Lightning: Ground discharge

(Section 3.2.4.1.1)

This type of lightning, popularly called a “thunderbolt” or “cloud-to-ground lightning” occurs between cloud and the ground. It typically appears to follow a tortuous course and is usually branched downward from a distinct main channel (streak, forked or ribbon lightning).

A ground discharge is typically initiated when a downward-moving, negatively charged “stepped leader” connects with a streamer of positive charge reaching upwards. Once this electrically conductive channel is established, a massive electrical discharge follows. This is the “return stroke” and is the most luminous and noticeable part of the lightning discharge. Most cloud-to-ground lightning flashes are made of several strokes, thus causing a flickering or strobe-light effect.

Although much less common than that initiated by upward-moving streamers, cloud-to-ground lightning can also be initiated by a downward-moving, positively charged streamer. This typically originates from high in a thundercloud rather than the lower part of the cloud. A positive cloud-to-ground discharge is usually very bright in comparison to other lightning. It may also travel many kilometres horizontally to strike the ground as the so-called “bolt from the blue” (clear-air lightning or anvil lightning).

Ground-to-cloud discharges initiated by an upward-moving leader can sometimes originate from objects on the ground, such as tall towers and skyscrapers.