> > In fact, Will, you have mis-remembered! Humid air is LESS dense than
dry
> > air, which is why the barometric pressure falls with approaching rain.
> > The molar mass of water is 18, compared to 28 for nitrogen and
> > 32 for oxygen.
> >
> AH - HA - Faulty logic: Dont confuse molar mass with density. By your
rationale
> a liter of air should be heavier than a liter of water! This is not
the
case
> because water is denser (more moles per liter than air). As for
barometric
> pressure falling - I am not sure of the mechanism for this but one
possibility
> may be the change of water from gas phase (same density as air in
moles/liter
> but less mass - therefore a cloud in the sky) - to water phase - now
dense
and
> falling out of the sky.
>
> Any physikist - meteorologikist types feel free to correct me.
>
>
> Thomas Wannenburg MD.
> Cardiology, Bowman Gray School of Medicine
> Wake Forest Univ
> twannen@isnet.is.wfu.edu
>
That a CLOUD is heavier than dry air I do not contest -- a cloud
contains droplets of liquid water but humid air
contains water vapor at a low partial pressure, which behaves very much
like an ideal gas, and therefore density is proportional to molar mass.
I stand by my original statement that humid air is less dense than dry
air, and this IS the correct explanation for a falling barometer. Now if Tom
or Will have actual cloud formation in their garage/basement, then
Will's
explanation may be correct after all!
Len Bugel
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