Farm Lake just outside the BWCA in Ely MN icing over for the winter

Winter is back!  It's that time of the year when the temperature dips down in the low teen digits at night and ice is being made.  

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. 

The picture above shows the main dock outside the lodge at Moose Track Adventures an Ely MN Resort just 7 miles outside of town.  We pull the pieces of the 'T' dock out of the water as much as we can so that come ice out in April the huge sheets of ice blowing around don't mangle it to pieces.  With the old crib dock that this replaced, every April we had to set aside a week to 'rebuild' what the ice sheets/chunks damaged.