smoke, air density, and convection

[superduper12]

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dry air/day: smoke from chimney rises as it leaves the chimney
damp air/day: smoke from chimney falls as it leaves the chimney

so for a dry day: dry air is more dense than the smoke: smoke< dry air
for a damp day: damp air is less dense than the smoke: damp air < smoke


so the trend for density is: damp air < smoke<dry air

why is damp air less dense than dry air...shouldn't it be more dense because there is more water vapor in damp air.

 

[engineeredout]

Molecular weight of water = 18g/mol. Molecular weight of air = 29g/mol. If there is more water vapor in the air it will displace air itself, leading to lighter air. At STP, 22.4 liters of dry air will weigh 29g. At STP, 22.4 liters of damp air will be less than that.

 

[superduper12]

ok got it, for some reason I thought air and water were close in MW.


ahh...so air has a higher MW, but since it occupies a much greater volume, it has a lower density...thanks.