The mechanism by which lakes freeze from the top down

Question

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.

In: 23

Answer 1

Water is weird in that it expands by about 4% when it freezes. So ice floats. So what happens is the cold air cools the water in the lake and the cooler water is denser and sinks and this goes on until the lake is pretty much all near freezing. Then as the water at the top freezes, it gets a bit less dense and floats!