Lightning

It’s the form of visible electric discharge between rain clouds or between a rain cloud and the earth. The discharge is seen in the form of a brilliant arc, sometimes several kilometers long, stretching between the discharge points. The discharge also sets up a sound wave that is heard as thunder. How rain clouds become charged is not fully understood, but most rain clouds are negatively charged at the base and positively charged at the top.

Studies with high-speed cameras have shown that most lightning flashes are multiple events, consisting of as many as 42 main “strokes,” each of which is preceded by a “leader” stroke. All strokes follow an initial ionized path, which may be branched, along with the current flows. The average interval between successive lightning strokes is 0.02 sec and the average flash lasts 0.25 sec. Because the duration of one powerful stroke is no more than 0.0002 sec, the intervals between strokes account for most of the duration of a lightning “flash.” So-called sheet lightning is simply the reflection of an ordinary lightning flash on clouds. Ball lightning is a rare phenomenon in which the discharge takes the form of a slowly moving, luminous ball that sometimes explodes and sometimes simply decays.

In cloud-to-ground lightning (CG), a channel of negative charge, called a stepped leader, will zigzag downward in roughly 50-yard segments in a forked pattern. This stepped leader is invisible to the human eye, and shoots to the ground in less time than it takes to blink. As it nears the ground, the negatively charged stepped leader is attracted to a channel of positive charge reaching up, a streamer, normally through something tall, such as a tree, house, or telephone pole. When the oppositely-charged leader and streamer connect, a powerful electrical current begins flowing. A return stroke of bright luminosity travels about 60,000 miles per second back towards the cloud. A flash consists of one or perhaps as many as 20 return strokes. We see lightning flicker when the process rapidly repeats itself several times along the same path. The actual diameter of a lightning channel is one-to two inches.

Cloud flashes sometimes have visible channels that extend out into the air around the storm(cloud-to-air or CA), but do not strike the ground. The terms sheet lightning or intra-cloud lightning (IC) refers to lightning embedded within a cloud that lights up as a sheet of luminosity during the flash. A related term, heat lightning, is lightning or lightning-induced illumination that is too far away for thunder to be heard. Lightning can also travel from cloud-to-cloud (CC). Spider lightning refers to long, horizontally traveling flashes often seen on the underside of stratiform clouds. 

(Britannica encyclopedia and Online sources)