Nature Notes – Lakes have frozen over, or will soon  Hilks Lake
and Goose Lake near the western edge of Waconia, and Swede Lake
near Watertown all froze over Nov. 24 and have kept their ice
covers. Some other area lakes froze over in late November and
opened up again a few days later, and some are freezing-up for
the first time this week. Freezing and the formation of ice
covers on lakes or other waters is a process controlled in large
part by a unique characteristic of water. Most materials, for
example mercury in a thermometer, shrink as they cool. Water
also shrinks as it cools from summer temperatures to 39 degrees
F. As the cool water sinks it mixes with the rising warmer water
until the lake becomes a uniform 39 degrees. However, as water
drops below 39 degrees it begins to swell. For this reason,
water cooler than 39 degrees is lighter than water at 39 degrees
and will float on the surface. Ice forms at 32 degrees, so on
the first calm, freezing day or night after the lakes and ponds
reach 39 degrees t