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

>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?

Yep, that does happen. The topmost layers of water get cooled first, but they don’t cool all the way to freezing point. They cool enough for their density to increase enough to sink to the bottom, but it hasn’t frozen yet. When that ‘layer’ sinks, the water just below it rises to the top, and the same thing happens again – it cools down enough to sink, but not until freezing point. This happens over and over until eventually, all of the water body reaches a temperature close to freezing point. Then, the uppermost layer of water becomes cold enough to go to the freezing point (naturally, since it’s directly in contact with the air to which it’s losing heat). So the uppermost layer freezes. Now for the slightly more confusing part – normally solids tend to be denser than the corresponding liquids, but in the case of water and a few other liquids, ice is actually *less dense* than water. As a result of this, the ice floats on the water, not sinking to the bottom. Ice is also a great insulator of heat, meaning that it doesn’t allow the transfer of heat very easily. So the water below the ice (which is close to freezing point), actually remains liquid without freezing, because the ice above doesn’t allow the remaining heat to flow out. So unless it gets *really* cold, normally you just have a layer of ice on top and just water beneath it. That’s why aquatic animals like fish can survive cold winters in lakes and ponds.

There’s a *bit* more nuance involved about *why* ice is less dense than water and about the exact temperatures which the water body is at, but that’s the basic explanation for why it happens the way it does.