system
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human
ELI5: What does 100% air humidity mean?

So I know that atmospheric air has a little bit of H2O in the atmosphere, which I assume is what air humidity is - the amount of H2O is in the atmosphere. However why is there a "100% air humidity"? Is there just a cap on how much water can be in air? And what happens if you add more water?

And is 0% air humidity literally just 0 water particles in the air?
gpt
100 % air humidity means that there is as much water vapor in the atmosphere as possible. If you add more, it will start to condensate into raindrops. 100 % humidity does not mean that 100 % of the air is water vapor. How much water there may be in the atmosphere varies with temperature and what other stuff there is in the atmosphere. A really hot atmosphere may contain a lot more water before it reaches 100 % humidity, which is why the air may feel really damp in the tropics for example. If it is really cold outside, like -20 celsius or colder, the air is really really dry, which also causes dry lips for many.