Lake Freeze-up records for area lakes

Although it's not quite over yet, I think it's safe to say that 2008 temperatures were cooler than average.  For the lakes, it started with a late ice out in the spring, and it looks like it will continue with earlier than average freeze-ups this fall as well.  Today I will talk about freeze-up records for Detroit Lake and Pelican Lake and how lakes freeze over.

First of all, Little Detroit Lake and some of the smaller lakes in the area froze over this week.  Larger lakes such as Big Detroit, have some ice around the edges.  Since freeze-up does not occur in one day like ice-out usually does, it 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 freeze-up and ice-out dates available on the Pelican River Watershed District website: http://www.prwd.org/?D=4. 

Since 1910, the earliest freeze-up for Detroit Lake was October 25 in 1919.  The latest freeze-up was December 13 in 2004.  The average freeze-up has been getting later as our climate warms.  The average freeze-up date in the 1990s was November 20, while the average date for the 2000s is November 28. 

Pelican Lake Association keeps records of their ice-in and freeze-up dates as well: http://www.pelicanlakemn.org/information.htm. The average freeze-up date for Pelican Lake since 2000 is December 1.  The latest freeze-up was December 16 in 2001 and the earliest was November 22 in 2002.

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.

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.  The cold wind this past week no doubt is helping the freezing process along.

Enjoy the lakes!

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