If it’s 80% humidity outside you may assume it’s quite damp, but what does the 80% translate to in numbers? I cant imagine 80% of the air to be moisture since it’s mostly nitrogen. And how is it calculated/measured?
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You have to look at how much water the air can hold. This is greatly dependent on the temperature.
Take the temperature outside and see how much water the air can hold in the air. The [vapor pressure](https://en.wikipedia.org/wiki/Vapour_pressure_of_water) is the maximum amount of water vapor in the air. The relative humidity takes the *actual* amount of moisture in the air and gives it to you as a percentage of the maximum.
Example. It is 68 degrees F outside here in Minnesota. Looking at an [online vapor pressure calculator](https://www.omnicalculator.com/chemistry/vapour-pressure-of-water) and converting the answers to “atmosphere” (normal total air pressure is 1 atmosphere by definition). Then we see that the maximum percentage of water vapor that the air can hold is 2.3%. So still over 97 percent of the rest of what air contains like Nitrogen and oxygen and stuff. If the relative humidity is 80% that means that actual amount of water value in the air is 80% of 2.3% which is 1.8%.
It’s easier to relate to relative humidity than it is the actual absolute percentages which are always less than 5%.
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