It is my understanding that within a given medium warmer substances tend to rise and cooler substances fall, i.e. warmer air above cooler.
Assuming the same is true with water then in a lake the cooler water should fall to the bottom while warmer water rises to the top, correct? If so, I would expect that lakes would freeze from the bottom up. Can you please explain why this isn’t the case?
Thanks for your explanation.
In: 23
Warmer substances *often* rise while cooler substances fall, but not *always*. It depends on if something expands or contracts with heat. Most things expand.
Even if water expanded with heat, since the cold air is at the top the ice would still form at the top. It would form a sort of snow or sleet and sink down to the bottom.
However, water is odd. Water is at its smallest between 2 and 3 degrees Celsius. When the air temperature drops, at first we see the water flowing up and down as the surface gets cold and falls to the bottom. Then, once everything has reached 2 or 3 degrees c, the water at the top continues to cool down, but now it expands so it continues to float at the top *even as it is colder*. Once it freezes it expands *even more* and now it is very much staying on top.
Warmer substances *often* rise while cooler substances fall, but not *always*. It depends on if something expands or contracts with heat. Most things expand.
Even if water expanded with heat, since the cold air is at the top the ice would still form at the top. It would form a sort of snow or sleet and sink down to the bottom.
However, water is odd. Water is at its smallest between 2 and 3 degrees Celsius. When the air temperature drops, at first we see the water flowing up and down as the surface gets cold and falls to the bottom. Then, once everything has reached 2 or 3 degrees c, the water at the top continues to cool down, but now it expands so it continues to float at the top *even as it is colder*. Once it freezes it expands *even more* and now it is very much staying on top.
The ELI5 version is that the whole lake has to get to 0C first before any ice can form at all (with the top water getting cooler, falling down, warmer water rising, cooling, falling down, etc), then once the entire lake is sufficiently cold, ice crystals will start to form and rise to the top as they are less dense (and the cold air/wind at the top help to wick away heat).
For a really cool book about this topic that explains it in a ELI18-ish way, check out [Lakes: Their Birth, Life, and Death](https://www.amazon.com/Lakes-Their-Birth-Life-Death/dp/1643260480)
Edit: and the reason the whole lake doesn’t freeze is that the top layer of ice provides an insulating effect
The ELI5 version is that the whole lake has to get to 0C first before any ice can form at all (with the top water getting cooler, falling down, warmer water rising, cooling, falling down, etc), then once the entire lake is sufficiently cold, ice crystals will start to form and rise to the top as they are less dense (and the cold air/wind at the top help to wick away heat).
For a really cool book about this topic that explains it in a ELI18-ish way, check out [Lakes: Their Birth, Life, and Death](https://www.amazon.com/Lakes-Their-Birth-Life-Death/dp/1643260480)
Edit: and the reason the whole lake doesn’t freeze is that the top layer of ice provides an insulating effect
Water is weird in that it expands by about 4% when it freezes. So ice floats. So what happens is the cold air cools the water in the lake and the cooler water is denser and sinks and this goes on until the lake is pretty much all near freezing. Then as the water at the top freezes, it gets a bit less dense and floats!
Water is weird in that it expands by about 4% when it freezes. So ice floats. So what happens is the cold air cools the water in the lake and the cooler water is denser and sinks and this goes on until the lake is pretty much all near freezing. Then as the water at the top freezes, it gets a bit less dense and floats!
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