 They occur when the electric field at the conductor exceeds the dielectric strength of the air, the "disruptive potential gradient", roughly 30 kilovolts per centimeter.At that voltage, electrons in the air are accelerated by the electric field to a high enough velocity that they knock other electrons off gas molecules when they hit them, creating ions and additional electrons, which go on to ionize additional molecules in a chain reaction.The electric field is highest at sharp points on the conductor, so discharges tend to form at these points.