system
Given the following post, respond as if you're commenting on Reddit:
human
Why is high barometric pressure indicative of good weather and low indicative of rainy weather?

Isn't an atmostphere with more water vapor within it more dense than the one without?
gpt
Actually its quite the opposite!  You may be thinking that water vapor is "added" to dry air which would increase the mass of a particular parcel of air but instead you should think of it as displacing the dry air instead.  As it turns out, Water Vapor is actually less dense than a parcel of completely dry air.  So the more water vapor you add to a parcel, the more buoyant it becomes.  If you think about the components of dry air and water this becomes a bit more evident.  A molecule of water is made up of 2 hydrogen atoms and a single oxygen atom.  Dry air is basically (not including trace components here and doing some slight rounding) made up 80% of Nitrogen (molecules of 2 Nitrogen atoms) and 20% Oxygen (molecules of 2 Oxygen atoms).  Both Nitrogen and Oxygen molecules weigh far more than a water molecule due to the very low atomic weight of hydrogen.  The molar mass of water vapor is ~.8g/mol while the molar mass of dry air is ~1.25g/mol.  So when that water vapor replaces other molecules in dry air, its easy to see why the density drops.

As a parcel of air rises, the water vapor will condense out due to the lower pressures and dropping air temperature.  However, condensation of water releases energy into the atmosphere which means the temperature of a more saturated parcel of air will drop more slowly as it rises and allows it to rise farther and faster.

So that addresses how air with a higher water vapor content rises but it doesn't really address your initial question about barometric pressure.  Areas of high pressure are generally associated with air that is sinking while the opposite is true for areas of low pressure.  Because rising air is necessary to form clouds and in turn form precipitation, areas of low pressure have a higher incidence of rainy weather.

That being said, low pressure isn't going to produce rainy weather if no water vapor is present, so the barometric guidelines are just used in a general sense.