I think it's more accurate so say that air density can affect aerodynamic drag significantly. Air density is a function of atmospheric pressure, temperature, and humidity. All three can have significant effects on air density.

For any volume of gas, there are an equal number of molecules (Avogadro's). When air is humid, water vapor (a gas) displaces dry air molecules. Because (counter-intuitively) water vapor has less molar mass than dry air, the density of that volume of gas decreases the more water vapor there is in it.