Why does all the water in a lake have to be cooled to 4 degrees Celsius before water can be cooled below 4 degrees Celsius?

Answer 1

Water in a lake is usually cooled by the cold air above its surface rather than from the ground underneath it. As the top layer of water cools its density increases and so it sinks to the bottom, bringing other water up to the surface. This water is then cooled. This carries on until the temperature reaches 4 deg C when the density reaches a maximum.

Further cooling lowers the temperature of the top layer but now its density is less than that of the water below so the top layer stays on top until it freezes (if it is cold enough). That is how you can get lakes with a frozen surface but liquid water below which can sustain aquatic life and also destroy human or other life forms that fall through the ice!