The ice cometh: Why won’t moving water freeze?

Yesterday strong winds blew across Birch Lake; as I stood on our frontage it smacked me straight in the face, the temperature in the single digits, the wind chill factor surely below zero. The near-shore water lay frozen; a few yards out, slabs of ice, like pieces of a crude jigsaw puzzle, rose and fell on waves. Other than that our lobe of the lake was open, choppy in the wind. Looking left toward the narrows leading to the other, more wind-protected lobe, I saw that it was frozen, covered in snow.

All it would take now, I thought, would be a cold, still evening, and the lake would freeze clear across. This morning confirmed my belief. Looking out from our hill as the morning brightened, I saw an unbroken sheet of white, a skin of ice dusted by the night’s light snow.

And I wondered: Why did it take a still night to make the lake freeze? Why wouldn’t it freeze despite the wind and waves? After all, 32 degrees is 32 degrees, and 8 degrees is 8 degrees. What difference should wind make in whether water gives up its requisite 80 degrees of heat energy per gram and freezes?

I searched all over the Internet without finding a satisfactory answer. One place said the motion of the water meant the molecular motion could not slow down enough to lower the temperature. I rejected this on the grounds it seems to confuse the macroscopic motion of the water with the sub-microscopic motion of molecules that correlates with temperature -- are we to believe that the mere fact of physical motion imparts heat? Another source said the mechanical motion of the water continuously fractures any ice crystals that form. I don’t have a technical argument against this; it just doesn’t seem plausible.

So I came up with a theory of my own, which is that the wave action creates a stirring effect, bringing up warmer, denser water from below and sustaining the surface temperature above freezing. Remember that, as earlier posts have said, water is the most dense at about 4 degrees C (39 degrees F). So the temperature gradient in a lake in winter runs from near zero degrees C at the surface (32 degrees F), gradually down to 4 degrees C. If the surface is heavily disturbed, as it was yesterday, then the agitation would be constantly bringing up warmer (or less cold) water from below, so that the surface water could not reach the freezing temperature. Then, once the wind calmed down and the water lay smooth, the surface water would readily give up its heat to the far-below-freezing air above and turn to ice.

Well, then, a critic might say, what keeps a river or creek flowing all winter, no matter how cold it gets, even if the river is rather slow-flowing and there is no wave action to speak of? Here we need to remember that a river or creek is not a closed system. In winter its source is likely, to a great extent, groundwater, which in northern latitudes is going to bubble out of the earth at temperatures in the low 40s F. Water at that temperature will enter continuously at numerous points along the stream’s route, so there is no real possibility of freezing. Of course many rivers do freeze on the surface, although the water beneath keeps flowing.

Now, as for Birch Lake, which has only a very small creek running in and out, and so is to a large extent a closed system in winter (absent snow melt and runoff) – what if the temperature remained at, say, 10 degrees indefinitely, and strong winds continued for weeks on end? Would the lake’s surface ultimately freeze?

That’s a good question. Does anyone have an answer? And would anyone care to confirm or shoot down my theories as to why rivers, streams and wind-blown lakes won’t freeze despite very cold air above them? I am all ears.