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
As water freezes, it expands. As it expands it takes up more volume with the same mass, so ice floats. So even if ice formed at the bottom it would float and collect at the top then freeze solid.
The top of the water has the most opportunities to shed heat compared to further down, so it’s the most likely place to freeze as well. But for the most part it’s really just that ice floats.
As water freezes, it expands. As it expands it takes up more volume with the same mass, so ice floats. So even if ice formed at the bottom it would float and collect at the top then freeze solid.
The top of the water has the most opportunities to shed heat compared to further down, so it’s the most likely place to freeze as well. But for the most part it’s really just that ice floats.
Water is at its densest at 4°c and gets less dense as you approach 0°c leading to the odd effect of bodies of water freezing from the top down as opposed to bottom up as that denser 4°c water settles at the bottom and the cooler 0°c water ends up at the top.
Water is at its densest at 4°c and gets less dense as you approach 0°c leading to the odd effect of bodies of water freezing from the top down as opposed to bottom up as that denser 4°c water settles at the bottom and the cooler 0°c water ends up at the top.
The air temperature can drop before the water temperature, and as a result, the surface of the water can freeze first where it is in contact with the cold air. Although warm water has a tendency to rise, once it freezes and turns into ice, it has a lower density compared to liquid water and therefore, it floats on the surface.
The air temperature can drop before the water temperature, and as a result, the surface of the water can freeze first where it is in contact with the cold air. Although warm water has a tendency to rise, once it freezes and turns into ice, it has a lower density compared to liquid water and therefore, it floats on the surface.
It’s not that warmer things rise, it’s that warmer things tend to “spread out” as the molecules get more giggly, and that makes them less dense. Denser objects fall. Less dense rise. That’s why a helium balloon rises without being cooler. Helium is less dense than nitrogen and oxygen, which push the helium upward as they fall below it.
Ok so what about ice? Water has a particularly weird, rare property that it actually is less dense when it’s frozen. This because it’s crystalline structure is less more widely packed than when it’s liquid. This is why ice cubes float. Also why, when it freezes on top (the area exposed to cold air) the ice doesn’t sink to the bottom. Luckily.
Because if it did sink, then more would freeze and sink. And more. And eventually it would all be frozen and never melt. And earth would be a giant ice ball. With no life.
Read a book called Cat’s Cradle. It’s about people who invent something called Ice 9 that basically works this way.
It’s not that warmer things rise, it’s that warmer things tend to “spread out” as the molecules get more giggly, and that makes them less dense. Denser objects fall. Less dense rise. That’s why a helium balloon rises without being cooler. Helium is less dense than nitrogen and oxygen, which push the helium upward as they fall below it.
Ok so what about ice? Water has a particularly weird, rare property that it actually is less dense when it’s frozen. This because it’s crystalline structure is less more widely packed than when it’s liquid. This is why ice cubes float. Also why, when it freezes on top (the area exposed to cold air) the ice doesn’t sink to the bottom. Luckily.
Because if it did sink, then more would freeze and sink. And more. And eventually it would all be frozen and never melt. And earth would be a giant ice ball. With no life.
Read a book called Cat’s Cradle. It’s about people who invent something called Ice 9 that basically works this way.
>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?
Yep, that does happen. The topmost layers of water get cooled first, but they don’t cool all the way to freezing point. They cool enough for their density to increase enough to sink to the bottom, but it hasn’t frozen yet. When that ‘layer’ sinks, the water just below it rises to the top, and the same thing happens again – it cools down enough to sink, but not until freezing point. This happens over and over until eventually, all of the water body reaches a temperature close to freezing point. Then, the uppermost layer of water becomes cold enough to go to the freezing point (naturally, since it’s directly in contact with the air to which it’s losing heat). So the uppermost layer freezes. Now for the slightly more confusing part – normally solids tend to be denser than the corresponding liquids, but in the case of water and a few other liquids, ice is actually *less dense* than water. As a result of this, the ice floats on the water, not sinking to the bottom. Ice is also a great insulator of heat, meaning that it doesn’t allow the transfer of heat very easily. So the water below the ice (which is close to freezing point), actually remains liquid without freezing, because the ice above doesn’t allow the remaining heat to flow out. So unless it gets *really* cold, normally you just have a layer of ice on top and just water beneath it. That’s why aquatic animals like fish can survive cold winters in lakes and ponds.
There’s a *bit* more nuance involved about *why* ice is less dense than water and about the exact temperatures which the water body is at, but that’s the basic explanation for why it happens the way it does.
>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?
Yep, that does happen. The topmost layers of water get cooled first, but they don’t cool all the way to freezing point. They cool enough for their density to increase enough to sink to the bottom, but it hasn’t frozen yet. When that ‘layer’ sinks, the water just below it rises to the top, and the same thing happens again – it cools down enough to sink, but not until freezing point. This happens over and over until eventually, all of the water body reaches a temperature close to freezing point. Then, the uppermost layer of water becomes cold enough to go to the freezing point (naturally, since it’s directly in contact with the air to which it’s losing heat). So the uppermost layer freezes. Now for the slightly more confusing part – normally solids tend to be denser than the corresponding liquids, but in the case of water and a few other liquids, ice is actually *less dense* than water. As a result of this, the ice floats on the water, not sinking to the bottom. Ice is also a great insulator of heat, meaning that it doesn’t allow the transfer of heat very easily. So the water below the ice (which is close to freezing point), actually remains liquid without freezing, because the ice above doesn’t allow the remaining heat to flow out. So unless it gets *really* cold, normally you just have a layer of ice on top and just water beneath it. That’s why aquatic animals like fish can survive cold winters in lakes and ponds.
There’s a *bit* more nuance involved about *why* ice is less dense than water and about the exact temperatures which the water body is at, but that’s the basic explanation for why it happens the way it does.
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