Despite what people say, Air is not 'like a sponge' which can 'soak up' Water. It does appear a bit like that, though, which is where the confusion arises.
Water vapour is just another gas which is part of the mixture of gases which make up air. The difference is that liquid and gaseous water exist in significant amounts at the temperatures we have on the Earth's surface so water appears to behave a bit differently.
Pressure in a mixture of gases is due to the sum of the individual pressures of all the different gases. If you take a sealed container with just water in it with a space over the top of the water surface, the liquid water will evaporate until the pressure of the vapour on top is equal to the vapour pressure of the liquid below. This will involve a certain mass of water in its vapour phase in order for this equilibrium pressure to be reached. The vapour pressure is, of course, very dependent upon the Temperature. At room temperature, the pressure in this container would be Atmospheric Pressure, after which, despite having started with 'a vacuum' over the water surface, no more water would 'boil off'.
If there happens to be air in the top of the container then you need less mass of water 'up there' to balance the vapour pressure of the water because the air gases are contributing to balancing the pressure.
SO... in atmospheric air, the amount of water vapour over a water surface (in an equilibrium condititon) will depend on the temperature and the air is described as 'saturated' because lowering the temperature will result in water condensing back onto the surface and raising it will cause more water to evaporate.
For air that is not close to a water surface there is not a mass of nearby water to supply extra molecules so the total pressure in the air is less than the vapour pressure of water at that temperature. Warm air will feel 'dry' because any liquid water (sweat etc) will have a high enough vapour pressure to evaporate quickly. The 'humidity' is just due to the difference between the pressure in the air and the vapour pressure of water at that temperature. If the temperature is low enough to reduce the vapour pressure to below that of the air, you will get condensation / rain. This is all 'independent' of the presence of the air - except in as far as the air molecules 'fill up' much of the volume of the air.
Unfortunately, I have gone and ruined all your future lectures because things are seldom presented in this (correct) way. Just go along with what you are told - it works ok in practice - and avoid being a smartarse with your lecturer by trying to put him right. He will HATE you for it!
