"Absolute" vs "Relative" humidity

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Muzhik
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"Absolute" vs "Relative" humidity

Postby Muzhik » Mon Jan 28, 2019 12:11 am UTC

Greetings! On another forum I'm trying to explain to another user the difference between "absolute" humidity (the amount of moisture by weight in a cubic meter of air at sea level pressure) and "relative" humidity (the percentage of moisture in a cubic meter of air based on current temperature and air pressure). The original question was why it was equally or more important to remain hydrated in cold weather (such as the center of the US is or will be experiencing) compared to when it's hot and humid out (as some forum members are in Australia and are dealing with drought.)

After I composed a very thorough and (I thought) clear explanation, the OP replied "Thank you for the explanation, you are very kind, but I still don’t understand. I’ll have to go and look at some stick figures somewhere." Of course, at the mention of stick figures, I thought of xkcd, but without being able to find a specific stip on the subject, I thought I'd turn to my fellow geeks for an explanation.

I'll post my original explanation here if you want to see it and use it as a jumping off point.

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Sableagle
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Re: "Absolute" vs "Relative" humidity

Postby Sableagle » Mon Jan 28, 2019 1:06 am UTC

It's 1am and I just glanced this way, so this may be b_____ks, but if you're inhaling very cold air even if it's saturated it contains very little moisture, because cold air has such a low moisture capacity. By the time it's done the gas exchange thing in your lungs, it's warmed up. Warmer air can hold a lot more moisture. That makes its relative humidity very low, so a lot of water can evaporate from the lining of your lungs and throat and into that air while you're breathing it back out.
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Eebster the Great
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Re: "Absolute" vs "Relative" humidity

Postby Eebster the Great » Mon Jan 28, 2019 3:14 pm UTC

If I evaporate an ounce of water in a cubic yard, I have an absolute humidity of one ounce per cubic yard. It's just how much water vapor there is. Sometimes it's cold and some of that water condenses and forms dew. That means the relative humidity is above 100%. But sometimes it's hot and the water doesn't condense, and even if I put more water in it will evaporate. That means the relative humidity is below 100%. In general, when water or ice dries up, humidity is below 100%, and when dew or frost forms, it's above 100%. If everything just stays the same, the relative humidity is exactly 100%.

Since more water will dry up if it's hotter, there needs to be more water vapor before it's at 100%. That's what makes it "relative," because it's relative to the temperature. Technically speaking, air has very little to do with it; the main effect is just the vapor pressure of water. You would get similar results in a vacuum.

By the way, if you keep the absolute humidity constant, the temperature at which relative humidity is 100% is called the dew point (or frost point if the temperature is below freezing). So for instance, if the absolute humidity is 12 g/m3, then the dew point is 13.8 °C, meaning if it is warmer than that, water will dry, and if it is colder than that, dew will form.

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SuicideJunkie
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Re: "Absolute" vs "Relative" humidity

Postby SuicideJunkie » Tue Mar 19, 2019 8:14 pm UTC

Muzhik wrote:The original question was why it was equally or more important to remain hydrated in cold weather compared to when it's hot and humid out.
After I composed a very thorough and (I thought) clear explanation, the OP replied "Thank you for the explanation, you are very kind, but I still don’t understand.

-On a cold day, you can see the moisture condensing from your breath, as the air can't hold all that moisture outside.
-You are breathing in non-foggy air, which obviously has less moisture floating in it.
-You're breathing out lots more moisture than you're breathing in and thus will dry out.
The idea of freeze-drying isn't the same thing, but could get the point across.

By comparison, on a hot day, you sweat out the moisture to try and stay cool. If it is humid, sweat becomes inefficient and you sweat even more to make it worse.

Different mechanisms, but the same problem of dehydration, and it sounds like it will catch them by surprise since they haven't grown up with it.


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