How is ice created in lakes?

The temperatures drop, and we all wonder, "When will the lakes freeze?"

My question today was, how does that actually happen?

Of course, ice is created when the water temperature reaches freezing. That is pretty obvious. We've all seen ice cubes created in the freezer. Yet, lakes don't freeze all at once, and they don't completely freeze all the way through. So how does that happen?

(See the dates the Iowa Great Lakes typically freeze here.)

It all starts with water density. As warm summer waters begin to cool, they will sink. Think of how the deeper that you go in the lake, the colder the water gets.

But once water hits 39 degrees (4 Celsius), the density actually begins to swing the other direction. At that temperature, the water begins expanding and becomes less dense as it gets even colder.

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That means that colder water begins to come to the surface, and the warmer water sinks to the bottom.

The water on the surface will then freeze to form a layer of ice, and the ice will have the least density, making it float on top of any cold water as well.

How does it freeze?

There are two different types of lake ice that are created in different ways.

Congealation ice is created from cold conditions. Spontaneous nucleation takes place on cold, calm nights when the surface of the lake drops below freezing and ice spreads across the surface. Heterogeneous nucleation takes place when strong winds blow dust, snow or frozen rain across the surface of a cold lake and as that hits the surface of the water, it freezes.

Snow ice occurs where snow falls on congealation ice and when it cracks, the water from below rushes to the surface and meets the snow, which will then freeze.

Why doesn't the water freeze all the way to the bottom?

In deep lakes, like the Iowa Great Lakes, water pressure also plays a role in why the surface freezes but water stays liquid underneath.

The weight of the water higher in the lake presses down on the water that is deeper. The pressure allows the water near the bottom to still get cold but does not allow the same expanding and rising as the water near the surface. Because of that pressure, the water at the bottom will not turn to ice even though it is at a freezing temperature.