Why the water in the Mariana Trench is warmer (34-39 F) than the water the victims of the Titanic fell in (28 F)?
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At depth water is under a lot of pressure, which causes its temperature to rise. That warm water in the Mariana trench would be close to freezing if it were brought to the surface in a thermally insulated balloon.
Oceanographers try to get rid of the pressure effect by computing the “potential temperature” which is the temperature the water would have at a common depth (like the surface or 1000 m). Potential temperature always decreases with depth, unless salinity makes the deeper water more dense.
We are used to this idea with air. Warm air rises but it is cold in the mountains because that warm air expands under lower pressure and becomes cold. If you took the air on Mount Everest and brought it to sea level it would be like 130 F. The “adiabatic lapse rate” is about 10 C per 1 km elevation. Meaning the same air would be 10 C warmer if you were 1 km lower in elevation.
The same thing is going on with ocean water. The Mariana trench is deeper than the Titanic, so the water is slightly warmer, even if it would be colder if it were brought up to the depth of the Titanic.
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