if heat rises, why do bodies of water freeze from the top down?


If I had to guess, it’s because the temperature above the water is constantly colder than the water itself – but that is what I actually don’t understand.

If the surface temperate is extremely colder than the temperature of the water itself, wouldn’t the cold “sink” that much faster, making the warm water “rise” at an equivalent rate, and thus forcing more of the cold water to the bottom?

Maybe a better post for stupid questions, but I’m very curious what causes this – perhaps this just applies to air, and not water? And if that’s the case, then also why?

In: 14

Deep water is insulated by surface water, the surface water has to cool to freezing before the deep water can get that cold. For small stuff. And yes, that’s connected to what’s in contact with the colder air.

For REALLY deep, not middle of the small river deep, pressure actually prevents water from freezing. Remember that it has to expand to form ice.

Because ice is less dense than water so it floats to the top.

The idea of “heat rising” isn’t directly true. What is true is that less dense things rise above more dense things. And generally speaking, stuff becomes less dense when it gets hotter. This is most pronounced when it comes to gasses, but it *does* affect liquids too (and even solids, but that isn’t really meaningful for most things).

So cold water is likely to “sink” that is true, but ice floating is more important.

Fun fact, for really deep water (like the ocean) the pressure keeps ice from forming. Because ice needs to expand in order to freeze, which it can’t do when under such immense pressure.

Heat rises because heat tends to make things less dense and so they float in the same fluid medium. Water, however, is different, while water does follow the same rule because of the crystalline structure formed water reaches its maximum density under normal atmospheric conditions at 4°C. So water at freezing temperatures is less dense than the relatively warmer waters underneath it, as a result water tends to freeze from the top down and act as an insulator for the liquid water underneath, this is an important factor for aquatic species surviving the freezing winters.

The reason is that ice is fairly unique among liquids, in that it expands when frozen. For this reason, ice is lighter than water. So even if the water near the bottom of the were freezing first, it would rise to the top once frozen.

As it is, the large body of water retains heat, as does the land beneath the water, so the surface that is in contact with the cold air freezes first. Ice being lighter than water, it stays on the top. If it didn’t, life on earth might not be possible. There was a short story about this written in the 1970s. *The Catalyst, by G.R. Yohe*

Water’s density is greatest at 4° C. As water’s temperature approaches 4° C it sinks, leaving either warmer or colder water to take its place.

So as a lake cools toward freezing, the coldest water is near the surface, and water closest to 4° C is at the bottom.