From the base of a cloud, usually negatively charged, a pre-discharge known as a stepped leader escapes. This stepped leader is barely luminous and its progression towards the ground is by leaps of several tens of metres; in reality, it is a succession of discharges travelling along the same ionized path at intervals of 40 to 100 µs (average progression speed of the order of 0.5 to 1 m/µs).

As it approaches the ground, the extremity of the highly charged stepped leader causes a significant increase of the average electric field strength along its length. This electric field can reach values of 400 to 500 kV/m. When the atmospheric air ionization threshold is reached (30 kV/cm) at high points, which are the preferred impact points for lightning (tree tops, chimneys, lightning rods, etc.), corona discharges occur. Locally, for higher electric field values, these corona discharges are transformed into positive rising discharges called “upward leaders”. The corona discharges with the best initiation characteristics or those progressing the most rapidly will join the downward stepped leader . The electrical connection between the thunder cloud and the ground is established by an ionized channel. Charge is then returned from the ground to the cloud (return stroke), which has a tendency to neutralize its own charge.

Several restrikes may be exchanged in the space of 0.2 to 1 second. Their progression is continuous and their propagation speed very high.