Field Notes: Why Ice is so Cool

Feb 11, 2020

In this month’s Field Notes, Susan Knight looks at the thin ice situation this year, and discusses why ice is so cool.

What a crazy year for ice. As you may remember, we had a cold snap in early November, and some of our smaller lakes actually froze early.  But then we had a big snow, and the ice was insulated from the above-ice temperatures, and the ice did not freeze very deeply.  The larger lakes froze on schedule but again we got a bunch of snow and mild temperatures, and the big lakes still have minimal ice.

For almost 40 years, our field crew here at Trout Lake Station ventures onto seven lakes to do water quality and biological assessments.  This week they visited Crystal Lake, Sparkling Lake and Big Musky in Boulder Junction.  On all three lakes examined so far, there was 27cm, or 10.6 inches of ice.  This is about 80% normal ice thickness for this time of year, but the stronger, clear ice is only about half what it usually is and the white ice, which is basically frozen slush, is about double the normal measure.

So why is ice so important and why is it so cool?

Everyone knows that ice floats.  Ice, of course, is the solid state of water, and liquid water is the, well, the liquid state of water. And because it floats, this means the solid state of water, the ice, is less dense than the liquid state, the water. You may not know that this is a very weird, but wonderful physical property of water.  Almost every other substance we know does just the opposite. That is, the solid phase is almost always denser than the liquid phase. 

Imagine what would happen if ice were more dense than liquid water. As the temperatures drop in the fall, the water cools.  As air temperatures fall below freezing, the surface of the lake freezes.  If the ice were more dense than the liquid water, that ice would sink to the bottom of the lake.  Then the surface would freeze again, and that ice would sink.  If you carry that little thought experiment to its conclusion, you would realize that before long, the lake would freeze solid.  And everything, pretty much, in the lake would die.  All the fish, for sure.  Instead, the ice floats.  

Why does ice float?  It comes back to the chemistry of water, hydrogen bonds, and kinetic energy. Within a single water molecule, the hydrogen and oxygen are tightly bonded together. But there are weaker, hydrogen bonds between the oxygen atom of one water molecule with the hydrogen of another molecule.  When temperatures are above freezing, the kinetic energy allows those hydrogen bonds to break and reform, break and reform and the molecules slide all over the place; it’s fluid!  But once the temperature drops below the freezing point, those hydrogen bonds between molecules do not break and the molecules no longer slide around – they become a rigid crystal. And in this crystal form the water molecules are farther away from each other than they are when they are liquid.  And voila! Because of the space between the molecules, the crystalline or solid form of water is less dense than the liquid form.

So back to the lake.  Once the ice forms a lid on the lake, the water is somewhat protected from freezing by the floating ice.  Most winters, the cold will penetrate down through the floating ice and ice will continue to thicken. But the water under the ice remains liquid and compatible with life, all winter.  

In the spring, the ice melts, but it also rots. First, the spring sun has to melt the snow on top of the ice and then the sun’s energy penetrates the ice. The ice, which acts like glass in a greenhouse, starts warming the water under the ice, and the ice melts from the bottom up.  As the ice gets thinner, and the sun gets more intense, the clear ice transforms into long crystals, called candles.   Meltwater fills in the gaps between the candles. The candled ice absorbs more light than regular ice and the lake starts to look dark. At this point, the ice is not safe, and the lake will soon be open once again.

In the Northwoods, unsafe ice has big consequences. You may have heard that many lakes are not yet safe for snowmobilers this year.  There is not enough ice to create the ice sculpture in Eagle River this year.  Ice fisherman are cautiously heading onto area lakes, but fewer are taking their trucks. Maybe the ice will thicken enough to be considered safe enough for travel but maybe not.  

I hope you will respect the ice, or lack of it. And hope that those hydrogen bonds continue to snap those water molecules into that crystalline structure we call ice.