Before an atmospheric discharge, a thundercloud - cumulo nimbus - emits a series of ions known as down tracers or leaders. The negative polarity leaders fall towards the ground in successive fifty meter drops at speeds of 0.15 to 1 m per microsecond (µs), whereas the positive leaders fall more steadily. As they approach the ground, the tips of the highly ionised down leaders generate a powerful electric field that can reach several hundred kilovolts per meter. This powerful disturbance triggers the creation of opposite polarity up leaders, primarily at high and prominent spots along the ground As the down and up leaders join, an ionised channel to the ground fills with cloud. A return arc, known as the atmospheric discharge, returns through this ionised channel. It consists of several arcs between the ground and the cloud.