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I know it's because of sweat, but why should the sweat over my skin get heat from the body, cooling it under the ambient temperature, instead of getting it from the hotter air all around me? If I think about it I would suppose that the sweat cools me down to ambient temperature, not lower.

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  • $\begingroup$ I'm voting to close this question as off-topic because it’s really about biology not physics. $\endgroup$ – ZeroTheHero Jun 29 at 11:43
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    $\begingroup$ @ZeroTheHero well I was concerned about the thermodynamic aspect of the cooling. I knew it was about the sweat, I just couldn't figure out how it works if my body is the coldest source of heat to interact with the sweat $\endgroup$ – mmarco Jun 30 at 12:13
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If I think about it I would suppose that the sweat cools me down to ambient temperature, not lower.

This is true if the ambient air is at equilibrium with your sweat as regards the evaporation of the sweat, i.e. if it is saturated and the partial pressure of water vapour in the ambient air matches the natural vapour pressure at the equilibrium temperature.

Generally speaking, this is not the case ─ ambient air essentially acts as a reservoir of dryness (rather than a thermal reservoir at a lower temperature). Simply put, in any situation where the water vapour that evaporates from your sweat is insufficient to meaningfully raise the humidity of the air around you, the ambient air acts as a reservoir that will suck away that humidity.

The reason this maters is because evaporation takes energy, in the form of the latent heat of evaporation. So long as the air around you is dry, molecules can evaporate, and in this process they remove thermal energy from your body.

This is the reason why humid heat feels that much more opressive than dry heat: sweat cannot evaporate, because the air is already saturated by water vapour (or rather, for every molecule of water that evaporates and takes away its latent heat of evaporation, there's another one which comes in from the ambient air, hits the sweat, and joins in, depositing its kinetic energy as an equivalent latent heat of condensation).

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  • $\begingroup$ So the latent heat is actually given by the body and not by the ambient air? $\endgroup$ – mmarco Jun 27 at 13:59
  • $\begingroup$ The lion's share comes from the body, simply because the sweat is in much tighter thermal contact with it than with the air. $\endgroup$ – Emilio Pisanty Jun 27 at 14:01
  • $\begingroup$ Oh right, the contact, I just ignored that. Thanks a lot $\endgroup$ – mmarco Jun 27 at 14:04
  • $\begingroup$ A big issue is that in this case, even though the heat comes from your body, the warm air around you will be acting to bring your body back up to those temperatures; whereas with ambient temperatures less than body temperature, it's only internally generated heat that the evaporation has to remove. $\endgroup$ – JMac Jun 27 at 14:22
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    $\begingroup$ @EmilioPisanty Not quite just that. When it's hot outside, you feel hot because your heat transfer is greatly reduced in most ways; but you still transfer heat to your surroundings through the main modes of heat transfer. Once you surpass body temperature, the heat transfer between your body and surroundings would reverse, so the environment is also heating you up, along with your biological processes. You get to the point where phase transition doesn't just help expel heat, it's the only real way. It points to the necessity of having a phase transition mode of cooling. $\endgroup$ – JMac Jun 27 at 14:29

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