I am looking to find the reason: why air pressure decreases with altitude? Has it to do with the fact that gravitational force is less at higher altitude due to the greater distance between the masses? Does earth’s spin cause a centrifugal force? Are the molecules at higher altitude pushing onto the molecules of air at lower altitudes thus increasing their pressure? Is the earths air pressure higher at the poles than at the equator?
Atmospheric Science – Why Does Air Pressure Decrease with Altitude?
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Air pressure is the intrinsic pressure in a quantity of air. It can come from any number of sources. Perhaps there is a closed cylinder of air with a piston compressing it isothermally. Or perhaps the air in the tire in your car is under pressure due to the weight of the car and the surface tension of the tire.
Atmospheric pressure is air pressure due solely to the weight of the air above you in the atmosphere. It is typically around $10^5\ \mathrm{N/m^2}$ on Earth, but can vary with altitude and weather. It is just a "natural," ubiquitous special case of air pressure in physical situations taking place inside an atmosphere.
Note that this is a very subtle distinction, and interchanging the terms "air pressure" and "atmospheric pressure" should cause absolutely no confusion in practice.
There are a couple of ways the atmosphere can be viewed as being "thicker" at the equator than at the poles. One sense is the mass per unit area of all of the air overhead, from sea level to outer space, at the equator vs at the North Pole. Atmospheric pressure at sea level, being essentially a potential, is on average pretty much constant worldwide when averaged out over the course of years.
Since atmospheric pressure is the weight per unit area of all of the air overhead, those places where acceleration due to gravity is low must necessarily support more air in terms of mass to achieve the same pressure than areas where gravitational acceleration is high. Gravitational acceleration at sea level is a bit less at the equator than at the poles, making the atmosphere at the equator a tiny bit "thicker" than the atmosphere over the North Pole, by about half a percent.
This is a tiny effect. It is not what people mean when they say the atmosphere is thicker at the equator. The tropopause is considerably higher over equatorial regions (more specifically, over the intertropical convergence zone, or ITCZ) than it is over polar regions. The tropopause can 20 km above sea level or higher at the ITCZ and only 4 km above sea level at the poles. The boundary between the troposphere and stratosphere is essentially non-existent at the South Pole during parts of the winter.
The small decrease in gravitation (including centrifugal acceleration) from the poles to the equator accounts for only a tiny bit of that large change in troposphere height. In fact, Of the five reasons listed in the question, none is primary. The key reason for the tropopause height reaching a maximum at the ITCZ is the general circulation of the atmosphere, with the ITCZ itself playing a key role.
The ITCZ is where the moisture-laden northern and southern hemisphere trade winds meet at the surface, are driven to great heights due to convection and latent heat, and finally separate to flow northward and southward near the top of the troposphere. It is the strong convection and associated very strong thunderstorms created by this meeting of these atmospheres that pushes the tropopause to great heights at the ITCZ. Tropical cyclones and CAPE-driven thunderstorms elsewhere can also push the troposphere to great heights, but these are transient local effects compared to globe-spanning ITCZ.
The polar regions generally have very little, if any convection. Oftentimes they suffer descending rather than rising air, pulling the tropopause downward. The tropopause is also pulled downward at the jet streams, particularly the polar jet.
Best Answer
The air pressure at a given point is the weight of the column of air directly above that point, as explained here. As altitude increases, this column becomes smaller, so it has less weight. Thus, points at higher altitude have lower pressure.
While gravitational force does decrease with altitude, for everyday purposes (staying near the surface of the Earth), the difference is not very large. Likewise, the centrifugal force also does not have significant impact.