If I had to guess, it’s because the temperature above the water is constantly colder than the water itself – but that is what I actually don’t understand.
If the surface temperate is extremely colder than the temperature of the water itself, wouldn’t the cold “sink” that much faster, making the warm water “rise” at an equivalent rate, and thus forcing more of the cold water to the bottom?
Maybe a better post for stupid questions, but I’m very curious what causes this – perhaps this just applies to air, and not water? And if that’s the case, then also why?
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The reason is that ice is fairly unique among liquids, in that it expands when frozen. For this reason, ice is lighter than water. So even if the water near the bottom of the were freezing first, it would rise to the top once frozen.
As it is, the large body of water retains heat, as does the land beneath the water, so the surface that is in contact with the cold air freezes first. Ice being lighter than water, it stays on the top. If it didn’t, life on earth might not be possible. There was a short story about this written in the 1970s. *The Catalyst, by G.R. Yohe*
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