Corona discharge only forms when the electric field (potential gradient) at the surface of the conductor exceeds a critical value, the dielectric strength or disruptive potential gradient of the fluid. In air at atmospheric pressure, it is roughly 30 kilovolts per centimeter, but this decreases with pressure, so corona is more of a problem at high altitudes. Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. The high curvature causes a high potential gradient at these locations so that the air breaks down and forms plasma there first. On sharp points in air, corona can start at potentials of 2–6 kV. In order to suppress corona formation, terminals on high voltage equipment are frequently designed with smooth large diameter rounded shapes like balls or toruses, and corona rings are often added to insulators of high voltage transmission lines.