Intracloud flash

From Glossary of Meteorology

intracloud flash

A lightning discharge occurring between a positive charge center and a negative charge center, both of which lie in the same cloud; starts most frequently in the region of the strong electric field between the upper positive and lower negative space charge regions.

In summer thunderstorms, intracloud flashes precede the occurrence of cloud-to-ground flashes; they also outnumber cloud-to-ground flashes. Intracloud lightning develops bidirectionally like a two-ended tree: one end of the tree is a branching negative leader, the other is a branching positive leader. Later in the flash, fast negative leaders similar to dart leaders (
also called K changes) appear in the positive end region and propagate toward the flash origin. In weather observing, this type of discharge is often mistaken for a cloud-to-cloud flash, but the latter term should be restricted to true intercloud discharges, which are far less common than intracloud discharges. Cloud discharges tend to outnumber cloud-to-ground discharges in semiarid regions where the bases of thunderclouds may be several kilometers above the earth's surface. In general, the channel of a cloud flash will be wholly surrounded by cloud. Hence the channel's luminosity typically produces a diffuse glow when seen from outside the cloud, and this widespread glow is called sheet lightning.