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
A lot of answers attribute it to the liquid density anomaly of water, that it is densest at 4°C. This is not the reason at all though. What somewhat matters is that ice does not sink, but even that is not that important as I will explain below. The same freezing from top would happen with almost any liquid and I have seen it with a variety of molten metals as well! You can actually compare it to sauces getting a skin layer on top first, if you want.
Reminder for the American people reading this, just to keep the following understandable: 0°C is where water freezes.
So what is actually going on? The **air is what gets below 0°C first**, both water and ground take a long time to catch up; in most areas the ground never really freezes below a few centimeters in, actually. The **cold air then cools the surface** of the pond/lake, while everything else is warmer than that.
It is important that **water can only freeze where it is colder than 0°C**, as freezing produces energy; water left at 0°C and perfectly isolated will never turn into ice (you could even go a bit below 0°C).
So where does it freeze? Well, only where it is cold enough: at the top. And unless the frozen liquid sinks down (it won’t for water, as ice floats; but even many other ones won’t as the surface tension and the rigidity of the solid version can keep it on top) **nothing will bring anything colder than 0°C downwards**. And as mentioned above, **liquid water cannot just spontaneously freeze at 0°C**, as that would produce energy and thus heat!
Even if water would sink at 0°C to the bottom, all that does is gradually cooling the entire lake to 0°C, then the above process starts. In actuality, the density anomaly causes a nice and cozy 4°C bed at the bottom for life to survive (this is why the anomaly is really important!), and then freezing at the top begins. The ice layer even makes a reasonable insulator towards that cold air. Regardless, only when this circulation cannot keep up with distributing the temperature anymore is where freezing starts.
The very same happens in any liquid. If the solid version does not float on top, it could however happen that it forms small clumps that then sink down and accumulate at the bottom, effectively filling it up. There is still some liquid in-between the clumps until the freezing form the top gets there.
Until now I have blissfully ignored the ground as if it never freezes. It may happen, though. This causes one mayor change: now also the bottom of the pond (for a full-on lake, this is extremely unlikely or just impossible) gets below 0°C. This causes freezing from the sides and bottom as well. And as mentioned for non-water liquids above, even with the density change it often will just stick there. In this case, the pond freezes from the outsides inwards.
**tl;dr**: unless the ground is rather heat conductive (compared to the liquid), that lake could just as well be alcohol, ammonia, liquid copper or liquid nitrogen; it would still freeze from the top.
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