How does the water take heat from the surface of our skin in the process of changing into water vapour? Does the water take heat out of the surrounding air above our skin as it changes also?
In: Physics
Going away from sweat for a minute:
Put a pot of water on the cooker and boil it. Once boiling the water is 100C, but it doesn’t all turn to steam immediately. That is because it takes a LOT of energy to turn liquid water into water gas. Significantly more than the energy required to heat it to start with. It takes more energy to go from 100C water to 100C steam than it does to go from a block of ice just above absolute zero all the way to water at 100C.
Water doesn’t need to be at 100C to transition to being in gaseous form, it just needs to not be in an equilibrium state with moisture already in the air. As long as the surface of your skin is warmer than the dew point of the water vapor at that temperature in the air some water will evaporate from you, taking a lot of heat with it as it transitions from liquid to gas, called the latent heat of vaporization.
The less moisture in the air, or the colder the air to start with relative to your skin, the more easily this process happens, which is why a “dry” heat is more bearable than an equivalent humid heat, and also why a dry-cold feels a lot colder than the equivalent temperature when the humidity is high.
Well, not a qualified response, but here goes.
moisture (or water) is a good absorbent of heat, so as you sweat, the moisture absorbs the heat away from your body and to the surface of your skin, where the resulting moisture then evaporates.
Of course this only works in environments where the surrounding air is drier than the moisture of your body. While the surrounding air will become more moist as the water evaporates, so you need a breeze to move the moist air away.
This does not work in HUMID environments, where the moisture cannot evaporate as efficiently. Which is why Evaporation Air conditioners do not work, in HUMID environments, and why you need a door or window OPEN, to allow excess moisture in the air to escape.
Refrigerated Air Conditioners work by REMOVING humidity from the surrounding air, and cooling the return air, making the air more drier, and allowing your body to remove the excess moisture more quickly. The trade off is they use MORE power than ‘Evap’ air conditioners.
The water takes heat to increase in temperature and additional heat to phase change into water vapor.
Water is made of molecules, and the different molecules all have different energies. Hotter water has molecules which are generally more energetic. This energy allows molecules to escape the liquid water, becoming a gas. However, the force holding them to the liquid is significant, and so they slow down as they escape, cooling them off.
So, the higher-energy water molecules escape the liquid, cooling it off. These molecules then lose energy as they escape the water, becoming a relatively cool gas.
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