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Premise: I live in a place where it is unquestionably wintertime in every aspect of the phrase. I was driving down the street and I drove by a lake. There was a recent and sudden temperature drop (from above freezing but still quite cold to well below freezing) overnight. The water had not yet frozen over.

It occurred to me while I was driving that the process of beginning the freezing process of the water at the interface of the water and the ambient air seemed to be counterintuitive. While I half (and admittedly naively) expected the ice to begin forming at the outside edges of the lake and continue to freeze inwards in a uniform fashion, the ice seemed to be beginning on one side of the lake only. I drove back by later on during the day, and the lack was nearly frozen over sans a bit of the side that was originally left unfrozen during my first observation.

The details are rather unimportant. I’m well aware of the fact that this was hardly a controlled environment, so the question remains to be how can we best describe or predict the nature of and changes within the interfaces of a substance and its surroundings during the a phase change of the substance? In particular, what’s prompting this question is the fact that there seemed to be a ‘preferred’ origin of the systematic spread of freezing. I’m curious as to why that might be and whether it is predictable. It could well just be dependent on the particular properties of the lake/general substance, but if so I’m curious as to what quantities influence these ready and obviously more rapid transitions.

My first intuition was that perhaps some parts of the lake are deeper and so there is a greater rate of heat transfer between the cold air/water interface and the volume of water below, resulting in that surface freezing more slowly in the long term. But, that doesn’t seem consistent with the fact that one of the edges of the lake remained entirely unfrozen until the end- most likely that edge was not a vertical drop and so would have fallen victim to earlier freezing as well. Presuming that the lake is a still body of water - are there any thoughts?

Somewhat related question here.

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  • $\begingroup$ If the water is slightly supercooled ice will preferentially spread from existing ice acting as nucleation centres $\endgroup$ – user56903 Sep 30 '15 at 13:29
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It was probably just the wind

Lakes freeze at their surface because between 0 and 4 degrees C water decreases in density with temperature. That means that the warmer water will sink and the cold water will rise to the surface. This convection will be much faster than any other mode of heat transfer. This means that in the cooling process the water will be well mixed until the surface temperature reaches 4 degrees and then from there onward, the heat transfer will slow.

Now if the air above the lake was constrained to a constant temperature below freezing, it would indeed freeze from the shallower regions first spreading across to the deeper ones.

However, if there is a wind across the lake, where the incoming air was at a constant temperature, then as the air moves across the lake cooling it, the lake also heats the air. This means that the air would be warmer at the far end of the lake, so the heat transfer would be slower. This will cause the upwind side of the lake to freeze before the downwind side.

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