However, ice is usually weaker near shore. It is continually breaking and refreezing during the winter as the ice surface on a body of water buckles.

So how thick should ice be? That's hard to say for certain, but much can be learned by cutting a hole in the ice and following these general guidelines: Four inches is marginal, 8-12 inches is recommended for skaters and hockey players, and 12 inches or more is recommended for all-terrain vehicles and snowmachines.

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