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Does heat always rise in a gravitational field?

I recently read that heat could be traveling down to the deep part of the ocean. Is there some new or old physics that makes heat sink?

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    $\begingroup$ If you mention that you read something, please try and cite it in some way, shape or form. $\endgroup$ – Signus Sep 24 '13 at 23:18
  • $\begingroup$ I think I read it in about ten recent news reports about some IPCC report that is coming out. $\endgroup$ – Jitter Sep 24 '13 at 23:24
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Heat does not rise or sink because it isn't actually a substance, it's energy being transferred. It is hot air which rises. The reason for this is that hotter air is more dilute than colder air. Or equivalently: colder air is denser than hotter air. The temperature of the air only plays a secondary role in determining which layer will be higher. The real decisive parameter is density.

In the sea, warmer water can sink because the density of water behaves peculiarly as a function of temperature. Water is densest at around 4 degrees Celsius. So if the rest of the water is cold enough, say at e.g. 1 degree Celsius, it will be less dense than the 4 degree water: the warmer water will be below the colder. This is related to the reason why ice floats on water: the ice is less dense than the warmer water below.

Read also this excellent answer by Lagerbaer on a related question.

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  • $\begingroup$ The way I would like to understand this is in terms of phonons. $\endgroup$ – Jitter Sep 24 '13 at 23:36
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    $\begingroup$ Phonons are not helpful here. You can understand heat well mostly in terms of classical physics. An explanation in terms of quantum field theory is an unnecessary complication, unless you have some specific need. $\endgroup$ – Dan Piponi Sep 24 '13 at 23:45
  • $\begingroup$ I assume that water vibrates and the extra volume created makes it rise. What hapens at 4C does some mechanism outdo the vibrations to expand the volume? Does ice sink as it gets towards absolute zero? $\endgroup$ – Jitter Sep 24 '13 at 23:46
  • $\begingroup$ My need is to understand the physics in regard to phonons or vibrations. To create the picture in my mind and "go for a ride on the rainbow". $\endgroup$ – Jitter Sep 24 '13 at 23:50
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    $\begingroup$ Another reason warm water can sink in the oceans is due to salinity differences. The sun heats water but also makes it evaporate, leaving saltier water, which is denser and so sinks. This causes deep convection cycles known as the thermohaline circulation. $\endgroup$ – Nathaniel Sep 25 '13 at 0:56

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