January 13, 2013

If it's not ice, stay off

Recent tragedies on a Maine lake spotlight ice's unpredictable nature and its consequent dangers.

By North Cairn ncairn@pressherald.com
Staff Writer

(Continued from page 1)

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Ice forms on the Androscoggin River in Brunswick last week. River ice in general is not as strong as lake ice.

Gabe Souza/Staff Photographer

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Deeply textured ice had formed along the banks of the Presumpscot River in Windham on Thursday.

John Patriquin/Staff Photographer

KNOW YOUR ICE

Here are some factors involved in understanding the strength of ice, how it forms on lakes and streams and some physical characteristics:

New ice is much stronger than old ice, as a rule. Ice from direct freezing of lake water will be stronger than ice formed from snow melt, refrozen ice or ice made by water bubbling up through cracks and freezing on the surface.

Judging the safety of ice requires consideration of several factors, including its appearance, the daily temperature, thickness, whether the ice is or isn’t covered with snow. The depth of water under the ice, size of water body, water chemistry, distribution of the ice and local climate also contribute to the equation.

Several inches of new ice may be strong enough to support you, while a foot or more of old ice might not.

Ice rarely freezes or thaws at an equal and uniform rate across a body of water. It can be a foot thick in one spot and only an inch thick a few feet away.

A layer of snow insulates ice, slowing down the ice forming process. In addition, the weight of snow can reduce the amount of additional weight the ice can bear.

Ice near shore is weaker, not stronger, than ice farther out. Buckling of the lake or stream over the winter breaks and refreezes ice continually along the shore.

“Booming” – cracking ice on cold days or still evenings – doesn’t necessarily mean the ice is dangerous, merely that it’s changing shape as the temperature changes.

Ice formed over flowing water can be dangerous near shore, around inflowing or outflowing streams, or on lakes containing large numbers of springs.

River ice is roughly 15 percent weaker than ice on lakes, and straight, smooth flowing stretches are safer than river bends.

A potential danger spot on lakes, such as occurred on Rangeley, is an open portion completely surrounded by ice. Winds will force exposed water beneath the ice and cause the ice to deteriorate from below.

Other factors that can weaken ice include water level fluctuations and the actions of birds and fish. As an example, schools of carp create thin ice spots or even open water by congregating in one location while circulating the water with their fins.

Source: U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory in New Hampshire

It's "that transition from water to land" that can be particularly difficult to discern, MacDonald said. Each day's weather conditions has differing variables that makes the thickness, quality, strength and color of the ice "different and fluctuating."

"People don't become experts at it (evaluating the ice), and our weather fluctuates so much," said MacDonald.

Now is an especially good time to be mindful of the dangers of ice. Jan. 25-27 is "reciprocal snowmobile weekend" in Maine, New Hampshire and Vermont, according to the Department of Inland Fisheries and Wildlife, when legally registered New Hampshire and Vermont snowmobiles can be operated in Maine without a current Maine registration, and vice versa.

MacDonald, a snowmobiler himself, said people should assume nothing.

"You could just as easily go through a thin spot walking" as riding in a vehicle, he said. "You have to be mindful of conditions even walking. Slush can get you into trouble, too."

Ice at the edge of a freshwater lake, for example, might not be as thick as ice farther out, which might run counter to common sense. Groundwater and runoff from the land could thin the ice at the lake's edge or even melt it, Hamilton said.

Likewise, a bitterly cold night doesn't necessarily mean a lake will have frozen over or have an ice cover thick enough to bear the weight of a person. It's more important to consider the temperatures in the preceding season -- how temperatures ran last fall, Hamilton said -- because that controls the temperature of the water for longer than people assume.

But most people don't think about temperatures several months ago when winter arrives and they want to use their cross-country skis, snowmobiles, skates or ice-fishing shacks, he said.

Ice must be at least 6 inches thick before snowmobilers, skaters or trekkers can venture out with confidence, Hamilton said. The wisest thing to do, he said, is "to talk to the locals. ... Check with the local game wardens."

Some local snowmobile associations also do regular checks, boring into the ice and measuring its thickness in several spots, Meyers said, so they can erect small flags to indicate safe crossing points.

Those are essential aides because other measures -- like "the sudden occurrence of super-cold weather" -- are not a guarantee that a lake's surface will be frozen to the thickness needed to bear much weight, Hamilton said.

The stalled recovery efforts on Rangeley Lake illustrate the difficulty of determining whether a frozen lake is safe, or whether it will stay frozen long enough to hold all the weight that stresses it: snow, sleet, people, animals and machines.

"If you load it slowly" with weight, Hamilton said, "it flexes. ... It bends and accommodates. But if you load it quickly, it fractures."

Hamilton, who studies ice as part of his work with the Climate Change Institute at the university, spends a month or two each summer in the Arctic and a month in Antarctica. His research focuses on changes to the Greenland Ice Sheet and how they might affect rising sea levels in the Gulf of Maine -- and by implication, globally.

He uses a snowmobile in his work, he said, and although he is experienced at using it under different weather conditions, he said, "It scares the crap out of me if I can't see where I'm going."

Staff Writer North Cairn can be contacted at 791-6325 or at:

ncairn@mainetoday.com

 

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