Saturday, January 23, 2016

The Chemistry And Physics Of Lake Ice

Skating on Thin IceLast night, the floodlights were on at my favorite skating lake. Several children wearing plastic skates and shiny helmets were gliding on the ice, shepherded by young parents. A father pulled a Nordic-looking sled with upturned runners, his bundled-up cargo insisting, “More!” each time he stopped. They were enjoying one of winter’s greatest gifts: the smooth, frozen surfaces of our northern lakes and ponds.

The gift is ephemeral. Some winters, our skates never leave the basement. Other years, the snow holds off and there’s black ice before Christmas. We skate as much as we can, knowing our days of clear ice are numbered. As winter progresses, rain may turn the surface to water — but the temperature plummets again and the resurfaced plane draws us back.

The ice was magic to me as a child. Lying on my padded belly, I stared down into it and wondered how all those little white bubbles got in there. I tested how far I could slide — then turned and lay watching the clouds until the cold began to seep through my wool coat. I was fascinated by what was caught in the ice: a leaf, a weed, a small fish.

But ice is not magic. It’s chemistry and physics. Ice formation begins in late summer, when the upper layer of the lake begins cooling. In this process, called turnover, the cooling water becomes denser and heavier, sinking toward the bottom and forcing warmer, lighter water to the surface. The cooling and sinking process continues until, one day in November or December, the whole lake reaches 39.2 degrees, the point at which water reaches its maximum density.

After the lake reaches this tipping point, as its surface water is exposed to freezing air, the water molecules expand and become less dense. This allows them to float above the 39 degree water. When they cool to 32 degrees, small discs form. If the wind stays down, these discs branch out into tree-like shapes that expand across the surface of the water until the whole cove or pond is covered by a thin skim.

Once the lake has frozen over, the progress of ice formation slows. As long as the air stays colder than the ice, the ice grows thicker; however, each new centimeter of ice requires more cooling than the last one to form, because the water below is better insulated. Eventually, the ice is thick enough to act as a complete insulator. Without further energy loss from the lake, the water beneath remains a chilly, but liquid, 39 degrees.

Incidentally, there’s beautiful language involved with the science of limnology, or freshwater lakes. Our northern lakes are monomictic, meaning they mix, or turn over, once a year. The layers have names, too: the surface is the epilimnion; the bottom layer is the hypolimnion, and the water in between is the metalimnion.

Black ice – hard, strong, and smooth – is the skater’s joy. It forms slowly under calm conditions. Individual crystals grow downward and are closely packed, which keeps out impurities (like air bubbles). It’s called black because it is transparent, and the water below absorbs most or all of the light. On black ice, every stroke of a skater’s blade leaves a tracing of white.

But large lakes rarely freeze black and smooth. A slight wind or wave motion breaks the skim into what is sometimes called frazil ice, a mush of needles that collect into round pans surrounded by water. Ice formed this way, or during a snowstorm, is white and rough.

The noise made by a frozen lake seems ominous to many, but it never sounded so to me. Growing up on a lake, I enjoyed the booming sounds the ice made at night as it contracted, and the groaning sounds of the ice expanding during the day.

Unfortunately, full freezes are becoming less frequent. While Lake Champlain froze over in 2014 and 2015, it had not frozen two years in a row for a decade before that. This year, the lakes got a late start. According to the Environmental Protection Agency, freeze dates have shifted later and thaw dates have shifted earlier, in a trend that began as early as the mid-19th century. The rate of warming in the northeastern United States has increased in the last four decades.

Science aside, though, I believe a little magic is a good thing. Especially if it can be captured on skates, or even on a padded belly.

Laurie Morrissey is a writer in Hopkinton, New Hampshire. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: wellborn@nhcf.org.

Photo by Kevin Boyle: Ice skater on the black ice of Lake Champlain in early January. 

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9 Responses

  1. Bruce says:

    It is magic. When I was a teen living next to Oneida Lake in the 50’s, my buddy Andy and I couldn’t wait for a clean freeze up with little snow. You see, Andy and his dad built and flew RC model airplanes which were huge in those days. We would take a couple of the big engines, fasten them and a gas tank to boards and use them to pull us around the lake.

