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Old 03-07-2021, 09:16 PM   #11
Join Date: Mar 2014
Posts: 1,476
Originally Posted by Justin View Post
Snow is a good insulator, and from what I was told once upon a time which seemed to make sense to me is that the water below the ice is above freezing temperature, and as that “heat” vapor escapes upward through the ice it gets trapped under the snow, causing the snow in contact with the ice to melt creating slush. Or something similar to that affect...
Pretty close, I think.

The ice and the water directly underneath, are at the same temperature. Water turns to ice at 0C at standard pressure, but there is a large amount of energy that needs to be removed to change phase.

In terms of heat flow - the liquid water has a large thermal mass (water has a high specific heat) so when the air is colder than 0C (32F), there is heat flow from the water to the air (temperature gradient must exist for heat flow).

The part you both are correct about is the snow trapping heat, or in other words creating a temperature gradient between the air and the top of the ice. Since ice and water can exist at 0C, there is a possibility that heat is being used for a phase change, rather than a change in temperature at the snow/ice interface. And likely the snow is melting - because it is ice crystals, but a much less dense structure than the ice, so the amount of solid converted to liquid is probably pretty minimal. The air pockets in the snow are what causes it to be a good insulator, but it's also not as dense as ice, so that in turn works to make it easier to melt because it has less thermal mass.

The part that I don't think is quite right is the "gas" part. I don't think anything is moving in the system except energy from the hot reservoir (the unfrozen lake - water is most dense at 4C so a gradient exists in the lake itself) to the cold reservoir (the air if it is less than 0C). Second law of thermodynamics. The magic is the just phase change process which happens at constant temperature but still exchanges energy and the fact that snow has a much higher thermal resistance and low thermal mass when compared to solid ice and water.
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