In polar countries, it only seems to be the water on the surface that freezes.
If I kept glass full of water in the freezer, then not only the surface, but the entire glass of water freezes, why?
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Shallow ponds will freeze all the way to the bottom (less than 5 feet), but deeper bodies of water will not. This is because the ice that forms on the surface insulates the water below. The temperature at the bottom of the lake is slightly above freezing. The ground is also just slightly above freezing. The water cannot be cooled down any lower since the heat needs to be taken away from the surface. Since the ice acts as an insulator (heat transfer is slow), the water cannot lose enough heat to freeze.
Because the surface of the ocean cools faster than the bottom, in addition the bottom has greater preassure so it freezes at a lower temperature, comething that doesnt happenin a glass of water where the preassure difference is minimal. Plus there are subsurface hot currents, so the two systems are not equivalent at all.