Naturalist has been watching freeze-up for decades; don't equate it with ice safety

While there have been some above-average high temperatures into this month, dozens of Minnesota lakes froze over by mid-November.

Sibley Lake, on the edge of the city of Pequot Lakes, and both Pickerel Lake and Stocking Lake just north of Park Rapids froze over Nov. 15 and are still ice-covered.

I record the official ice-on date for a lake when it's at least 90% ice-covered and stays covered until spring. So far the freeze-up date for Lake Vermilion near Tower on Dec. 1 and Lake Phalen in St. Paul on Dec. 2 have held-up.

The phenomena of lakes freezing over is interesting and quite spectacular. During the fall season, as the angle of the sun drops, lake water cools. It shrinks as it cools, becoming more dense. Once the temperature drops below 32 degrees, however, the water begins to swell, and this cooler water, having become less dense as it swells, naturally rises to the surface. Ice forms at 32 degrees.

On the first calm, freezing day or night after a particular pond or lake reaches 39 degrees in all parts, an ice cover will form. 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, reaching 39 degrees on the bottom.

Soon after freeze-up, ice sheets on lakes are heard cracking, thundering and rumbling. These loud, long roars and rolls don't necessarily mean that the ice is unsafe, but the eerie sounds remind us to respect the ice. Once liquid water turns to ice, this solid expands with warmth and contracts when cooled, causing the ominous booms.

Even with freezing night temperatures, ice is taking a much longer time to grow in thickness because of the above-normal high temperatures the last half of November into the second week of December.

Remember, it takes at least 4 inches of new solid ice in contact with stationary freshwater for safe walking, skating and ice fishing; 8 to 12 inches for a car or small truck. We all need to warn children, and those not familiar with ice safety, of the dangers ice covers present. Even neighborhood ponds. Falling through the ice is a huge shock to the body. Cold water saps body heat 25 times faster than air of the same temperature.

Jim Gilbert's observations have been part of the Minnesota Weatherguide Environment Calendars since 1977. He taught and worked as a naturalist for 50 years.