Humid air is less dense than dry air. Moving from a Dry condition to a Humid one is technically 'asking' you to lean the condition. The humid air is harder to pull the airframe through as well, so a hot humid day isn't the day to be over compensating with the needles on a more heavily loaded engine. Moving a properly tuned engine from a humid environment to a dry one, would 'ask' you to richen the condition. Within reason, temperature has less of an affect on my well tuned engines, than moisture does, in the air.