Q & A: Pressure around the world

Q:
Where is there more pressure on the equator,in the south pole or in the north pole.
- Lynn (age 12)
Sagesse High School, Lebanon
A:
Hi Lynn,

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 out.

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.

Tom

(published on 10/22/2007)