From Glossary of Meteorology
A transient, high-current electric discharge in the atmosphere with path lengths measured in kilometers. Lightning is comprised of dendritic networks of leader and streamer discharges that form the physical dimensions of the flash and determine its emission spectrum. Natural lightning arises from the strong electric fields that exist between separate regions of thunderstorm (cumulonimbus) clouds whose hydrometeors have a surplus of either positive or negative electric charge. The initial breakdown occurs when the local electric field overcomes the electrical impedance of the air and typically leads to the development of bidirectional leaders that self-propagate through the positive and negative charge regions—though, at different stages of the process, unidirectional leader development can occur. While most of these networks of lightning channels remain in the cloud as cloud-to-cloud flashes or intracloud flashes (generally, cloud flashes), leaders can connect with the ground to form a cloud-to-ground flash, leave the cloud to terminate in the atmosphere as an air discharge, or produce transient luminous events such as sprites and jets that occur in the upper atmosphere.
Lightning serves to neutralize the charge imbalances that build up in thunderstorms over time. Through the induction process, lightning transfers the stored electrical energy of the electrified cloud into electrical currents that traverse the conductive ionized lightning channels. Most of the lightning energy goes into heating the atmospheric constituent gases, which causes them to undergo dissociation, excitation, and recombination. This generates intense emissions at atomic lines for the atmospheric gases. The rapidly expanding gases surrounding the channel also generate a shock wave that we perceive acoustically as thunder. Because sound travels more slowly than light through the air, the thunder will be appreciably delayed. The time difference between the flash of light and clap of thunder can be used to gauge the distance to the flash. Only when the leader connects to the ground does the ground potential wave (return stroke) affect the lightning process.
The appearance of lightning to human observers and lightning-detecting instruments depends on the types of discharge processes involved. Observers have long described lightning as streak lightning, forked lightning, bead lightning, sheet lightning, or spider lightning, among other terms, depending on the behavior of the leaders as they propagate through the clouds or make their way toward the ground. Cases of self-initiated upward lightning and lightning-triggered upward lightning have also been documented that resemble cloud-to-ground flashes that originate from a tall object on the ground and strike the cloud. The largest flashes—known as megaflashes—can develop horizontally over hundreds of kilometers through the cloud while initiating multiple cloud-to-ground strokes along their paths. Distant lightning that is too far away for the thunder to be heard has been called heat lightning and other terms that do not represent distinct types of lightning. Reports of ball lightning have also been made, but the existence of this phenomenon and its underlying physical processes have not been verified.
In addition to neutralizing local charge imbalances within the thunderstorm, lightning is also linked to the broader Earth–atmosphere system through the global electric circuit. The global circuit provides a framework for measuring and tracking changes in convective weather worldwide over a long period of time, making lightning measurements a useful tool for climate monitoring.
Term edited 3 March 2022.