All the water of a lake is not at the same temperature, which implies the stratification of zones of different densities over the lake's depth. The densest zones (at 39.2° F / 4° C) migrate to the bottom while the less dense remain on the surface. In summer, the warmest water, which is also the least dense, rises to the surface and the coldest water rests at the bottom. On the other hand, in winter, when the temperature drops below 39.2° F (4° C), the stratification is reversed. The warmest (39.2° F / 4° C at most) and the densest water migrates to the bottom of the lake while the coldest water (at least at 32° F / 0° C) rises to the surface and freezes.