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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Relative humidity is a comparison between the current moisture content of the air as a percentage of the maximum moisture content possible at that temperature/pressure.
it’s not that there is 80% gaseous water + 20% atmosphere. It means that air (the mixture of oxygen, nitrogen, etc) can transport a certain amount of moisture. Think of it like a glass if water can take a certain amount of salt withiut you seeing any. 80% means that the current water level in the air is 80% of whats the maximum of water the air can contain at tis temperature. The same amount of water would yield a higher percentage if the air was cooler or a lower percentage, if you heat that air up. This can be seen in condensation: air with a relatively high waterpercentage cools down (eg when in contact with a surface) and isn’t able to hold all of it anymore. The surplus water then gathers in drops you can actually see.
Realative humidity is tricky and this is a great question. Humidity is defined as the amount of water in the air divided by the maximum amount of water that could be in the air.
The place that gets tricky is the last part. The amount of water that can be held in the air is a function of temperature. The warmer the air the more water that can be held in suspension.
This is also related to dew, which is what happens when the capacity for water holding decreases as the air gets cooler and the water drops out of suspension and ends up on the ground.
The same process can produce fog.
https://www.edinformatics.com/math_science/what-is-humidity.html
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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