Ice fishing and snowmobile enthusiasts should presume that ice on lakes and rivers is unsafe, and use extreme caution when traveling across lakes, the Department of Natural Resources (DNR) reported January 23 while issuing ice safety guidelines.

"Just because a lake or stream is frozen, doesn't mean the ice is safe," said Lieutenant Creig Grey, a marine safety and education supervisor for the DNR Law Enforcement Division.

The strength of ice cannot be determined by its appearance, its thickness, the temperature, or whether or not it is covered with snow, Mr. Grey said. New ice is generally much stronger than old ice, and a couple of inches of clear ice may be strong enough to support a person's weight, while a foot of old, air-bubbled ice will not.

"Clear ice that has a bluish tint is the strongest," Mr. Grey said. "Ice formed by melted and refrozen snow appears milky, and is very porous and very weak."

Ice covered by snow should always be presumed unsafe because snow acts like an insulating blanket and slows the freezing process. Ice under snow will be thinner and weaker. Asnowfall can warm up and melt existing ice.

If there is slush on the ice, stay off, Mr. Grey advises. Slush ice is only about half as strong as clear ice and indicates the ice is no longer freezing from the bottom.

Anglers should be especially cautious in areas where air temperatures have fluctuated in recent days, Mr. Grey said. Any newly formed ice that is created after a cold front moves through should be regarded with caution. A cold snap with very cold temperatures quickly weakens ice and can cause large cracks within half a day. A warm spell may take several days to weaken the ice.

When temperatures vary widely, causing the ice to thaw during the day and refreeze at night, the result is weak, "spongy," or honeycombed ice that is unsafe, he said.

The DNR does not recommend the standard "inch thickness" guide used by many anglers and snowmobilers to determine ice safety, because ice seldom forms at a uniform rate.

Three or four inches of ice on a shallow farm pond with no inlets or outlets, for example, cannot be compared to the same amount of ice formed over a river with strong current, or to ice covering the bays of the Great Lakes, where ice cover will always be more fragile, Mr. Grey said.

Deep inland lakes take longer to freeze than shallow lakes. Ice cover on lakes with strong currents or chain-of-lakes systems is also more unpredictable.

"Always presume that no ice is safe," Mr. Grey said. "Do not venture out onto the ice unless you test the thickness and quality with a spud, needle bar, or an auger. Ice that is six or seven inches thick in one spot can be only two inches thick close by."

On the big lakes, conditions can change within just a few feet because of currents under the ice. Be especially careful around pressure cracks, Mr. Grey advised.

Ice near shore tends to be much weaker because of shifting, expansion, and heat from sunlight reflecting off the bottom. If there's ice on the lake, but water around the shoreline, proceed with caution.

Avoid areas with protruding logs, brush, plants, or docks. These structures can absorb heat from the sun, weakening the surrounding ice.

"I personally would never recommend that you take a car or truck onto the ice," Mr. Grey said. "But those are personal decisions. I would urge that anyone wear a life jacket when walking onto a frozen lake or river."