A naturalist thinks about these things as November ends:

We’re reminded that immersion heaters, available commercially, keep birdbaths ice-free. Birds need water year-round, and other wildlife such as deer, red squirrels and gray foxes will come for water, too.

And speaking of freezing, it’s time for many ponds and lakes to go hard. In general, Minnesota lakes freeze over from late November into early December.

During the fall season, as the angle of the sun drops, lake water cools. With that, the water also shrinks, becoming more dense. Once the temperature drops below 39 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.

The median average freeze-up date for Shagawa Lake at Ely has been Nov. 20 for the past 50 years. For Lake Waconia, the second largest of the Twin Cities area lakes, the median freeze-up date since 1940 is Nov. 30.

Last year, after a much warmer than normal November, I observed and recorded the freeze-up date for Lake Waconia as Dec. 9.


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.