Why ice freezes on the top of a pond

[bobsmith76]

I'm having a little trouble understanding why water freezes on the top. Take this passage:

We can use this unusual thermal expansion behavior of water to explain why a pond freezes slowly from the top down. When the atmospheric temperature drops from 7°Celsius to 6°Celsius, say, the water at the surface of the pond also cools and consequently decreases in volume. This means the surface water is more dense than the water below it, which has not yet cooled nor decreased in volume. As a result, the surface water sinks and warmer water from below is forced to the surface to be cooled, a process called upwelling. When the atmospheric temperature is between 4°Celsius and 0°Celsius, however, the surface water expands as it cools, becoming less dense than the water below it.

Why I don't understand is that the cold water should be on the bottom of the pond since it is more dense and the warm water on top. Given that I would think that ice would form at the bottom of the pond, then rise to top since ice is less dense than water, but that is clearly what does not happen. But why doesn't it happen? I'm guessing the surface water has to first reach 0 Celsius before it can freeze, but when the surface water reaches 0, the bottom water should be about -2 or so since the less dense warm water will be on top. So why isn't the colder water at the bottom freezing than moving to the top, like a backwards ice storm?

 

[K^2]

Water has lowest density at 4°C. As you cool water bellow 4°C, long before it begins to freeze, liquid water already starts getting less dense. As a result, if the water in the pond is all between 0°C and 4°C, the least dense water in the pond is at 0°C, just ready to freeze. And so it surfaces and freezes at the surface.

The reason water close to 0°C is less dense than water at 4°C has to do with how water molecules align. At 4°C, the alignment is more or less random, but as temperature drops, the hydrogen bonds start aligning water molecules into lattice they will form once the freezing process begins. So you end up with clusters of "almost ice" in the fluid, which are significantly less dense than liquid water. The average density begins to decrease.

This effect is present above 4°C as well, but isn't significant, allowing normal thermal expansion to dominate temperature-dependence of density. So at 4°C you end up with the maximum density.

 

[atlasman84]

good questions.. i also have trouble to understand

 

[tiny-tim]

hi bobsmith76!

… When the atmospheric temperature is between 4°Celsius and 0°Celsius, however, the surface water expands as it cools, becoming less dense than the water below it.

Why I don't understand is that the cold water should be on the bottom of the pond since it is more dense and the warm water on top.

no, when the water is below 4°, the cold water is less dense, and should be (and is) on the top …

the bottom water stays at 4° until the ice reaches it!​

 

[bobsmith76]

thanks, i get it now.