When the air warms up, it expands. But the physical properties ie the moisture (RH) content cant expand as they are a given density so what happens is they disperse themselves more into the expanded air thus spreading themselves further and lowering the relative humidity. But whilst its worth noting that the humidity is lower the moisture content will stay the same, so once the air cools down again the rh will rise inline with dry bulb temp dropping.
100% rh is when they wet and the dry bulb are of equal temperature, normally chucking it down with rain then.