### Question

If the pressure at the bottom of the ocean is high, why is it so cold?

The lack of heat at the ocean bottom is due to the lack of thermal radiation from the sun. Water is generally incompressible, i.e. a lot of pressure is required for very little volume change. Thus, one doesn't appreciably increase the intramolecular collisions by applying pressure to the water. Therefore the lack of thermal radiation far out weights the increased pressure with respect to temperature.

Pressure and temperature only affect one another when changing (and that is only true at constant volume for an ideal gas, which here is not the case). If you were to suddenly put pressure on the bottom of the ocean, it would heat up. The difference is that the pressure is fairly constant, and therefore whatever heat was given to the ocean from its pressure, has reached equilibrium a long long time ago.

Temperature is only affected by a change in pressure. The ocean's pressure is constant and does not change much, so therefore the temperature is constant and does not change much. After that fact, the reason the bottom of the ocean is so cold is because of heat currents, which is to say that hot water flows to the top and cold water flows to the bottom.

Are you referring to the ideal-gas equation:

PV = NkT ?

If so, please remember that water is not an ideal gas, and that the ocean is not an isolated system.

For example, as you go higher from the sea-level the atmospheric pressure is declining as well as the temperature. That's because what you call "temperature" relates to the "energy" of the medium around you (the atmosphere) which is lower where the air is thinner.

In answer to your question, as you go deeper into the ocean the pressure increases but the temperature is getting lower because the dense water are preventing the radiation of the sun from heating the lower levels of the water. The lower you go the less radiation is available to heat the water.

Ocean water becomes cold by evaporation from its surface. Water which converts to vapor by evaporation carries away heat of liquid water with it leaving liquid water of ocean colder. This cold water from the surface of water being denser goes down to the bottom and hot water from bottom come up which becomes cold on evaporation. Thus ocean water becomes cold.
Answered by: Madhukar Daftary, B.S., Chemical Engineer and teacher of physics, India

Having spent much of my life in submarines, however, I have found that the oceans are not simple contained structures. As we traveled the ocean bottoms, we encountered streams and currents within the ocean structure; therefore, temperatures varied despite constant depths. One of the most surprising was entrance into the Mediterranean from the Atlantic Ocean. At constant depth, the temperature dropped almost twenty degrees. Yes, it was colder in the Med than in the Atlantic Ocean at the same depths.
Answered by: James LeVangie, M.S., Retired Navy Science and Technology Specialist

First off, water is a polar molecule. Secondly, the Earth's gravitational field produces a downward pressure gradient. Therefore it is unwise to invoke the formula PV ~ NT, because what we are talking about is far from an ideal gas in a box. Infact, a rise in pressure on a given volume will increase the density (and therefore the number of particles, N) of that volume by an unspecified amount (that is, not specified by the ideal gas law), so the effect on temperature is ambiguous. However, One consequence of polarity is that individual water molecules are subject to a mutual Coulomb attraction. Under conditions of relatively high pressure (and therefore density) the Coulomb attraction between neighboring water molecules will be relatively high (since electrostatic forces are inversely proportional to distance). Thus, a given volume of water will be more tightly bound to itself. The result is that a mechanical disturbance at the surface of the volume is damped faster, the effect of the disturbance on the water molecules' velocities is minimized because the molecules are holding each other in place tighter (kind of). Now if the particle velocities are (on average) minimized, then , by definition, the temperature is minimized.

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