How does the lake freeze?

When the temperature of the water in the lake near the surface reaches 39°, we know that freeze-up will begin soon. This is because 39° is the temperature at which water reaches it's maximum density.

As the weather gets colder in the fall, the lake begins to cool. The surface water, being in contact with the cold air, drops in temperature until it reaches 39°. At 39° water reaches maximum density, so it's heavier that the water below it, and it sinks. This displaces the warmer, deeper water, which rises to the surface. At the surface, it's exposed to the cold air, and it cools until it reaches 39°. It then sinks, and displaces warmer water below, which is forced to the surface. This process continues until all the water from top to bottom in that area has reached 39°, so it's all the same density. This is called “lake turnover”. This happens more quickly in shallow areas, which is why the shoreline and shallow areas typically freeze first.

At that point, the top layer of water won't sink. It will remain in contact with the cold air, and will continue to cool off until it reaches 32°, and can begin to freeze. In cold temperatures, this can happen in a matter of hours. If the air is calm, ice will begin to form, and freeze-up begins.