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?