of minus one is significant in the design of gliders because this indicates a perfect location for a "Total energy" port for supply of signal pressure to the Variometer, a special Vertical Speed Indicator which reacts to vertical movements of the atmosphere but does not react to vertical maneuvering of the glider. In the fluid flow field around a body there will be points having positive pressure coefficients up to one, and negative pressure coefficients including coefficients less than minus one, but nowhere will the coefficient exceed plus one because the highest pressure that can be achieved is the stagnation pressure. Compressible flow 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. greater than one indicates the freestream flow is compressible. Perturbation theory The pressure coefficient can be estimated for irrotational and isentropic flow by introducing the potential and the perturbation potential , normalized by the free-stream velocity Using Bernoulli's equation, which can be rewritten as where is the sound speed. The pressure coefficient becomes where is the far-field sound speed. Local piston theory The classical piston theory is a powerful aerodynamic tool. From the use of the momentum equation and the assumption of isentropic perturbations, one obtains the following basic piston theory formula for the surface pressure: where is the downwash speed and is the sound speed. The surface is defined as The slip velocity boundary condition leads to The downwash speed is approximated as Pressure distribution An airfoil at a given angle of attack will have what is called a pressure distribution. This pressure distribution is simply the pressure at all points around an airfoil. Typically, graphs of these distributions are drawn so that negative numbers are higher on the graph, as the for the upper surface of the airfoil will usually be farther below zero and will hence be the top line on the graph. Relationship with aerodynamic coefficients All the three aerodynamic coefficients are integrals of the pressure coefficient