The mechanism by which lakes freeze from the top down


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

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!

The ELI5 version is that the whole lake has to get to 0C first before any ice can form at all (with the top water getting cooler, falling down, warmer water rising, cooling, falling down, etc), then once the entire lake is sufficiently cold, ice crystals will start to form and rise to the top as they are less dense (and the cold air/wind at the top help to wick away heat).

For a really cool book about this topic that explains it in a ELI18-ish way, check out [Lakes: Their Birth, Life, and Death](

Edit: and the reason the whole lake doesn’t freeze is that the top layer of ice provides an insulating effect

Warmer substances *often* rise while cooler substances fall, but not *always*. It depends on if something expands or contracts with heat. Most things expand.

Even if water expanded with heat, since the cold air is at the top the ice would still form at the top. It would form a sort of snow or sleet and sink down to the bottom.

However, water is odd. Water is at its smallest between 2 and 3 degrees Celsius. When the air temperature drops, at first we see the water flowing up and down as the surface gets cold and falls to the bottom. Then, once everything has reached 2 or 3 degrees c, the water at the top continues to cool down, but now it expands so it continues to float at the top *even as it is colder*. Once it freezes it expands *even more* and now it is very much staying on top.

>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.

It’s not that warmer things rise, it’s that warmer things tend to “spread out” as the molecules get more giggly, and that makes them less dense. Denser objects fall. Less dense rise. That’s why a helium balloon rises without being cooler. Helium is less dense than nitrogen and oxygen, which push the helium upward as they fall below it.

Ok so what about ice? Water has a particularly weird, rare property that it actually is less dense when it’s frozen. This because it’s crystalline structure is less more widely packed than when it’s liquid. This is why ice cubes float. Also why, when it freezes on top (the area exposed to cold air) the ice doesn’t sink to the bottom. Luckily.

Because if it did sink, then more would freeze and sink. And more. And eventually it would all be frozen and never melt. And earth would be a giant ice ball. With no life.

Read a book called Cat’s Cradle. It’s about people who invent something called Ice 9 that basically works this way.