[Phys-L] Re: A "simple" physics problem?

[Robert Cohen <Robert.Cohen@PO-BOX.ESU.EDU>]

Brian Whatcott wrote:
> > > I may be missing something.  Moist air is less dense:
> > > its columnar weight is less than dry air, so a water
> > > level beneath it would rise.

to which I responded:
> > Assuming the pressure is the same.  As you point out, the
> > surface air
> > pressure is closely related to the total columnar mass. Evaporating
> > water into the column will *increase* the air pressure, won't it?

to which Brian Whatcott responded:
> I used the idea of a surface air pressure being calculable from
>   the gravitational pull on the mass of the column of air out
> to vacuum - which I called its weight.
>
> I asserted that water vapor is lighter than dry air, which is
> true - other things equal - so I implied that moist air tends
> to go with lower air pressure, and hence  higher water level
> in the region.
>
> So I say again, adding humidity tends to decrease air
> pressure. But I will listen to reasons why you are right!
> :-)

Is it appropriate in this case to say "other things equal"?

Imagine that the earth is covered by water.  If all of the water
warms up and thus increases evaporation, does the surface air
pressure go up or go down?

____________________________________________________
Robert Cohen; 570-422-3428; www.esu.edu/~bbq
East Stroudsburg University; E. Stroudsburg, PA 18301
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