Why lakes freeze from the top down

Most liquids get denser all the way down to freezing. Water stops at about 4 °C, and that small quirk keeps lakes liquid underneath through the winter.

Most substances behave in a tidy way as they cool. Their molecules slow down and pack closer together, so the material gets denser, and the solid form is usually the densest of all. For most materials, a frozen chunk sinks in its own melt.

Water breaks that pattern twice. The well-known part is that ice floats: it is about 9 percent less dense than the water it froze from. The less known part happens before it freezes. Liquid water is at its densest at about 4 °C. Cool it further, toward 0 °C, and it starts to expand again.

The cause is the hydrogen bond, a weak pull between neighbouring water molecules. Each molecule can hold on to four neighbours, and when those bonds are in place they form an open, spacious network. In ice the network is complete and fixed, which is why ice is so light. In very cold liquid water, more and more of it forms. Warm the water a little and some of the bonds break, letting the molecules crowd closer together.

What happens in a lake

In autumn, the surface of a lake loses heat to the cold air. That cooler water is heavier, so it sinks and warmer water comes up to replace it. The stirring continues until the whole lake is close to 4 °C. Then the pattern flips. Surface water that cools below 4 °C is now lighter than the water beneath it, so it stays on top, keeps cooling, and finally freezes.

The ice works as a lid. It floats, keeps the wind off the water and slows the loss of heat from below. In deep lakes, the water at the bottom stays at about 4 °C all year. Even Lake Baikal in Siberia, with its long, hard winters, freezes to a depth of only about a metre.

Now picture water behaving like most liquids. The coldest water would sink, cooling would keep stirring the lake from top to bottom, and it could freeze from the bed upward until nothing living in it survived. Fish spending the winter in the dark, still water under the ice owe their lives to a small kink in the way one molecule packs.