On the first calm, freezing day or night after the lakes and ponds reach 39 degrees throughout, ice covers can form. Already reports have come in of this happening the last week of October.

The freezing and final formation of ice covers on our ponds and lakes are processes controlled by the unique characteristic of water. Most materials, for example mercury in a thermometer, shrink as they cool. Water also shrinks as it cools to 39 degrees. At 39 degrees, however, water goes into reverse and swells as it cools below that temperature. For this reason, water colder than 39 degrees is lighter than water at 39 degrees, and so will float on the surface. Ice then forms at 32 degrees.

If water cooler than 39 degrees continued to shrink and to become more dense and sink, ice would form from the bottom of the lake upward.

Listed below are freeze-up dates for eight of our Minnesota lakes last fall. Freeze-up is defined as the first day at least 90% of a lake is frozen over, and stays frozen over until the next spring:

• Sibley Lake (near Pequot Lakes), Nov. 9

• Lake Hendricks (Lincoln County), Nov. 10

• Budd Lake (Fairmont), Nov. 13

• Lake of the Woods, Nov. 18

• Leech Lake, Nov. 20

• Lake Mille Lacs, Nov. 25

• Green Lake (at Spicer), Nov. 27

• Lake Minnetonka, Nov. 28

Yes, sheets of ice may be forming on ponds and lakes, but remember it takes at least 4 inches of new, solid ice in contact with water to safely walk, skate or ice fish. A snowmobile takes 6 inches of ice, 8 to 12 inches are needed for a car, and 12 to 15 inches for a 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.

If you live near a Minnesota lake and observe the freeze-up, as defined above, report your observation on the State Climatology Office’s Facebook page, and it will become part of the permanent state records.

Jim Gilbert’s observations have been part of the Minnesota Weatherguide Environment Calendars since 1977.