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Why does water below the surface of a lake not freeze like it does on top? Is water underneath the surface warmer?
Good question, Christina.
Ice is less dense than liquid water (water is very odd in this
way). So any ice below the surface of water will tend to rise. You can
see this all the time in a glass of ice water. The ice cubes float as
close to the surface as they can.
When water freezes on a lake or pond, though, the ice will actually
form at the surface, not just rise to it. When the temperature of water
is below 4 degrees Celsius it becomes less dense the more you cool it.
This is another strange property of water.
So as a body of water cools below 4 degrees, the cooler water (less
dense) will rise to the surface where it will turn into ice. Hockey
players everywhere are very glad for this peculiar effect of water.
I hope this answers your question.
(republished on 07/25/06)
Follow-Up #1: on thin legal ice
I understand about cooler water freezes at the top and all, my question is how much ice is made in an overnight period when the temperature is say 10 degrees. I live on a lake and wonder how many days of cold temperatures will make enough safe ice to be on. Can you help? Thank you.
Questions about rates are often hard to answer, because rates usually don't come straight out of simple physics. here a key question is how fast the heat releaed by the freezing water is carried away. That depends a lot on how windy it is, not just the temperature. So I really don't know the answer. The one thing we can be sure of is that University lawyers would be extremely upset if I guessed an answer, on the assumption that we'd be unlucky and someone would fall through thin ice and their survivors would sue.
Do your neighbors ever test the thickness with a portable drill with a long bit?
(published on 03/03/07)
Follow-Up #2: forming lake ice
My question is how does lake ice form and how did it begin to come about on Earth.
- Camie Haney (age 13)
North Wilkesboro, NC
I've marked your question as a follow-up to ones where we discuss the formation of lake ice. I don't understand the part about how it began. Are you asking how there came to be water on the surface of the earth?
(published on 10/07/11)
Follow-up on this answer.