why does the belt come off every time it rains?

Suspect belt running out of alignment .. Possible harmonic balancer outer ring has slipped out of position .. One of the driven accessories is not aligned properly .. Belt tensioner is poor quality .. Belt is now damaged and needs to be replaced again .. Can spray water on belt while engine is on to see if it's obvious where it runs off AT .. Water just amplifies the problem and let's belt slip off of the *flat backside* pulleys/idlers more easily.