Last year, Lake Waconia froze over Nov. 23 and opened again the next day. The official freeze-up — the first day when at least 90 percent of a pond or lake freezes over and stays that way — would come Dec. 7.
During the fall season, as the angle of the sun drops, lake water cools, shrinks and becomes more dense. Once the temperature drops below 39 degrees, however, the water begins to swell. The cooler water, having become less dense as it swells, naturally rises to the surface. Ice forms at 32 degrees. An ice sheet forms on the first calm, freezing day or night after a particular pond or lake reaches 39 degrees in all parts. The temperature of the water in contact with the ice sheet is 32 degrees, but a few feet below the ice the temperature remains above freezing.
Soon after freeze-up, ice sheets on lakes and ponds expand and contract, owing to the temperature. Ice will emit cracking and rumbling sounds. These loud, long roars and rolls don’t mean that the ice is unsafe to walk on, but the activity is a reminder to respect the ice.
A lake needs at least 4 inches of new solid ice in contact with stationary water for safe skating, walking and ice fishing. A foot to 15 inches is needed to hold a medium-sized pickup truck.
You don’t want to fall through ice; cold water saps body heat 25 times faster than air of the same temperature. In 32-degree water, a person will last about 15 minutes before losing consciousness.
Jim Gilbert’s Nature Notes are heard on WCCO Radio at 7:15 a.m. Sundays. He taught and worked as a naturalist for 50 years.