When you have a hotter and/or more humid environment the molecules needed for combustion are spread further apart (less dense) so you need to lean your needles to get the optimum ratio again. The opposite for cold dry environments.
Temperature has the largest effect if I remember correctly, not humidity. Someone correct me if I am wrong, I haven't flown full scale for 20+ years or been in school for longer than that.
Your plane will also have to fly faster to create the same amount of lift. If you have ever flown full scale you can look at graphs/charts that show you take off/landing distances, mixture leaning etc. d/t temp and elevation changes.