The mechanism by which lakes freeze from the top down

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It is my understanding that within a given medium warmer substances tend to rise and cooler substances fall, i.e. warmer air above cooler.

Assuming the same is true with water then in a lake the cooler water should fall to the bottom while warmer water rises to the top, correct? If so, I would expect that lakes would freeze from the bottom up. Can you please explain why this isn’t the case?

Thanks for your explanation.

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26 Answers

Anonymous 0 Comments

As water freezes, it expands. As it expands it takes up more volume with the same mass, so ice floats. So even if ice formed at the bottom it would float and collect at the top then freeze solid.

The top of the water has the most opportunities to shed heat compared to further down, so it’s the most likely place to freeze as well. But for the most part it’s really just that ice floats.

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