Why does the bottom of a lake not freeze?

Water has a remarkable characteristic.  

It has its highest density at 4 °C.  

This is when it is heaviest.  

Not when it freezes, which is at 0 °C. 


When the surface of a lake cools down to 4 °C 

it sinks to the bottom.  

If the air temperature cools the lake further,  

the colder surface water becomes lighter.  

It lies as a layer on top of the four-degree water. 

This is why lakes freeze from the surface and then downwards. 

© 2020 av Johan Hammar