Barometric pressure in Philly 28.29 ??

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12. Are you sure? I've always understood it to be based on atomic weight

And it is the way we studied it in Meteorology and flight school. A barometer, along with weather balloons, can test surface and aloft air pressures, giving an indication of how much water vapor is above a location. And since water vapor is lighter than air, changes in barometric pressure readings over time, can indicate an increase or decrease in water vapor.

Water vapor - H2O - is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 18 atomic units. With 18 atomic units water vapor is lighter than diatomic Oxygen with 32 units and diatomic Nitrogen with 28 units.

Note! Water vapor in air will replace other gases and reduce the total density of the mixture. Dry air is more dense than humid air!

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