Ummmm.... humidity hurts FE and robs power from the engine.
This is the only reference I can find at the moment.
"Fog and humidity affect the performance of aircraft. During takeoff, two things are done to compensate for their effect on takeoff performance. First, since humid air is less dense than dry air, the allowable takeoff gross weight is generally reduced for operations in areas that are consistently humid. Second, because power output is decreased by humidity, pilots must compensate for the power loss."
SPECIFIC HUMIDITY
Not only for that reason but it also decrease the specific heat ratio of the combustion gases.