How is evaporation related to humidity?
Well, the rate of course...
I'd say that the rate of evaporation is inversely proportional to the rate of humidity. This is because humid air contains lots and lots of water molecules, therefore forming a region of higher concentration. When water evaporates, the water molecules want to move away from each other and into the air, but since there is a region of higher concentration there, it is unlikely that they'll try to "fit into" that area. That's because of diffusion.
Diffusion states that molecules will move down a concentration gradient to maximize equilibrium between two sides.
I hope this helps, otherwise check these answers: