How does a pond or a lake freeze over? This article will explain the science behind it.

When the air temperature falls the bodies of water lose their energy and begin to cool. The cold air causes the surface water to cool first. As the surface water cools it becomes denser (heavier) and sinks into warmer less dense (lighter) water. This process of colder surface water sinking into the body of water cools the lake/pond until it reaches 40 degrees. 40 degrees is the magic number the entire lake/pond has to reach because that is when water is at its most dense. Because the core water temperature is at 40 degrees, water will no longer be able to sink. It will remain at the surface and continue to cool until it reaches 32 degrees. When the water reaches 32 degrees it begins to freeze into ice.

Ice will form first around the shoreline then spread inward. Once the ice takes over the surface of the lake it will begin to move downward causing the ice to thicken. The deeper parts of the bodies of water will remain around 40 degrees and not freeze over. This is where the fish swim and survive.