I live in a town with a lot of humidity in summer. I perspire a lot, but restaurants around here have misters outdoors to “cool things down”. How does that work? How can adding more humidity make the heat more tolerable?
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So there’s two effects working simultaneously here.
Mainly, air has a certain capacity for how much water vapour it can hold. So humid air makes sweating less efficient, as the sweat finds it more difficult to evaporate into air already full of water, and evaporation is how sweat cooling works. It’s also how evaporative coolers work.
The other fact is that water has a far larger heat capacity than dry air for the same temperature. So cool water will drain far more heat from you than cool air, while warm water will make you far hotter than warm air.
So whether water in the air is good or bad depends on if it’s warm or cool. Warm moisture in the air prevents you from cooling down by sweating. And if it’s hot and humid enough, it will dump massive amounts of heat into your skin, while preventing you from sweating. That’s the “wet bulb temperature”, and if it’s above your body temperature, it’s lethal.
But cool moisture added to relatively dry air will make you cooler, like sweating but without the part where dried sweat is sticky. Artificial, clean sweat if you will.
On the other hand, humid winters also feel much colder than dry winters, for this reason. The water in the air will happily infiltrate all your clothes and sap the heat from you. And if you try to wrap yourself against it, you’re trapping your own sweat. It’s nasty and almost as bad as hot, with the one difference that it’s easier to insulate and heat yourself than cool down. You can wrap against cold, you can’t tear your skin of against heat.
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