Why don't lakes freeze solid?

  • #1

Main Question or Discussion Point

Why is there liquid water under the floating ice layer?

I know the answer has to do with ice being less dense than water and its high specific heat(H bonds), but I cannot understand how this layer regulates the temperature of the liquid water that exists under it.
 

Answers and Replies

  • #2
Borek
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It doesn't regulate temperature per se. It isolates and slows down freezing of the water, but in general if you wait long enough and the temperature is low enough, every lake will freeze, fro top to bottom.

Sure, there can be source of heat below, and if the ice layer is thick enough freezing point goes down, so in some particular places water will not freeze, bute these are special cases.
 
  • #3
Thanks for your answer.

So, If I understood it correctly, water slows down freezing because of its high specific heat, H fond release energy when they form(iff temperature drops) and they absorb energy when they break(iff temperature increases), right?

I still have problems understanding it considering the fact that these H bonds last only 10^-11 seconds, which was my other question[1], it would be great if you could take a look at it too. :)

[1] - https://www.physicsforums.com/showthread.php?t=234950
 
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  • #4
mgb_phys
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Another important factor is that water has it's lowest density at 4deg C
so as water cools it falls to the bottom of the lake, but when the water drops below 4C it will rise toward the surface and so lakes freeze from the top down.
Because the warmer water is denser there is no convection current and the water at the bottom can only cool through conduction which isn't very effective.

This is an important feature for life - otherwise many lakes/seas would freeze continually.
 
  • #5
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the layer of ice formed above acts as an insulator to minimise further freezing downwards.
 
  • #6
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You are all wrong!
Water's volume decreases upto 4 degree C
And below that it increases all of a sudden(Anomalous expansion of water)
This stops the total freezing
 
  • #7
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You are all wrong!
Water's volume decreases upto 4 degree C
And below that it increases all of a sudden(Anomalous expansion of water)
This stops the total freezing
Are you saying that a lake cannot freeze solid? What about a pond? A skating rink? An ice cube?
 
  • #8
DaveC426913
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You are all wrong!
Water's volume decreases upto 4 degree C
And below that it increases all of a sudden(Anomalous expansion of water)
This stops the total freezing
Correction:

Liquid water has its volume at a minimum at 4C, true. Below that, it begins expanding - until it reaches 0C. Then it freezes - and then its volume begins to decrease again.

But ... this is not what stops total freezing. Nothing stops it. But the effects of floating ice do slow the process greatly.
 
  • #9
Chronos
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Ice floats, as already noted. More importantly, it inhibits convection of heat to the atmosphere, as also noted. As water underneath the 'skin' of ice cools, it sinks, mixing with warmer subsurface waters. This inhibits the freezing process by roughly the square of the surface ice thickness.
 
  • #10
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Are you saying that a lake cannot freeze solid? What about a pond? A skating rink? An ice cube?
You got it right. Small lakes (think depth less than frost line) do freeze solid.
 
  • #11
DaveC426913
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You got it right. Small lakes (think depth less than frost line) do freeze solid.
Yes, it's Drik Paul that needs correcting.
 
  • #12
Borek
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Chronos said:
This inhibits the freezing process by roughly the square of the surface ice thickness.
Perhaps my English fails me, but I don't understand what you wrote. As far as I understand word "inhibit" it means in this context "to stop freezing". You can't stop freezing by some amount? Do you mean it slows down the freezing? Square suggests you simply refer to the speed at which heat is transferred through the layer of ice of given thickness?
 
  • #13
tiny-tim
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(of course, I am inhibited …)

Perhaps my English fails me, but I don't understand what you wrote. As far as I understand word "inhibit" it means in this context "to stop freezing".
According to my Pocket Oxford Dictionary, "inhibit" can also mean "hinder" or "restrain" … in my experience, that is the usual meaning. :smile:
 

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