After you take a shower your hair is still wet. When there is humidity there is more water vapor in the air. So the air around you carries more water. If there is little to NO humidity your hair dries faster because there is less water vapor in the air and the wetness of your hair can escape faster in the form of water vapor. But in humidity the water from your hair needs to "fit into the air" which takes longer because of the WV already in the air. I hope this helped:-)!