An airplane flying at low, subsonic, speeds causes air-pressure changes because of its movement but they are relatively small. It is convenient, therefore, to treat the air as though it could not be compressed.

However, the air-pressure changes caused by the movement of an airplane at high speed, close to the speed of sound, are considerable and cannot be ignored. The air is compressible and its effect on the airplane must be accounted for when it is travelling at high speed.

An airplane moving through the air creates a pressure wave around it, which is propagated away from it in all directions simultaneously at the speed of sound. For an airplane flying at a relatively low speed, less than the speed of sound, the air-pressure wave ahead of the airplane can move away from the airplane. Consequently, air-pressure, air-density, air-temperature, and air-velocity changes take place ahead of the leading edge of the wings and are a gradual process.

If, however, the airplane is travelling at the speed of sound then the air-pressure wave ahead of the airplane is unable to move away from the airplane and a wave of compressed air builds up at the leading edge of the wing.

This compression wave will create a discontinuity plane, where the airstream experiences the following changes:

Air Temperature - Increases (due to compression)
Static Pressure - Increases (due to compression)
Total Pressure - Decreases (Normal shockwave suffers greater loss than an oblique shockwave)
Air Density - Increases (Oblique shockwave has less compression)
LSS - Increases (due to the temperature increase)
Speed of Airflow - Decreases to less than Mach 1 (normal shockwave). Decreases but still above Mach 1.0 (oblique shockwave).

Minimum energy loss through a normal shock wave will occur when the Mach number of the airflow in front of the shock wave is small but supersonic.