When and how do lakes freeze over?

Now that it's too cold to swim in our lakes, we can look forward to the next great lake season – winter sports!  Bring on the cold and snow so we can ski, skate, snowshoe, ice fish and snowmobile.

This week, I noticed that some small ponds have a thin layer of ice on them.  This prompted me to think about when and how our lakes freeze over in this area. 

As you all know, water freezes at 32 degrees Fahrenheit.  That doesn't mean, however, when the air temp reaches 32 the lakes freeze.  Water is a great insulator and good at holding heat, which is why the lake temperature doesn't fluctuate much day to day like the air does.  Therefore, below freezing temperatures are needed for a week or more to form ice on a large lake.

As I have mentioned before, water is a unique substance in that the solid form (ice) is lighter than the liquid form (water).  For most substances, the solid form is heavier.  Our lives would be much different if ice sank instead of floated.  If ice sank, lakes would freeze from the bottom up and the fish and other aquatic creatures wouldn't survive the winter!

Since water is good at holding heat, the more water there is, the more heat it will hold.  This is why large deep lakes take longer freeze and melt than small shallow lakes.

Water freezes from the perimeter of the lake to the center.  It happens this way because the water is shallower at the lake's edge so it cools off faster.  Water is most dense at 39 degrees Fahrenheit, so when it gets colder than that, the cold, lighter water floats on top of the lake.  This top layer of water interfaces with the cold air, which cools the top of the lake even further until it freezes.  Windy days cool the lake surface off faster because the cold air moving over the water cools the lake faster.

Since ice-in does not occur in one day like ice-out usually does, it is hard to keep accurate records.  The ice can form around the edge of the lake, and then a warm sunny day can come along and melt it again.  There are historical records for Detroit Lake of ice-in and ice-out dates available on the Pelican River Watershed District website: http://www.prwd.org/?D=4.  For the last five years, the ice has formed on Detroit Lake sometime between November 25 and December 13.  The date depends on the weather patterns and how cold and windy our fall is.

I took the temperature of the water at the surface of Detroit Lake Thursday morning (Nov. 8) and it was 40 degrees Fahrenheit, so it still has a ways to go before freezing.  St Clair Lake had a thin film of ice on it Thursday morning however, so small shallow lakes may have already begun to freeze.

Keep an eye on the lakes in the next couple weeks and see how and when the ice forms.  I'll keep you updated right here in my column.

Until next week, enjoy the lakes.  Enjoy looking at them anyway, since the only way you can actually use them right now is for fishing or boating!

Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465, lakes.rmbel@eot.com.