humidity does affect peformance--INDIRECTLY.
the only real thing that matters is whether or not the air is hot or cold.
hot air has the ability to hold more moisture, thats all. cold air holds very little moisture and sometimes none at all. just because air is hot though doesnt mean it has to be humid..there has to be a water source present for the air to 'suck' up that moisture (sorry if this is confusing).
like i said though..the only thing that matters is whether the air is hot or cold. to go a bit further in details..it's only OXYGEN that really makes a difference. the more oxygen you can stuff into your engine the better it performs because it will mix with more fuel to make more power.
hopefully this can explain what im talking about: basically..hot air is bigger (if you heat air, it expands and the molecules are spaced further apart). cold air is dense (if you cool air, the molecules cluster closer together). air is made up of about 18% oxygen. that means for every 100 molecules of miscellaneous air molecules, 18 of those molecules will be 02.
now, think about your Intake
). it has a fixed volume--it only has so much space that air can be transferred to the engine in, no more, no less. so obviously if the air is colder it is therefore denser and therefore more air can fit into your Intake
which therefore means more oxygen gets into your Intake
which therefore means you can burn more fuel which then makes more power. the opposite is true of hotter air (hot air means 'bigger air' which means less can fit into your Intake
which means less oxygen gets into your Intake
which means less fuel is burnt which means less power)
ah, i'm gonna shutup now. i'm sure i confused enough people with that but i hope you got what i was talking about