My take would be that underwater temps cannot get below freezing. So the surrounding temperatures above water can be lower than under water.

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Pooge a day ago | parent | next [-]

> My take would be that underwater temps cannot get below freezing

Yep, density of water changes with temperature—it's the only thing that has that phenomenon—and therefore the colder water is always at the surface. That's actually what made life possible since life could survive despite a frozen surface.

Happy to be proven wrong, but that's what I learnt making cocktails and investigating clear ice :)

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dotancohen a day ago | parent [-]

Lots of things change density when temperature changes. Water is unique that below 4°C, it gets less dense as it cools - and that trend continues even through the phase change. That's why, as you mention, ice floats, and the surface freezes first, and fish can live below a frozen surface.

	▲	Pooge a day ago \| parent [-]
		> it gets less dense as it cools Right!! Thanks for correcting me.

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nh23423fefe a day ago | parent | prev | next [-]

plus thermal conductivity of water vs atmosphere

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mrloopex a day ago | parent | prev [-]

Freezing point of water changes with salinity and impurities. By definition a liquid cannot be below its freezing point until it has completed its phase change.

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RetroTechie a day ago | parent | next [-]

> By definition a liquid cannot be below its freezing point until it has completed its phase change.

Supercooled water is a thing: (and I've seen it a few times myself)

https://en.wikipedia.org/wiki/Supercooling

	▲	wahern a day ago \| parent \| next [-]
		Superheating, too, and that's something that can happen in a microwave, occasionally causing severe burns when superheated water flash boils. AFAIU, it's the same phenomenon--lack of nucleation sites to kick-off the phase transition.
	▲	mrloopex a day ago \| parent \| prev [-]
		Does that mean every TTT diagram is wrong? Nevermind I just learned about CCT diagrams. I guess having only one year of materials engineering means I should STFU. :)

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FromHoiPolloi a day ago | parent | prev [-]

Yes. My point was that it's easier(?) to get below freezing point above water than below. So while the cold air keeps the upper part of the iceberg (relatively) intact - it will slowly melt below the sea until the center of gravity changes enough to flip over.