Freezing is a process controlled in large part by a unique characteristic of water. Most materials — mercury in a thermometer, for example — shrink as they cool. Water also shrinks as it cools from warm-weather temperatures as low as 39 degrees. During the fall, the cool water sinks as it mixes with rising warmer waters to reach a uniform lake temperature of 39 degrees.

But then, something unusual happens when water temperatures drop below 39 — the water begins to swell. For this reason, water cooler than 39 degrees is lighter than water at 39 degrees. The cooler water floats on the surfaces of lakes and ponds. Then, on the first freezing day or night, ice will form on the surface. Water touching the ice will remain 32 degrees all winter long. Temperatures are 35 to 38 degrees a few feet below the ice and 39 degrees at the very bottom.

Imagine if water cooler than 39 degrees continued to shrink. As it becomes more and more dense, it would sink to the bottom of lakes, ponds and streams, rather than floating at the top. Just think; our bodies of water would have permanent ice covered by a layer of water all summer long.

Last year the freeze-up dates of some Minnesota lakes included: Caribou Lake near Lutsen in Cook County froze on Nov. 23, Lake of the Woods on Nov. 26, Lake Mille Lacs and White Bear Lake on Dec. 10 and Lake Minnetonka on Dec. 22. The freeze-up date is the first day when at least 90 percent of the lake is frozen over and stays frozen over.



Jim Gilbert’s Nature Notes are heard on WCCO Radio at 7:15 a.m. Sundays. His observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota. He taught and worked as a naturalist for 50 years.