What happens to Lakes during the Winter.
During Winter, it is common for temperatures to be below freezing, 32° F. Lakes will lose energy to the atmosphere which causes water near the surface to cool and freeze quicker than the water beneath it. Water is most dense at 40° F.
Cool water is denser and heavier which will cause it to sink under warmer water. Warm water will rise because it is less dense and will replace the cool layer above, much like how air moves in the atmosphere. When the entire lake reaches 40° F, the surface water will continue to cool below 40° F. The only way the entire surface water will freeze is if the temperature of the water falls to 32° F.
Ice is physically less dense than water which is why ice will float above water. The layer of ice prevents cold air from reaching the water below it which actually keeps the water below the ice warmer! Overtime, the freezing process spreads downward into the lake and the ice thickens. Shallow lakes will freeze more quickly than deep lakes. Once the atmosphere warms up, the ice will melt!