Since down here the humidity is high 98% of the time I have been asking this question also. HP goes down with high humidity. The highter the humidity the less room you have for oxygen in the air. Air/fuel ratio would be close to the same so motor will have less oxygen, more fuel & end result is less HP. Blower motors will see a bigger % drop in HP that a natural aspriated motor will. In drag racing where a nitrous motor is racing a blower motor the nitrous guys like high humidy since is does not bother them a much as a blower motor will.