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Old 12-16-2017, 04:55 PM   #31
DickR
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Quote:
Originally Posted by PaugusBayFireFighter View Post
....
For anyone who wants to read more about 'the flip' this article provides good information on what's happening to the lake water on these last days of autumn.
http://onthelake.net/fishing/turnover.htm
I have to dispute a small part of that article, where it says:
"....The water that cools below that temperature, to 32 degrees, freezes and stays on the top, effectively capping the lake. It also stops further energy loss from the lake. Everything beneath the surface of the ice never gets any colder than 39 degrees Fahrenheit."

Energy loss from the lake does not stop with the formation of an ice layer on top. The idea that it does is actually refuted further down in the article:
"As winter approaches, the water that has now reached 39 degrees sinks to the bottom, allowing colder and less dense, buoyant water to remain at the surface to freeze. The ice thickens because it is not a good insulator; water in contact with the underside of the ice cools further and freezes, adding to the surface layer."

More ice cannot form without heat loss; the heat of fusion of water to ice is approximately 144 BTU/lb. As long as the ambient air is colder than the surface of the ice, there will be heat loss to the air by conduction, resulting in further ice formation for a while. However, while ice is not a good insulator, it does have limited thermal conductivity, as do all things. In terms of insulation value, the "R" value of the ice layer increases with thickness. With slow heat loss, both the water in contact with the lower surface of ice and that ice surface itself are in equilibrium, at 32 F. Thus there would be a temperature gradient, from 32 F at the lower surface down to whatever the upper surface exposed to the air is, except that the air temperature keeps changing up and down. Still, as ice thickens, so does the layer's insulation value, slowing down the rate of heat loss and retarding the rate of formation of more ice. Ultimately, the rate of further heat loss approaches the rate of heat gained by conduction from warmer water below, and the ice cannot get thicker without the air temperature getting even colder. With sufficiently cold air for a long enough time, a very shallow body of water can freeze entirely to the bottom. As spring approaches, the process reverses, and conduction of heat from below the ice melts from the bottom up, in addition to surface melting from warmer air.

As to that part about water below the ice never getting colder than 39 F, while the most dense water, at at about that 39 F, settles to the bottom, water between that and the bottom surface of the ice can be progressively cooler and thus less dense, and in the vicinity of that bottom surface it must decrease to precisely 32 F, the freezing point, where liquid and solid forms of water are in thermal equilibrium.

BTW, the area between Long Island and the end of Moultonborough Neck mostly froze over toward daybreak today and consolidated to a mostly unbroken sheet of ice wall to wall. The very thin coating of snow flurries confirmed that late today.
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