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Asked by francesca_sweet to Anna, George, Jodi, Rob, Stefan on 13 Nov 2014. This question was also asked by sammy.
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Rob Appleyard answered on 13 Nov 2014:
They do, if it gets cold enough. The Arctic ice cap is made of frozen seawater.
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George Ryall answered on 13 Nov 2014:
As Rob says, at the ploes, where it gets really cold, they do. The reason that on a cold day the ocean doesn’t freeze over is essentially because it is massive (probably the salt content and the fact the surface is moving probably also contribute when the water is close to freezing). The atmosphere is warmed by the ocean on a cold day, and it would have to do a lot of warming to drop the temperature to freezing point. Imagine taking a cup of tea and leaving it to cool, it would take a while for it to reach room temperature. Take a really large cup of tea (like the one on my desk right now) and it would take even longer. Now imagine a similar experiment, but with tap water and your freezer, again the larger the cup of water the longer it will take to freeze. The ocean is a large cup and by the time it would reach the freezing air temperature it’s summer.
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Stefan Lines answered on 13 Nov 2014:
There are four main factors that typically stop the sea from freezing; salt, currents, water volume and geothermal heating. The salt changes the freezing point of water, from 0 Celsius to about -2 Celsius. The tides and currents keep the water moving and give it enough kinetic energy to stop freezing. As George mentioned, the large water volume makes it difficult for the whole body to freeze before the air temperature rises again. And finally, the water is heating from the ground underneath it which is warm from the heat at the centre of the earth.
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