Glenn Research Center Shock waves occur whenever an object moves faster than the speed of sound and the object abruptly constricts the flow.
Shock waves are very small regions in a gas where the gas properties change by a large amount.
Scientists call this a high gradient region, which just means a large change of variable over a very small distance. Across a shock wave, the air pressure increases almost instantaneously. The air temperature and density also increase across a shock wave, while the Mach number and speed of the flow decrease. If the shock wave is oriented perpendicular to the flow direction it is called a normal shock. If the shock wave is inclined to the flow direction it is called an oblique shock.