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I was wondering why freshwater freezes from up to bottom and not the other way around. The reason I question this is because in a Video I came across, I discovered water freezes from top but surely with the current knowledge I would say it should be opposite because we know cool air drifts down while hot air drifts up so similarly the heat should fall down and this that the freezing should start from bottom and not up? Unless my knowledge applied is wrong and potentially stupid so I want to ask:

Why does water freeze from top to bottom?

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  • $\begingroup$ in the typical case, where is the surface? .. there you go $\endgroup$
    – Bort
    Jul 24, 2015 at 8:54
  • $\begingroup$ see also: physics.stackexchange.com/q/91637 $\endgroup$
    – user32229
    Nov 28, 2016 at 0:52

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The density of water rises from 0 to 4 degrees celsius. So you would have a gradient of water temperatures. On the bottom 4 degrees celsius and colder water above it. This is also the reason why fish survive in winter, they just dive to the bottom where the temperature stays warm longer. Also the density of ice is smaller than water so it will float. enter image description here

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  • $\begingroup$ There's also the contribution of evaporational cooling,which only takes place at the surface. So, assuming you don't have a refrigeration unit at the bottom of the pond :-) , the water doesn't necessarily have a monotonic thermal gradient from top to bottom. $\endgroup$
    – Carl Witthoft
    Jun 16, 2014 at 19:03
  • $\begingroup$ This is interesting if you think about a water bottle in the freezer. Despite the uniform cooling (let's assume anyway), it still freezes at the top first because the water under 4°C will rise to the top. $\endgroup$
    – Armatus
    Jul 19, 2015 at 21:21
  • $\begingroup$ @CarlWitthoft - You might want to rethink that one. At zero C, evaporative cooling is, well, "miniscule" comes to mind. As an overstatement. Furthermore, as soon as the first few molecules of the top surface freeze, no liquid phase is exposed to the air. $\endgroup$
    – WhatRoughBeast
    Jul 23, 2015 at 23:11
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    $\begingroup$ This is good, but doesn't answer the question of why. $\endgroup$
    – Steeven
    Jul 23, 2015 at 23:48
  • $\begingroup$ @WhatRoughBeast agreed - but consider the evap cooling before the water reaches zero C, and the relatively slow transfer of heat from the putatively warmer water below to the surface layer. $\endgroup$
    – Carl Witthoft
    Jul 24, 2015 at 12:25
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I think it depends from pressure,that at the bottom is higher. So we need to reach a temperature lower than at the top of the liquid. You know,higher the pressure of water,lower the equilibrium temperature liquid-solid. So while at the same temperature the freezing starts from top,the lower layers are "heated" from the layers are above,and can't reach the temperature of freezing until the upper layer is freezed and his temperature has begun to drop below the 0°C.

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Because it has less density then liquid water at 0 degree and empty spaces are created. Therefore, ice freeze from top to bottom.

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Because the water-air boundary has a higher heat exchange rate. Thus a thin ice layer forms there first. Since ice is less dense than water, it floats on the water. Once the ice forms, the ice-water boundary has a higher heat exchange rate than the lower regions of the water.

As heat energy continues to leave the water, more ice crystals form. This can sometimes create a condition in which striking the container (assuming your water is in a container, for example) can cause very rapid formation of ice crystals when cooled carefully. This video shows how to make instant ice with purified water or carbonated water: https://www.youtube.com/watch?v=RMzio5sLN48

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Zero degree water is slightly lighter than 4 degree water, so if we take a body of water above freezing temperature and drop the temperature around it (usually above it in regards to a lake as the air temperature drops), the colder water should be on top and in contact with the colder air, and freeze first.

Water has a tiny bit of air in it. If you look at icecubes, you can see that ice cubes freeze from the top and from the sides (this makes sense cause the cold is all around the ice cube tray. This traps the air bubbles in the middle, cause air doesn't freeze at those temperatures.

enter image description here

You can see the clear ice (which froze first) and the white ice - white because of tiny air bubbles, that froze last. Water freezes where it's in contact with cold. It will also float upward if it can because it's lighter, ice is often structured and in place, so it doesn't always float upwards, but instead, will freeze in place.

Clear ice, which is sometimes used in bars is frozen from the bottom up. This is done by freezing from the bottom and, often, keeping the water circulating.

enter image description here

There's no mechanism for a lake to freeze bottom-up, unless you found a way to make the Earth colder below the lake. Water is also a very rare substance that expands and becomes lighter when it freezes. Most materials condense and sink when they freeze.

Clear ice is made by a method called directional freezing. So it really is a two part answer. Yes, the ice and zero degree water floats on top when the lake is close and into the freezing range, but also, where the cold is in contact with the lake matters. Air gets below freezing quite often. Ground below the lake, much less often if at all.

If you pour a bucket of water into a very cold depression on the ground in below freezing temperature and watch it freeze, there's a good chance you'd observe freezing from the bottom up.

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Due to anamolous expansion of water, it has an ability to decrease its density when cooled below 0 C. As the density of ice is less than water, it start to freeze from top to bottom .

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