We all know that when our sweat evaporates to the air we feel colder, but since this process is a latent heat transfer our body temperature will not change, then if our body temperature is constant how do we cool down?
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2$\begingroup$ "but since this process is a latent heat transfer our body temperature will not change' I don't understand your reasoning. Latent heat changes produce tremendous thermal changes in the adjacent materials. $\endgroup$– ChemomechanicsCommented Jul 4, 2021 at 5:51
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$\begingroup$ Because a latent heat transfer is a process where heat is supplied from a body or absorbed by a body without changing its temperature. $\endgroup$– Carlos WerbockCommented Jul 4, 2021 at 5:55
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3$\begingroup$ When evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling. from;en.wikipedia.org/wiki/Evaporation $\endgroup$– Adrian HowardCommented Jul 4, 2021 at 5:56
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2$\begingroup$ Boiling of water takes place at a constant temperature (of 100°C) but evaporation of sweat is not the same as boiling and it can result in a temperature change. $\endgroup$– John RennieCommented Jul 4, 2021 at 7:01
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1$\begingroup$ "Because a latent heat transfer is a process where heat is supplied from a body or absorbed by a body" This may be the source of confusion. The "body" or system here is the water, which doesn't change temperature as it evaporates. This cools down your literal body. Identifying and distinguishing the different systems here is essential. $\endgroup$– ChemomechanicsCommented Jul 4, 2021 at 15:02
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2 Answers
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Evaporation occurs at all temperature and it that way it is unlike boiling.
Consider your body as the system with some water (sweat) on the skin.
On average the faster molecules in the sweat "evaporate" lowering the average kinetic energy of the molecules in the sweat and so the temperature of the sweat decreases.
As the sweat is cooler than the skin heat is transferred from the skin to the sweat.
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To expand slightly upon Farcher's answer:
At any air temperature at which it is a liquid, water molecules will be striving to evaporate from that body of liquid at a rate which depends on the ambient temperature (higher means faster) and on the relative humidity (i.e., moisture content) of the air at that temperature (lower humidity means faster). In so doing, they will carry away the latent heat of vaporization from your body and deliver it to the surrounding air.
So for example if your skin and the air surrounding it is at 75F and low relative humidity (as in a desert) and it is covered with a film of water (as sweat), then your skin will cool down by evaporation.
The cooling effect (delta T) you can achieve by this method is thermodynamically limited to the dew point temperature of the air, at which your skin is so cold that the relative humidity of the air at that cool temperature is 100%, and net evaporation ceases.
In practical terms, you can never achieve that number, and the usual figure for delta T assuming desert conditions (<10% relative humidity) is about 20F.
answered Jul 4, 2021 at 16:34