If you're curious as to why that is, the Wikipedia articles on evaporation and vapor pressure are a good start. Basically, liquid water and water vapor will reach an equilibrium once water vapor makes up a certain proportion of the surrounding air. High humidity indicates that the air's water vapor content is at or near that equilibrium point for the current temperature.
If you're curious as to why that is, the Wikipedia articles on evaporation and vapor pressure are a good start. Basically, liquid water and water vapor will reach an equilibrium once water vapor makes up a certain proportion of the surrounding air. High humidity indicates that the air's water vapor content is at or near that equilibrium point for the current temperature.