Why do lakes and rivers only freeze on the surface and not all the way through?
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1. The air is what gets cold, the ground is almost always a couple degrees above freezing, so the lake cools from the top down.
2. When a part of water freezes, it floats to the surface no matter where the ice forms, this is due to ice being less dense than water.
3. Once the surface is frozen it acts as an insulator for the rest of the water below.
This mechanism is very important for life in the water. If it froze all over, fish would have a very bad time over the winter.
The freeze/ thaw change for water requires transferring enormous amounts of energy and the surface ice acts as an insulator preventing that energy transfer.
That being said some shallow lakes do freeze completely.
A lot of water bodies also end up being used by people who dump relatively warm into them from showers, laundry, dishes, power plants, etc
The reason for this is closely related to the density of ice and the properties of water. After the liquid water becomes solid ice, the spacing between water molecules will become larger, that is, the density will become smaller, from the weight of ice becomes lighter than water, and because of this, the ice will float on the surface of the water. This is why ice floats on the surface of the water instead of sinking to the bottom. Thus, when rivers and lakes have a layer of ice on the surface, the ice floating on the water’s surface will isolate the outside air from the water below. At this time, the temperature of the ice surface and the temperature of the outside air will remain the same, the bottom layer of ice and the top layer of water, which is often referred to as the ice and water mixture temperature of zero degrees, and further down, the temperature of the water will gradually increase. In addition, water and ice are poor conductors of heat, not good at conducting heat, and the temperature of the upper layer of ice can not be effectively transferred downward. Therefore, in rivers and lakes’ ice can only stay in the surface layer, if the external temperature is low ice will be thicker, not so low ice will be thinner, but not all frozen into ice.
Normally things get lower density when heated and higher density when cooled. So if you cool a liquid from the top the cold liquid falls down to the bottom pushing up the hotter liquid, so it all keeps the same temperature. This is why ACs are usually mounted high on the wall and heaters are mounted low. But water have the property that the ice is less dense then water and therefore cooling down water to ice makes it float to the top. This is even noticable at temperatures a bit above freezing. So when you cool a lake from the top and the water is almost at freezing the cold water stops falling to the bottom of the lake and just stay on top. The water at the bottom of the lake therefore does not cool down any more. On the surface however the water might freeze into ice and even be cooled far bellow freezing but none of that ends up at the bottom of the lake.
Because ice floats.
That’s the short answer. Let’s unpack this a little.
Water is one of a very few substances that expands when it freezes. This means that frozen water, i.e. ice, is less dense (i.e. lighter, for the same volume) than liquid water. And therefore, when you freeze water, ice floats to the top.
Why doesn’t all the water freeze all at once when the temperature falls below 0°C? Well, because it takes a lot of energy to freeze water. That is, to turn a drop of liquid water at 0°C into ice, a lot of heat energy needs to escape. For comparison: to lower the temperature of 1 g of water by 1°C, requires 4.18 J of energy to be lost (to the environment). On the other hand, to freeze 1 g of water that is at 0°C (i.e. just to turn it into ice that is still at 0°C), takes a whopping **334** J of energy loss.
So, when water is placed in an environment that is below 0°C, its temperature will drop to 0°C fairly quickly, as it only takes about 4 J per °C, per gram of water. But then when it hits 0°C, now it has to overcome this barrier of a 334-joule energy loss in order to turn into ice.
Now let’s think about where the freezing is happening. This is mainly at the surface of the water, where the water is exposed to the cold environment. This is the main place where energy loss happens. And here’s the kicker: once ice is formed, it will float to the top, and form a layer of ice exactly on the surface where energy loss happens. This layer of ice insulates the liquid water below from the cold air above. Not perfectly, of course, but it does slow down the transfer of heat energy out of the liquid water. And as more ice forms, the insulating layer on top of the water only grows and slows the freezing process even more. If any snow happens to fall on the ice, then this provides even more insulation.
If the environment stays below 0°C for long enough, then eventually an entire lake will in fact freeze from top to bottom. And sometimes, in shallow waters, this does indeed happen. But your typical lake is too deep, and your typical winter is not long or cold enough, for it to freeze solid before spring comes and the ice starts to melt.
If ice didn’t float, then it would not form this insulating layer, and freezing would continue at a constant rate instead of slowing down as the ice grew, causing bodies of water to freeze faster.
Add this to your list of questions. If heat rises and ice floats, how does ice even form on lakes?
In addition to the forementioned answers: water is heaviest at +4°C. Water freezes at 0°C, which is lighter than the +4°C water, so the ice floats on top. Once you hit a certain point that the outside temperature can’t affect the water through the thiccccc layer of ice, because the weather is not cold enough, the water stops freezing due to some laws of thermodynamics I’m not super familiar with.
1. Water has an extremely high heat capacity, it is higher than almost every common substance. That means it takes a huge amount of energy to change the temperature of water. The air is one of the few systems with enough available energy to cause this sort of process
2. Water is one of the few substances on Earth where the liquid form is more dense than the solid form. This is why ice cubes float in your drink. If ice were to form at the bottom of a body of water, the lighter density will cause the newly formed block of ice to float towards the surface.
Because Ice is less dense that water and floats upon it. Once it forms a layer of ice then the ice provides insulation to the water below it preventing it from getting cold enough to freeze.
If ice sank then we’d be in for a world of trouble. The sinking ice would not insulate the water from the freezing temperatures and more would freeze and sink until all of the water was frozen. No liquid water is not a good thing. There is a short story called “The Catalyst” (I think) that I read in elementary school that basically covered this.
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