    As spring approached, and the breakup started piling up on shoreline trees, we would start preparing our fishing gear.

  2. Curt Austin says:

    I will add that water is a peculiar substance, having a maximum density at 39F. Most liquids freeze to a denser phase, so its “ice” sinks. Lakes might freeze solid otherwise, which would greatly disrupt the sort of life they could sustain. I recall reading something that suggested that “life as we know it” would not have evolved if not for this peculiarity of water.

    Water is peculiar in other ways, too. Most molecules of its size (e.g., ethane) are gases at room temperature. It’s an odd molecule, having hydrogen atoms attached at an angle; this is what leads to its special properties, including the electromagnetic property exploited by microwave ovens.

    You can get real metaphysical about water. Physicists believe it is possible we inhabit a particular universe out of many others that have different fundamental constants. Our universe features constants that allow the formation of a very peculiar substance that is very important to us: water. These constants give us many other good things, too, like coffee and warm fires.

    I saw the first fishing shack on Loon Lake only yesterday. Very late.

    • S. Sampson says:

      Curt, you say:

      “Physicists believe it is possible we inhabit a particular universe out of many others that have different fundamental constants.”

      Please point me to some of these physicists and their writings…. I would think such physicists would be ridiculed by the mainstream physicist academic community…

      • Curt Austin says:

        Just google for “multiverse”. It’s actually a mainstream idea, one of the many wacky consequences of quantum theory. Somehow, the mass of the Higgs boson determines whether we have a multiverse or not. Physicists are anxiously waiting for more results from the big collider in Europe; the early results favored the multiverse theory. But who knows; they’re still mystified about dark matter and energy.

        This is coming from someone firmly rooted in reality. Reality, however, is not what it seems from everyday observation. Einstein delivered this message first, over 100 years ago. He also gave birth to Quantum Theory but it evolved into something so strange that even Einstein couldn’t accept it.

  3. Cranberry Bill says:

    Several hundred years from now Einstein may well be remembered as we now remember Galileo and Newton, as a foundation builder upon which all of modern physics and astronomy rest. What I read about the Big Bang, the ever expanding universe, and dark matter seems to be in a state of constant change. The idea of parallel universes has serious proponents and skeptics. See their names in a Wikipedia article – “Multiverse”. I don’t even know if there is a single mainstream physicist community now. Thence I shall just crawl back into my black hole and imagine skating on Planet Nine.

    • Curt Austin says:

      Well, I believe frozen methane is naturally slippery like water ice (the old notion that the 39F density maximum thing is involved – pressure-induced melting – has been deprecated). But let’s keep Planet Nine forever wild, shall we?

      Apologies, Laurie, for taking the subject so far afield. I need to get out more. Perhaps I’ll take my fat bike out on the ice today.

  4. Ed says:

    This is an interesting piece, but I think it confuses a few bits:
    -most of our lakes are dimictic, turning over in both spring and fall
    -the metalimnion is specific to the thermocline and is often relatively thin; the hypolimnion is the bottom layer, but generally will make up the majority of the depth of a decent sized, stratified water body
    -while ice does become slightly more dense as temperature drops (i.e. it contracts), the noises it makes are dominated by the formation of pressure cracks as the ice itself “grows” under cold conditions

  5. Cranberry Bill says:

    I watch Lake Erie from our “crib-cam” located on the water intake for our city. The lake was somewhat frozen before clearing earlier this year, but now looks to have the frazil ice you describe. And then it occurred to me, practically 3 weeks after reading your article, and Curt’s reply, that if water did not expand under 39deg, lakes would freeze from the bottom up, and ice skates probably would never have been invented. Ice fishing would consist of netting the cold blooded creatures from my canoe, and no ice skater would ever fall through the ice.

    • Cranberry Bill says:

      And then, about a month later, I come to learn that some, or many, lake bottoms would not even melt in the summer – nowhere to hook my canoe anchor. Laurie Morrissey’s article has been freeze-thawing my brain cell for weeks.