It is because water reaches its maximum density at 39 degrees F (4C).

Most things simply contract as they get colder. Water does too until it reaches 39 F/4 C, then it starts to expand a little. When it freezes, it expands a lot so ice floats.

Since water starts to expand below 39 F/4 C, cold surface water stops falling to the bottom of the lake in cold weather. In other words, it stops convecting. Instead it forms a coating of floating ice that insulates the remaining water. There is also a lot of heat available from the ground below and the heat from below can still convect up, keeping the water at a constant 39 F/4 C.

To freeze water, you also need to remove a great deal of heat, 144 BTUs/pound (80 calories/gm).

This is why the water in lakes and ponds is typically 39 F/4 C throughout the Winter.

If this were not the case, lakes and ponds would freeze solid in the winter and most fish would be frozen in.

Fergus S Smith

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