Even thick ice can cause a problem because there are a lot of variables involved in how strong ice is, and the ice can be very unpredictable. Last year, a snowmobiler drowned in late November in Lake Wissota in Chippewa County after his snowmobile broke through thin ice and plunged into 20 feet of water. Three other snowmobilers who were riding on frozen Wisconsin lakes in early 1997 drowned in separate incidents when they accidentally rode their snowmobiles into open water on days with poor visibility. DNR safety specialists do not keep track of pedestrians or ice anglers who drown after breaking through ice, or die of hypothermia after breaking through ice, nor does the state health division. Ice strength depends on a complex mix of factors including its thickness, the daily temperature, whether the ice is covered with snow, the surface size of the river or lake, its depth, the load placed upon the ice, and how fast that load is moving, Thoresen said.
Ice bends up and down in long waves that roll away from a car as it drives across the ice.
Because of this wave action, drivers who are taking their cars or trucks onto the ice shouldn't follow closely behind other cars or they could cause a breakthrough on otherwise safe ice, Thoresen said, and even thick ice may crack if a car drives above a certain speed on it. People can follow some general guidelines to help them decide whether ice is safe enough to walk or drive upon, Thoresen said. New ice is much stronger than old ice, as is ice formed when the lake freezes instead of ice that forms when snow melts and freezes. Clear ice is generally stronger than ice with air bubbles in it. Lake ice is often stronger than river ice, because currents moving under river ice can weaken it.