Extreme high-speed footage of a lightning strike. Lightning is an electrical discharge that occurs when a thundercloud accumulates a high enough electrical charge. The base of the cloud produces several negatively-charged leaders, which form channels of ionised air as they move towards the ground. Many of these leaders do not make contact with anything and fade out. If a leader reaches the ground, or makes contact with an induced upward-growing positively-charged leader, a massive discharge occurs through the channel of ionised air. This is called the return stroke, and is the bright bolt that is usually observed. There are usually several discharges down a channel once it is established, as seen here. This gives the lightning a flickering appearance when viewed at normal speed.