- water vapor in the mix acts as a coolant, both to the charge itself, and to the cylinders. That has the same effect as a cooler day (denser mixture charge). It also evaporates to steam, increasing the cylinder pressure on the power stroke, or adding "reaction mass" to a jet's exhaust.
That may well be true in racing, where *liquid* water is injected into the engine, but has no bearing at all to "humid days". The water in the air is already in *gas form* and cannot evaporate any further. Unless you confuse "humid" with "mist" or "fog".