system
Given the following post, respond as if you're commenting on Reddit:
human
How do you measure humidity?

I always hear on the news that today has 75% humidity or whatever, but I have no idea how it is calculated. First thing that came to my mind was that the percentage represented the percentage of H2O relative to air, but that doesn't seem right. I imagine that if something is by volume 75% H2O(g), and the other 25% is air, it would be a lot worse than what I feel outside. Anyone care to enlighten me? Thanks :).
gpt
Typically the "humidity" reported is a type of humidity referred to as relative humidity - this is a measure of how much water vapour is present in the air compared to what the air could hold before it became saturated (100% humidity). When air saturates you get cloud or fog (its a little more complicated than that, but essentially).

Hot air can hold a lot more moisture than cold air, so 98% humidity in say 30˚C/86˚F feels much more uncomfortable than 98% humidity in 10˚C/66˚F. Above 36˚C/97˚F the air is almost impossible to saturate...

Another type of humidity is called absolute humidity, which is the amount of water present compared with the volume of dry air (dry air is made up of a number of things, mainly nitrogen, and oxygen, and a small amount of carbon dioxide, argon and other trace gases), but I've never really seen this used.