Science Puzzle
Why Does a Lake Freeze from the Top Down?
In a cold winter, a lake freezes over from the top down. The ice gets thicker week by week, but the water underneath stays liquid and fish survive all winter near the bottom, where the water sits at about 4 °C.
Why does the coldest water stay at the top, where it freezes, instead of sinking to the bottom?
The Answer
Water is strange: it's at its densest at about 4 °C. Water colder than that is lighter, so it floats on top, and that's where the ice forms.
Most liquids get steadily denser as they cool. Water does too, but only down to about 4 °C. In autumn the surface of a lake cools, becomes denser and sinks, and warmer water rises to take its place. This churning carries on until the whole lake is about 4 °C. Then something odd happens. As the surface cools below 4 °C, the water expands slightly and becomes less dense, so it stays on top instead of sinking. The surface keeps cooling until it reaches 0 °C and freezes.
Ice is lighter again, about 9% less dense than water, so it floats. And the layer of ice, often with snow on top, acts like a blanket, slowing the loss of heat from the water beneath. The bottom of the lake stays at about 4 °C right through the winter.
If water behaved like most liquids, the coldest water would sink, lakes would freeze from the bottom up and many would freeze solid. The reason for water's odd behaviour is the way its molecules link together as it gets near freezing, into an open, spacious arrangement that takes up more room.
The principle: The density anomaly of water. Water is densest at about 4 °C and expands as it cools further, so near-freezing water and ice float, and lakes freeze from the top down while the depths stay liquid.