In the flow of compressible fluids such as air, and particularly the high-speed flow of compressible fluids,  (the dynamic pressure) is no longer an accurate measure of the difference between stagnation pressure and static pressure.  Also, the familiar relationship that stagnation pressure is equal to total pressure does not always hold true.  (It is always true in isentropic flow, but the presence of shock waves can cause the flow to depart from isentropic.)  As a result, pressure coefficients can be greater than one in compressible flow.