visibility3miles
Humidity affects density [altitude?] too, as water molecules weigh less than nitrogen or oxygen molecules
Yes indeed, but remember that the maximum amount of water vapour strongly depends on the temperature :
The maximum mass fraction of water in saturated air is below 5%, in which case the density of the mixture will only be 2% less than the density of dry air in the same conditions. This figure will drop to 0.5% at 20 deg C.
No big influence on aircraft performance at ground level (and nothing at all at high altitude)