Freezing Lakes
Most recent answer: 10/22/2007
Q:
Assuming a lake is made up of different layers which one freezes first. I said, "the bottom layer because as water gets colder, it sinks,the warm layers rise and when all the layers have sunk and the bottom one is on the top it can’t sink anymore because the layers below are already sunk, then the bottom layer freezes.
My teacher peer said "the top layer freezes first" Who is right and why. I hope you will let me know when you write the answer and where to find the answer.
Bob Kowal
- Bob
arcadia elementary, deer park, WA
- Bob
arcadia elementary, deer park, WA
A:
What I’m sure of here is what you can easily observe: lakes with a
layer of ice on top of liquid water. In that very common case, the top
layer obviously has frozen before the lower layers, although I don’t
know what’s happened on the bottom in the middle of the lake.
Your argument might sound reasonable, but it can’t trump direct observation. And in fact there are some flaws in it. When water gets almost down to freezing temperature, it actually expands rather than contracts on further cooling. So very near freezing, it’s the COLDEST water which would tend to rise to the top. In addition there are all sorts of complications which I don’t know much about, such as different salt concentrations in different parts, comparative rates of freezing and of temeperature-driven circulation, etc.
That’s why it’s nice to know the answer by observation. And this is an important answer- if you thought the process went the other way, you’d assume that when the top was ice, it was supported by ice all the way down. Many people have died from making that assumption.
Mike W.
Just to add a couple of details: Most lakes freeze over because the ambient air temperature gets below freezing. That, combined with evaporation from the surface and radiation of heat energy to space makes heat loss from the surface greater than heat loss through the bottom. In fact, because the earth generates thermal energy due to nuclear decay, the ground warms up the water rather than cooling it off, at least in the wintertime. So the ice forms on the surface because that’s where the heat loss is, rather forming first on the bottom and then floating upwards.
Tom J.
Your argument might sound reasonable, but it can’t trump direct observation. And in fact there are some flaws in it. When water gets almost down to freezing temperature, it actually expands rather than contracts on further cooling. So very near freezing, it’s the COLDEST water which would tend to rise to the top. In addition there are all sorts of complications which I don’t know much about, such as different salt concentrations in different parts, comparative rates of freezing and of temeperature-driven circulation, etc.
That’s why it’s nice to know the answer by observation. And this is an important answer- if you thought the process went the other way, you’d assume that when the top was ice, it was supported by ice all the way down. Many people have died from making that assumption.
Mike W.
Just to add a couple of details: Most lakes freeze over because the ambient air temperature gets below freezing. That, combined with evaporation from the surface and radiation of heat energy to space makes heat loss from the surface greater than heat loss through the bottom. In fact, because the earth generates thermal energy due to nuclear decay, the ground warms up the water rather than cooling it off, at least in the wintertime. So the ice forms on the surface because that’s where the heat loss is, rather forming first on the bottom and then floating upwards.
Tom J.
(published on 10/22/2007)