Air Pressure and Elevation


Turns out that the gravity of the earth actually does cause the gradual change in the density of air as you go from the surface of the planet upwards to outer space. And since density of gases is really just another way of expressing pressure, we can say that the air pressure of the earth is very dependent on the height (elevation) at which you take that pressure. The good news is that down here in the troposphere where we all live and breathe, the pressure is pretty much a directly linear function of elevation.

Below is a plot of altitude vs pressure up to around 12 km which means this is pretty much for the first layer of our atmosphere known as the troposphere.

Plot of Elevation vs Pressure
 

The relationship is very close to linear for the first 4-5 km. You can see that the pressure drops about 10 kPa for every 1 km in altitude rise. After this linear region, it gradually changes over to more of an exponential fall off in pressure as the altitude reaches to the boundaries of our atmosphere.

It is also worth noting that the plot does extend to negative elevations a bit. There are places on earth that are considerably below sea level and therefore have higher pressures. Death Valley here in the United States is like that at 282 ft (86 m) below sea level. The lowest elevation on earth is the Dead Sea which is 1388 ft (423 m) below sea level.

There are many physical and chemical phenomena that are affected by differences in pressures. A very common one is the boiling point of a liquid. Lower applied pressures result in lower boiling points. This is why many directions for preparing pasta have a note about boiling for a longer time (2-3 minutes longer) if you are at a higher elevation.



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