Atmospheric Pressure

What is it ?

Atmospheric Pressure is the pressure due to the gases in the atmosphere pushing on a point.
If we climb from Sea Level upwards through the atmosphere the pressure decreases. If we know numerically how the pressure varies as we ascend then we can use that information to calculate our altitude (or get an altimeter to do the conversion for us).
For the purposes of the JAR-FCL PPL examinations it is enough to say the the pressure measured in millbars (mb) decreases by 1mb per 30ft of ascent.
For the purposes of the JAR-FCL CPL examinations it is enough to say the the pressure measured in millbars (mb) decreases by 1mb per 27ft of ascent.
Neither of these is actually true, but they are close enough to come out with usable figures. The ft per mb ratio changes as we get higher in the atmosphere.

What really happens ?

The pressure of the air is really the weight of that column of air. The weight of anything depends on its mass and gravity so if we now how much air we are dealing with then we can calculate its weight and work out the pressure it exerts at its base.

Comparison To Fluid Hydrostatic Pressure

 

See the definition of hydrostatic pressure.
The pressure is just the weight of the column of air spread over the area at the base of the column.
There is however one small problem, the density of the air is not constant because air is not incompressible.
However If we make h very small we can calculate the weight of thin slivers of air each of different density and sum the up the weight of all the tiny slivers of air which make up a column and calculate its total weight.
So how do we know the density ?

Example Content

Equation for atmospheric pressure.

 
PAlt = PSeaLevel e -mgh/kt
PAlt
Pressure At Altitude [Nm2]
PSeaLevel
Pressure At Sea Level [Nm2]
e
exponential constant
m
mass of one Mole of gas [g]
g
Acceleration due to gravity [m s-2]
h
height [m]
k
Boltzmann Constant
t
Temperature [° K]
Example Content

Web References

Hyperphysics derivation of the Barometric formula