Physics Q & A - How does a lake freeze?
Lam Chun-hung (Translation by Wong Ka-lei)

The cooling of a lake
Fig. 1   The cooling of a lake
The freezing surface of a lake
Fig. 2   The freezing surface of a lake

Why does freezing always start from the lake surface? How does this phenomenon affect the fish?

Water density changes with the water temperature. The water density is highest at 3.98óJ, therefore water molecules at 3.98óJ are more likely to sink than the ones at other temperatures.

Let us consider the situation with a water temperature higher than 3.98óJ. In Fig.1, the lake's surface temperature is 20óJ, the air temperature is 5óJ lower than the water temperature, that is, 15óJ. Part of the thermal energy of the water molecules on the lake surface will flow to the cooler air. As a result, the temperature of the water molecules drops and the density rises and therefore these water molecules sink. The warmer water under the surface will rise gradually because of its smaller density, convection thus takes place and the whole lake will be cooled down.

Let us see the situation with a water temperature lower than 3.98óJ (Fig. 2). As water with a lower temperature has a lower density, the cooler water molecules will flow on the water surface. On the contrary, the molecules that are closer to 3.98óJ will sink due to their higher density. When the water temperature is lower than 0óJ, water with a lower temperature on the lake surface will freeze first and then the freezing will spread to the lake bottom. Because heat is transmitted by conduction, the cooling rate is very slow. The water temperature under the ice layer usually maintains at 3.98óJ. Fish can still survive. If water does not have this special property, we can imagine that water will start to freeze from the bottom and convection will increase the rate of freezing. After a very cold winter, we are afraid that all fish and other living things in the lake will all be dead.