WeatherTalk: When the lakes freeze over, it's quite a process
One key is the fact that water reaches maximum density at 39 degrees Fahrenheit.
Area lakes have been reluctant to freeze early this winter due to not enough cold weather. The process of lakes freezing over is as complex as it is fascinating. The Earth under the ice is warm, which creates an interesting calculus problem as cooling rates from cold air do battle with heat rising from below. Adding to the mystery is the fact that water reaches maximum density at 39 degrees Fahrenheit.
Once the surface of the water reaches 39 degrees, it sinks and is replaced by warmer water rising up from below. This process continues until the entire system is colder than 39 degrees and, only then, can the lake begin to freeze. This is part of why shallow lakes freeze much more quickly. Once ice forms, it acts as an insulator, which slows the thickening of the ice. Snow cover on top of the ice is an even better insulator and can greatly restrict ice formation underneath.