ORIGINAL: Hossfly
Relatively warmer air restricts the number of air molecules entering the engine via a given carb throat. For a given needle setting the engine will be richer. The lower amount of O/2 will require less fuel (leaner needle) thus reducing the power output. A leaner needle will help the running, however the overall power output is still reduced.
Yesterday was the hottest, and most humid, day I've flown so far this year. I did have to lean the engine a bit before flying. I didn't really notice a lack of power, but that's possibly due to the extra power this plane has.