How does ice on a lake grow thicker from below?

Once an initial layer of ice has formed on a lake surface, it grows thicker from below through the process of thermal conduction.1

Because the boundary between the water and the ice remains at the freezing point (0° C), heat is continuously lost from the water to the air above by passing through the existing ice layer. As long as there is no significant heat flow from the water below, this heat loss through the ice results in the freezing of additional water at the bottom surface of the ice cover.1

The rate of this thickening is proportional to the temperature difference between the top and bottom surfaces of the ice and is inversely proportional to the current thickness of the ice layer. As the ice thickens, the growth rate typically slows, eventually proceeding in proportion to the square root of time. If a layer of snow is present, it acts as an insulator, offering resistance to this heat flow and thereby reducing the rate of ice growth.1