A good share of the lakes in southern Minnesota froze over before Thanksgiving but opened up again. In fact, many of the lakes have frozen over and reopened several times.

Up North, some small and moderate-sized lakes froze over the first 10 days of November and have stayed ice-covered up to now. Still, conditions widely vary.

Always remember that it takes at least 4 inches of new, solid ice in contact with stationary water for safe walking, skating and ice fishing.

Soon after freeze-up, ice on lakes is heard cracking, thundering and rumbling. These roars and rolls don't necessarily mean that the ice is unsafe, but the eerie sounds remind us to respect the ice. Once liquid water turns to solid ice, it expands with higher temperatures and contracts with lower temperatures, causing the ominous booms and moans.

A year ago, after a much warmer-than-normal late autumn, I recorded dozens of late freeze-ups for Minnesota lakes including: Lac qui Parle Lake near Milan on Dec. 8; White Bear Lake and Lake Bemidji on Dec. 10; both Upper and Lower Red lakes on Dec. 12; Lake Mille Lacs, Leech Lake, Lake Vermilion and Lake of the Woods on Dec. 13; and lakes Calhoun and Harriet in Minneapolis, and Minnetonka on Dec. 15.

Jim Gilbert was a naturalist for 50 years.