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

What many people know about ice can be boiled down to what they may have learned in elementary school: Ice is water that has changed from a liquid into a solid.

click image to enlarge

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


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

But in the outdoors, such a simple explanation can be dangerously misleading.

"Ice is kind of an unusual material," said Gordon Hamilton, professor of earth and climate science at the University of Maine in Orono. The quality, thickness, character and reliability of ice as a surface to walk, skate or snowmobile across depends on a host of conditions, including air temperature, the temperature beneath the water's surface, sunlight and wind. Such factors can combine to turn a magical moment -- standing on ice -- into a nightmare.

"There's no way to know that the ice is safe," said Hamilton. "It's really tricky to do it on the basis of the weather." And, he said, it takes experience and substantial skill to look at an expanse of ice and be able to distinguish between areas that are safe and those that are perilous.

That dynamic has been tragically illustrated recently as searchers unsuccessfully tried to locate three snowmobilers, all in their 40s and now presumed dead, who were reported missing in the early morning hours of New Year's Eve on Rangeley Lake.

A fourth snowmobiler, a woman from Yarmouth, died when her snowmobile broke through the ice at the lake. She had been out on the ice with her 16-year-old son, whose snowmobile also started to break through the ice, though he managed to make it to safety. Her body was recovered the next day.

The three missing snowmobilers were snowmobiling in darkness under whiteout conditions and likely ran into open water, officials have said. Bitter cold and a frozen lake surface in recent days have hampered the search for bodies.

"You really can't rely on anything, basically," said Bob Meyers, executive director of the Maine Snowmobile Association in Augusta. When in doubt, he said: "Stay off the ice, stay off the ice, stay off the ice."

"We have 14,000 miles of trails" on land in Maine, Meyers said. "That is the solution -- staying off the ice."

Many variables contribute to what creates a safe or hazardous frozen lake surface.

"Some of the (Maine) lakes are very large, and if there's open water and wind, the water is circulating and may be different in different places," Hamilton said. That translates into different thicknesses of ice, or even a mixture of ice and open water.

"Wind is bad for lake ice growth and longevity," Hamilton said, because it mixes water near the freezing point of 32 degrees Fahrenheit with water that might well be warmer. Frequently, a thin layer of the surface can be frozen, but water below can be eight degrees warmer or even higher. At about 40 degrees Fahrenheit, water reaches its densest point and sinks, pushing colder water upward.

That's a blend that can lead to dangerous conditions because it may create a paper-thin surface, strong enough to hold falling snow but not much more.

And if a lake is capped with ice, Hamilton said, "You keep the water away from the (cold) air and it takes longer for the ice to thicken. And if it snows . . . it further dampens the transfer of heat" from the water into the air, thwarting the formation of ice or its growth, either horizontally or vertically.

Cpl. John MacDonald of the Maine Warden Service has been a spokesman for the agency during the recent search and rescue efforts for the three missing snowmobilers. He said snow and cold weather may give a false impression that ice is safe to walk on.

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