Shock diamonds were first described by Moravian-born Austrian physicist Ernst Mach in the late 19th century, hence the nickname Mach diamonds. They form when the supersonic exhaust from a propelling nozzle is slightly over-expanded, meaning that the static pressure of the gases exiting the nozzle is less than the ambient air pressure. The higher ambient pressure compresses the flow, and since the resulting pressure increase in the exhaust gas stream is adiabatic, a reduction in velocity causes its static temperature to be substantially increased. The exhaust is generally over-expanded at low altitudes, where air pressure is higher. As the flow exits the nozzle, ambient air pressure will compress the flow. The external compression is caused by oblique shock waves inclined at an angle to the flow. The compressed flow is alternately expanded by Prandtl-Meyer expansion fans, and each “diamond” is formed by the pairing of an oblique shock with an expansion fan. When the compressed flow becomes parallel to the centre line, a shock wave perpendicular to the flow forms called a normal shock wave or Mach disk. This locates the first shock diamond, and the space between it and the nozzle is called the “zone of silence”.