Static Pressure
The operation and performance of an aircraft and an number of instruments greatly depends on pressure differences. Not only does a wing produce lift by the effect of pressure differences, also a flight instrument may require pressure in order to present correct flying indications (i.e. the airspeed indicator ASI). Therefore it is necessary to understand the basic principles of pressure. First I will explain what static pressure means and how it is related to pressure differences.
Atmosphere Build-up
Where ever you are and at any point in the atmosphere static pressure is exerted in all directions equally. As the name "static" implies, it does not involve any relative movement of the air. A static vent is usually used to measure static pressure in an airplane. As altitude increases static pressure decreases. This can be explained by the basic build-up of the atmosphere which consists of a large number of molecules pressing down on eachother. The higher you get the less molecules press down and the static pressure is less (figure 1.1).

Static pressure cannot be calculated. Since pressure itself is form of energy it is possible however to apply Bernoulli's principle to it. Bernoulli states that the total energy in a streamline flow always remains constant. This means:

Pressure energy
+
Kinetic energy
=
Constant total energy
Static Pressure
 
Dynamic Pressure
   

This theory can only be applied below 0.4 mach (300 kts cut off) where density changes due to compression of air will not occur.
Before we can continue it is necessary for you to understand what dynamic pressure is and how it relates to kinetic energy. Therefore the dynamic pressure where we'll continue.
Figure 1.1 - Atm. Build-up
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