if we could magically shut down human sweating process when humidity is 100% and presumably no evaporation from the skin can occur, would that be somehow harmful? In other words, is there actually any use in sweating in such conditions or does it happen solely because our body does not know how to measure humidity and evaporation rates?
Sweating is the body’s way of cooling when getting overheated. If you were to shut this down, either the body would need to figure out another way to cool (like panting like dogs do, or maybe excessive peeing), or people would be more prone to heat stroke/heat exhaustion.
Your body doesn’t start sweating in high humidity “just because” - it’s because your body is trying to protect itself from overheating.
I think you misunderstand, and that the OP is asking whether sweating in 100% humidity actually provides any cooling.
If humidity was exactly 100% humidity, there’d be no benefit to sweating. The old psychrometers have exactly the same temperature on the dry and wet bulbs at 100% humidity.
However… humidity is very rarely 100% when it’s warm outside. The warmer air gets, the more water it can hold. To stay at 100% humidity as the temperature increased, evaporation from the environment would have to be fast enough to keep the air saturated. If it was 100% humidity at the hottest part of the day, you’d see condensation before sunset.
So you’re much more likely to be dealing with humidity somewhere below 100% when it’s warm. Thus, sweating is better than nothing in virtually all natural circumstances.
During the day, that’s very likely true.
However, on a steamy, humid night, the relative humidity might well get very close to 100%, if not at 100%.
I don’t look at relative humidity, I look at dewpoint. Yesterday around here, the dewpoint was 79 degrees F. My skin was warmer than 79 degrees, so even though I was a sweaty mess, some evaporation was occurring. I think you might find dewpoints higher than skin temperature only on very rare occasions in some tropical jungle.
Why not? Sweat removes heat from inside our bodies, to outside.
I remember being taught in grammar school that it was the evaporation of the sweat, but I don’t believe that is true.
(And no, I don’t have a cite; I probably hear it on NPR years ago.)
What you’re talking about would remove heat without changing temperature, though.
If I pour half a kettle of boiling water into a teapot, I have removed half the heat from the kettle. But it’s still boiling (or nearly so).
If sweat doesn’t evaporate, it’s not going to help change your temperature.
I’m pretty sure you can believe it: evaporation occurs because the moisture on your skin obtains enough energy (heat) from your body to attain escape velocity and leave your skin, taking that little bit of energy with you and leaving you a tiny bit cooler. Repeat several hundred thousand times, and you don’t die of heat stroke. 
Have you been to Florida?
I’ve experienced many 100% humidity days, usually in the height of hurricane season, which is June 1 - November 1. Pretty much any place in Florida is “warm” from June through November.
Or a cornfield in Tennessee after a morning rain?
Back to the OP- if there is some sort of air movement across your sweat covered body, some evaporation would occur, even at 100%.
Of course, but an equal amount of condensation onto your body will occur, gaining you nothing.
Only if your skin temperature is less than the dew point.
You don’t need to go to FL, just to the east coast. It’s 94% humidity and rising right this minute - really, it’s 1% up from 15 minutes ago - and I’m over 1400 miles from FL. 96-99% humidity is pretty common most places on the east coast in July or August.
Equilibria work both ways. Even if there is net evaporation or none at all, condensation still occurs. However, if your skin temp is below the dew point, the rate of condensation will be greater than the rate of evaporation. But your post leads to some scenarios to consider.
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Air temp = 98.6 °F, 100% humidity. Surface water on your skin is in equilibrium w/atmospheric water vapor. Rate of evaporation = rate of condensation
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Air temp > 98.6 °F, 100% humidity. Evaporation < Condensation. Atmospheric water condenses on your skin and warms you up :eek:
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Air temp < 98.6 °F, 100% humidity. The water on your skin doesn’t have anywhere to go. However, contact between the air and your skin will warm the air directly around you (cooling you slightly), allowing a small amount of evaporation (cooling you some more). Move that warmer, saturated air away from you (a fan?), and you can get cool.
Looks like in (2) you’re hosed. Sweating won’t reduce your temperature. However, drinking cool water and excreting hot aqueous liquid through various mechanisms will help you stay cool.
Put your :dubious: on when reading this; it’s not my specialty.
With respect, your analogy is flawed; we are not tea-kettles.
For one thing, we produce our own heat, and we have several means for removing it from our bodies, most of which involve water.
Furthermore, our heat is not evenly distributed throughout our bodies; our core is higher. Core heat can be transported by blood to our surface, where it dissipates through our skin, but this is not effective enough sometimes.
Heat on the skin (in sweat) is not the same as heat within the body; the primary purpose of sweat is to transfer heat out of the body.
And none of you are going to believe me, I am going to waste hours fruitlessly trying to find a cite, and, if I ever do find one it will be in February and no-one will care.
I did a google search of “at what temperature humidity is sweating ineffective” and was surprised by the results.
Essentially it looks like that at 75% humidity, sweating becomes rather ineffective at cooling the body, as it just rolls off the skin and doesn’t evaporate anymore.
I really would have thought that sweating would be effective until 100% humidity, and even then still be somewhat effective simply due to the massive amount of moisture that warm air can hold.
[quote=“dracoi, post:8, topic:547659”]
The water in the kettle is still boiling. But the temperature of the whole region of space occupied by the kettle (or I guess the average temperature of that region?) has gone down quite a bit, hasn’t it?
No it hasn’t. When a kettle is filled with boiling water, any excess space is occupied by steam.
But let’s not get into the finer points of kettle emptying because it’s just a metaphor meant to illustrate the underlying physics. If the metaphor didn’t help explain it, hopefully some other poster can come along with an explanation that will make it more clear.
If the surrounding air is cooler than your skin, some theoretical evaporation can occur - or, what j66 said.
However, as one who grew up in hot and humid weather (including FL), I can assure you that, if the relative humidity is over 80% on a hot day, that evaporation is going to be minimal. This is why Og invented clothing - it soaks up your sweat LONG before the evaporation under those conditions possibly could. If you refuse to wear clothes, then the sweat will run down or fall off your body.