Warmer and more humid reduces aerodynamic drag and tire rolling resistance but destroys power output. If I am going to do a 10 mile time trial, I want it 85-90 F as, humid as possible, and low atmospheric pressure like right after a T-storm. By the end of the race, my power is still good but starting to fade. On long rides, I prefer colder temperatures, certainly below 70F. 60-70F is a nice range.