Lakes freeze from the surface downward to the lake bottom. Ice floats because it is less dense than liquid water.
The density of liquid water depends on the water temperature. The density of water is highest at a temperature of about 40 degrees Fahrenheit. So, why is that important?
As winter sets in, lakes lose energy to the atmosphere as the water near the surface cools. The density of the water near the surface increases and this surface water sinks because it is more dense than the warmer water below.
Warmer water under the surface rises to replace this sinking water because of its smaller density. When all the lake water reaches a temperature of 40 degrees, further cooling of the surface water temperature makes it colder than 40 degrees and, because it is now less dense than the water around it, it will stay on the surface of the lake and continue to cool.
Once this surface water decreases to 32 degrees, the water freezes. The freezing then spreads downward into the lake and the ice thickens. This is why you find liquid water below the ice, unless the body of water is very shallow.
The water temperature below the ice is about 40 degrees. Fortunately fish can live in temperatures this cold.
Freezing also first occurs along the shoreline, where the water is shallow. Before ice can form on the surface, the entire water column must first reach a temperature of 40 degrees, and this is likely to first occur along the shoreline.
Lake Mendota typically freezes over on Dec. 20; while Lake Monona’s typical freeze date is around Dec. 15. This year, Lake Monona froze over on Dec. 10. With the cold weather southern Wisconsin has been experiencing, it is likely that Lake Mendota will also freeze over a bit earlier.