The Rankine-Hugoniot equation governs the behaviour of shock waves (node/1245#comment-2327) normal to the oncoming flow.
http://en.wikipedia.org/wiki/Rankine-Hugoniot_equation
Hugoniots equations are thermodynamical equations derived from the laws of conservation of mass, momentum, and energy, that relate the velocity of a shock wave and the pressure, density, and enthalpy of the transmitting fluid before and after the shock wave passes.
http://www.answers.com/topic/rankine-hugoniot-equation