Intermediate 7 minutes
Temperature and heat transfer in water
Conduction, convection, and thermoclines: the physics behind the cold you feel
You're descending calmly along a wall and suddenly, within the space of a meter, the water turns noticeably colder and the view distorts as if you were looking through rippled glass. You've just crossed a thermocline, and there's interesting physics behind that invisible border.
In this article you'll see why water pulls heat away so fast, what the transfer mechanisms are, why the sea takes so long to change temperature, and what thermoclines are.
It's the physical counterpart to the thermoregulation article: there you saw how your body responds, here you'll see why the medium demands so much and how water temperature is organized.
Why water cools you so much
Two properties of water combine against the diver. The first is its thermal conductivity: water conducts heat roughly twenty-five times better than air, so it pulls heat away from your body much faster at the same temperature.
The second is its heat capacity, which is also very high: it takes a huge amount of energy to raise the temperature of a mass of water. Applied to the body, that means the water around you absorbs an enormous amount of your heat without warming up appreciably. There's no way to "warm your surroundings": the sea is a practically infinite heat sink.
That's why 25-degree air feels pleasant while 25-degree water ends up chilling you after a long dive. It's not the temperature that changes: it's the rate at which the medium pulls heat away from you.
The four transfer mechanisms are the same as in any physical system: conduction, convection, radiation, and evaporation. Underwater, radiation is negligible and evaporation doesn't apply; all the action is in the first two.
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