Answer: The temperature of the lake gets colder over the course of the winter, until (if the lake is far enough to the north) the surface of the lake freezes over. The decreasing temperature of the lake is caused by the fact that, in winter, the angle of the sun’s rays are less direct (acute angles), thus less solar radiation is reaching the lake.

The farther north you are from the equator, the more pronounced is this effect, due to a more pronounced decline in the angle of the sun in relationship to the surface of the lake.