Why do lakes and ribers freeze first at their surfaces?

Question

Why do lakes and ribers freeze first at their surfaces?

Answer 1

Let us consider the situation with a water temperature higher than 3.98¢J. In Fig.1, the lake's surface temperature is 20¢J, the air temperature is 5¢J lower than the water temperature, that is, 15¢J. 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.

Answer 2

The surfaces are in contact with the cold air.  Heat radiates from warm to cold and  the greater the difference between the 2 the higher the rate of radiation.  The water below the surface is protected from the cold air by the water above it.

The ice will work its way down to depth as the heat from directly beneath the ice comes into contact with the encroaching ice.

Answer 3

Because unlike most liquids, which continuously increase in density as their temperature drops, the density of water actually decreases once its temperature drops below 3.98 °C, so it rises to the surface. The closer it gets to freezing point, the lower the density (and when it does freeze, the density of ice is lower still, which is why ice floats).