Pressure coefficient

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The pressure coefficient is a dimensionless number used in aerodynamics and fluid mechanics, most often in the design and analysis of an airfoil. The relationship between the coefficient and the dimensional number is:
$C_p={p-p_\infty \over \frac{1}{2} \rho V^2}$

where

$p_\infty$ is the free stream pressure
$ρ$ is the fluid density (sea level air is 1.225kg/m^3)
$C p$ of zero indicates the pressure is the same as the free stream pressure
$V$ is the velocity of the fluid flow

[edit] 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 $C p$ for the upper surface of the airfoil will usually be farther below zero and will hence be the top line on the graph.

[edit] $C l$ and $C p$ relationship

The coefficient of lift can be calculated from the coefficient of pressure distribution by integration, or calculating the area between the lines on the distribution.
$C_l=\int_{LE}^{TE}C_{p_l}(x)-C_{p_u}(x)\,dx$
where:
$C_{p_l}$ is pressure coefficient on the lower surface
$C_{p_u}$ is pressure coefficient on the upper surface
$L E$ is the leading edge
$T E$ is the trailing edge
When the lower surface Cp is higher(more negative) on the distribution it counts as a negative area as this will be producing down force rather than lift.