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The question came to while hiking. We are often told that we must drink at least 8 glasses of water regularly to compensate water loss by evaporation from the skin.

So how does one compute the rate of evaporation of water from the surface of the skin? Say in Liters per $ s^{-1} m^{-2}$. A bigger person, who has more surface area would suffer more loss (hence $m^{-2}$ and staying longer means more loss, hence $s^{-1}$. Weather also plays a significant role here, especially temperature.

I think we have a Maxwellian distribution of molecules, the loss will somehow happen through diffusion and some loss from the tail of the distribution.

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  • $\begingroup$ The 8 glasses bit excludes food, which unless you are a complete carnivore, is mostly water. You might think of including physical activity, as the main reason for evaporation. $\endgroup$
    – user108787
    Commented Dec 11, 2016 at 13:55
  • $\begingroup$ In physical activity, rate of sweating depends on the intensity of the activity. To simplify the problem, we need to identify the simplest condition first $\endgroup$
    – wander95
    Commented Dec 13, 2016 at 12:12

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