The Lake Where You Live

All right, it’s a bit early to think about our lakes freezing over — but then, that’s about the best seasonal thing to look forward to since the leaves have fallen and laid the landscape bare.

It turns out there’s lot more to making lake ice than just low temperature. The first step is for ice crystals to begin forming in the cold water at the surface. That process is similar to the way raindrops form in the air.

Water vapor doesn’t create droplets spontaneously. It need particles of something to condense around, like free-floating microscopic specks of dust. And water molecules also need something to grab onto before ice crystals can form. For that purpose, lake water is loaded with solid particles too small for us to see, including living organisms like bacteria and algal cells.

Snow falling into the water can also accelerate crystal formation. After all, what are snowflakes but delicate structures of ice? In the right conditions they can seed the crystals that eventually consolidate into an ice sheet.

That sheet forms when cold water at or near the freezing point is exposed to cold, still conditions, which normally occur overnight. Then the sheet has to thicken to where it can support us as we engage in our favorite winter lake pastimes.

I’ve written before that the rate of ice thickening is related to the number of freezing degree days in a 24-hour period. By that measure, ice will thicken by one inch for every 15 freezing degree days, which for the sake of simplicity means a day with an average temperature of 17 degrees F (15 degrees below the freezing point of 32 degrees F).

That process slows down as the ice gets thicker, and a layer of snow and even a heavy frost on the surface will impede the rate of thickening. Another school of thought says this formula is too simplistic. 

An article on the Lake Ice website http://lakeice.squarespace.com describes the Swedish Ice Growth Prediction Method. This method takes into account that cooling occurs by convection (the movement of air via wind) and radiation. It says that the rate of growth in thin ice not covered by snow is the sum of:

• 0.05 millimeters per hour for every degree Celsius of negative air temperature (below the freezing point of zero degrees C).

• 0.02 millimeters per hour times the product of wind speed in meters per second and the negative air temperature.

• 0.7 millimeters per hour for completely clear skies, and nothing for full overcast.

The article doesn’t explain why clear versus cloudy sky matters so much; it simply says that in clear conditions there is more cooling by radiation. 

Additional factors come into play. In daytime, the angle of the sun makes a difference, and so does the sun’s presence or absence. Cold wind helps sweep heat from the ice. Snow is not the only insulator that retards freezing — slush on the surface or as a layer between surface ice and more ice below “stops growth on the bottom of the ice sheet until the slush layer is fully frozen,” according to the Lake Ice article. 

So there will be a lot to contemplate as we watch for the ice to take shape on our lakes.

Ted Rulseh is a writer, author and lake advocate who lives on Birch Lake in Oneida County. His new book, “Ripple Effects,” has been released by UW Press. You can learn about it by visiting my website at https://thelakeguy.net.