Watch your step: Why thin ice is hard to detect

Ice fishers walk on Gull Lake near Brainerd, Minn., Dec. 15, 2011.
Steve Kohls / Brainerd Dispatch via AP 2011

You've heard the warning: Don't tread on thin ice. But it's not that easy to tell when and where the ice is safe.

Why it's tricky to know:

Ice isn't uniformly thick. Ice on a single lake might be six inches thick in one spot, but only one inch thick in another part.

Not all lakes are the same. Even if it's been cold for several days in a row, some lakes will take longer than others to freeze. It all depends on their size and depth.

Moving water is not good for ice. Don't walk on areas where streams feed into lakes. The rest of the lake might be safe, but ice formed over moving water is not stable. It might not be easy to discern where these areas are, so get to know the lake before venturing out on the ice.

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Ice can't be measured from a distance. Satellites can tell us about ice cover, but not about thickness. There aren't obvious visual cues that can tell you if ice is 4 inches or 12 inches thick. The only way to know is to go out on the ice and use a tape measure.

How thick does ice need to be before going on it?

The Department of Natural Resources recommends 4 inches for walkers, 5 inches for ATVs or snowmobiles, 8-12 inches for cars and small trucks and 12-15 inches for larger trucks.

To check thickness, measure the ice every foot or two as you walk out onto the lake — or check on conditions with a local bait shop. If ice near the shore is cracked or mushy, do not go out on the lake.

New ice is stronger than old ice. Jay Austin, a physicist at University of Minnesota Duluth's Large Lake Observatory, said this is partially due to the lake's sediment. "Over the course of the winter, the lake actually heats up below the ice because of the heat stored in the sediment below the water."

The science of freezing

Water reaches its maximum density at 39 degrees Fahrenheit — so as water freezes it gets less dense. This is why frozen water (aka ice) floats.

So as water gets below 39 degrees, the colder water floats to the top of the lake, where it eventually freezes and forms a layer of ice.