Lakes freeze from the top because water is heaviest at 4 °C
Liquid water reaches its maximum density at about 4 °C. Colder water floats, so a lake loses heat at the surface and ice forms there, insulating the life below.
Established2 min read
Hydrogen bonds hold water in a structure that is more open in ice than in the cold liquid. As you cool from room temperature the molecules pack closer, until near 4 °C the open structure starts to win and the density falls again. Ice, at 0 °C, is lighter still, so it stays on top.
If water behaved like a normal liquid, ice would sink, lakes would freeze from the bottom, and a winter would sterilise them. The anomaly is a peculiarity of one molecule and the condition of freshwater life in a cold climate.
Caveats
- Seawater, loaded with salt, has a lower freezing point and a shifted density curve. The top-freeze still happens, with a different profile.
- The familiar 4 °C is for fresh water at ordinary pressure.
Sources
- 1Eisenberg D., Kauzmann W.. The Structure and Properties of Water. Oxford University Press, 1969.