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Imagine there are two rooms kept at the same temperature but with different humidity levels. A person is asked to stay in each room for 5 minutes. At the end of experiment if we ask them which room was hotter, they will point to the room with the higher humidity. Correct right?

How does humidity cause this feeling of hotness?

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When the ambient humidity is high, the effectiveness of evaporation over the skin is reduced, so the body's ability to get rid of excess heat decreases.

Human beings regulate their body temperature quite effectively by evaporation, even when we are not sweating, thanks to our naked skin. (This, supposedly, is also what made it possible for early hominids to become hunters by virtue of being effective long-distance runners.)

Humans are so good at this, we can survive in environments that are significantly hotter than our body temperature (e.g., desert climates with temperatures in the mid-40s Centigrade) so long as the humidity remains low and we are adequately hydrated. (Incidentally, this is also why we are more likely to survive being locked in a hot car on a summer day than our furry pets.) In contrast, when the humidity is very high, even temperatures that are still several degrees below normal body temperature can be deadly already.

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    $\begingroup$ Is there a typo in the last sentence? $\endgroup$ – Virgo Jul 27 '15 at 5:21
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    $\begingroup$ I don't believe so, why? $\endgroup$ – Viktor Toth Jul 27 '15 at 5:22
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    $\begingroup$ I am sorry but I don't know what you mean. To paraphrase what I am saying above, at low humidity, $T\gg 37^\circ$C is survivable; at high humidity, never mind $T>37^\circ$C, you can already die at $T\lesssim 37^\circ$C. Not sure what "almost/just as deadly" would refer to, as the only deadly thing mentioned in my answer is $T\lesssim 37^\circ$C, combined with high humidity. $\endgroup$ – Viktor Toth Jul 27 '15 at 5:35
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    $\begingroup$ Your point is perfectly clear, as is the whole excellent answer. It's the "already" that looks a bit odd grammatically. Just remove it. $\endgroup$ – Matt Thrower Jul 27 '15 at 8:13
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    $\begingroup$ I'd reword that sentence to "In contrast, when the humidity is very high, even temperatures that are several degrees below normal body temperature can be deadly." The way it's written now, it initially seemed to me like you were saying low temperatures could be deadly at high humidity. (E.g. 34 C could be deadly, 15 C would definitely kill you.) Hence the confusion. $\endgroup$ – Ajedi32 Jul 27 '15 at 13:42
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It because the higher humidity makes it more difficult to cool the body. Even though the room temperature is below body temperature, you generate more heat than needed to maintain your body temperature. To help cool you down, you sweat, and the water evaporates from your skin. The evaporation of the water cools you down.

When the humidity is higher, the sweat does not evaporate as fast, so your skin temperature goes up. This makes you think the temperature is higher when the humidity is higher.

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For the thought experiment in your question, the answer is: it depends on the temperature of the room!

Cold Room

If the room is cold, humidity will actually make it feel colder! This is because water vapor has a much higher heat capacity than dry air, meaning that it takes more heat to raise or lower its temperature. So a volume of air with a lot of water vapor can transfer a more heat to (or from) your skin than the same volume of dry air.

Warm Room

If the room is moderately warm (around 27-37°C, I think), it's close enough to your own body temperature that radiative heat transfer is minimal, so the main difference in perceived heat is due to the water vapor's effect on your body's ability to use evaporation (of sweat) to cool itself. This is what the other answers are talking about.

Hot Room

If the room is hot, heat capacity again plays the dominant role. A given volume of high-humidity air contains much more heat than an equal volume of dry air, so it is able to transfer a lot more heat to your body. This is why dry saunas are frequently operated in excess of 100°C, while a steam sauna at that temperature would be instantly scalding!

In a hot room there is also the issue of condensation. If your skin is at or below the dew point, water will condense out of the air onto you. Condensation is an exothermic phase change, so it will transfer additional heat to your skin.

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    $\begingroup$ +1 for the first to mention the effect of temperature. Indeed humidity in Winter ups the hypothermia hazard greatly if you're out in the elements. $\endgroup$ – WetSavannaAnimal Jul 28 '15 at 5:14

protected by Qmechanic Jul 28 '15 at 10:46

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