Good thing you asked this question! This is a great example of how physicists set up problems in order to solve them.
On average, the pressure in the air at the surface of the ocean
should be the same around the world. If it weren't, the air would blow
from low-pressure zones to high-pressure zones in order to even things
Now things are more complicated than that. The biggest variations
in pressure in the atmosphere are due to storms. There may be more
storms near the equator than at the poles, giving rise to lower
pressure zones. But then again, there are also pockets of high pressure
in between the low ones, so I cannot honestly say I know that this can
possibly affect the average pressure.
One effect of the Earth's rotation is that the local "gravity" is
a little less at the equator than it is at the poles for two reasons.
One is that as the Earth turns, a centripetal force is needed to keep
an object following the circular path around the center of the earth
(in the rotating frame of reference, this is called the "centrifugal
force") which lightens up objects. Another effect is that the Earth
itself bulges a tiny bit at the equator because of this very effect,
and so an object on the surface is farther away from the center of the
Earth at the equator than at the poles (the Earth is an "oblate
spheroid"). But does this cause a pressure difference at the surface?
Water flows from high spots to low spots, and the Earth is covered
in water. The condition here is that no energy can be gotten from
gravity+centrifugal force by moving water from one place to another on
the Earth to another. So the pressures are all the same for water at
the surface of the ocean.
That equilibrium situation is never seen -- it is modified by the
fact that water does in fact circulate around the Earth, a feature of
convection. Water of different temperatures and salinity will be more
dense or less dense, and the heating up of water near the equator will
set up gigantic convection currents in the ocean moving water from one
place to another. In addition, the tides keep water sloshing around
from one place to another, creating local pressure differences (which
translate into water height differences), particularly around tight
spots such as the opening to the Mediterranean sea and places where
rivers meet the ocean. But the surface pressures are all the same (it's
just that the water surface moves up and down).
If we are above sea level, then the altitude is an important
factor. Right at the South pole is land covered with ice, so it is
actually quite a bit above sea level. The North pole is also covered in
ice, which is floating on seawater; the surface of the ice there is
probably at a lower altitude than the surface of the ice at the South
pole. Around the equator there are deserts, oceans, and mountains. The
air pressure high up the side of a mountain will be less than at the
base, so depending on where you stand, the air pressure at the equator
can be any of a great many numbers.
(republished on 08/02/06)