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



Brian Whatcott wrote:

I first imagine I am on Earth, where the water warms up.

I lack the model for the notoriously problematic dynamics of
the atmosphere, but still, I hazard the guess: the pressure
over oceans drops, while moist airmass circulates to dry
land, which sees increased precipitation, and higher surface pressure.

I next attempt to visualize a water world whose water mass
warms throughout. Increased evaporation, reduced integrated
atmospheric density for that reason. Now, does pressure rise,
or fall at sea level? Not sure: what's your view?

My view is that the pressure rises at sea level.

Here is my take on the problem: with increased evaporation
into the air, the air pressure will increase over the oceans
(due to added mass in the air).

That then leads to divergence away from the oceans and
convergence over the land.*

Once the pressure equalizes, the density of the air over the
oceans will be less** than that over land (by some small amount)
because the average mass per molecule is less.

-----

*Getting back to the original discussion, if the equilibrium
process is slow, I suppose the oceans will be pushed downward
but it seems that wouldn't lead to a sea-level change unless
the oceans are somewhat compressible.

**Using the ideal gas law (PV=nRT), if the temperature and
pressure are the same, the number of molecules per volume
(n/V) is the same. Since the air over the ocean contains
a higher percentage of H2O molecules and a lower percentage
of N2 and O2 molecules (compared to over land), the mass
per volume must be less.

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