During the fall season, as the angle of the sun drops, lake water begins to cool. And as it cools it shrinks, becoming more dense. Once the air temperature drops below 39 degrees, however, the water begins to swell, and this cooler water, having become less dense, 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 water remains above freezing, reaching 39 degrees at the very bottom.

Remember, it takes at least 4 inches of solid ice in contact with stationary freshwater for safe skating and ice fishing. A snowmobile requires 6 inches of ice for safe riding. Eight to 12 inches are needed for a car or small truck, and 12 to 15 inches for a medium-size pickup.

You don't want to fall through the ice as cold water saps body heat 25 times faster than air of the same temperature. In 32 degree water, you have about 15 minutes before losing consciousness.

Freeze-up is the first day when at least 90 percent of a lake is frozen over and stays frozen over. Freeze-up was two- to three weeks ahead of normal for many Minnesota lakes this year, especially for small- and medium-size lakes. Due to heavy winds, many large lakes are still wide open.

Here are a few freeze-up dates from 2013: Sawbill Lake in Cook County, Nov. 12. Lake Hendricks in Lincoln County, Nov. 22. Woman Lake at Longville, Nov. 24. Lake Minnetonka, Nov. 29. Lake Mille Lacs, Dec. 4. Lake Calhoun and Lake Harriet, December 6.