How does water vapor affect air pressure?

Question

How does water vapor affect air pressure?

Answer 1

The effect of water vapor is evident within an air mass.  Parcels of air that contain water vapor within a drier air mass are more buoyant and tend to have less air pressure and tend to rise.  You can see evidence of this on a sunny day by looking at the scattered cumulus clouds.  These clouds are a result of a parcel of air that are moister (contain more water vapor) than the surrounding air.  They rise, and when they reach a certain cooling temperature, the water vapor in the column of rising air condenses into the clouds you see.  

The following is an explanation from wikipedia.  At the same temperature, a column of dry air will be denser or heavier than a column of air containing any water vapor. Thus, any volume of dry air will sink if placed in a larger volume of moist air. Also, a volume of moist air will rise or be buoyant if placed in a larger region of dry air. As the temperature rises the proportion water vapor in the air increases, its buoyancy will become larger. This increase in buoyancy can have a significant atmospheric impact, giving rise to powerful, moisture rich, upward air currents when the air temperature and sea temperature reaches 25°C or above. This phenomenon provides a significant motivating force for cyclonic and anticyclonic weather systems (tornado's and hurricanes).

Answer 2

Air pressure at a place is the weight of the overlying column of air over a unit place at a particular level.Moist air(air containing watr vapour) is lighter than dry air.So the air column containing moist air will have comparatively less weight and hence less presure.

Hence,the presence of water vapour in air over a particular place reduces the air pressure at that place.