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Lightning is a sudden electrostatic discharge during a thunderstorm. This discharge occurs between the charged regions of the cloud (called intracloud lightning or IC), between two clouds (CC lightning) or between the cloud and the ground (CG lightning).
Charged areas in the atmosphere temporarily self balance through this discharge, which is called an impact (if it strikes an object on the ground) and a flash (if it happens in a cloud). Lightning produces light in the form of plasma and sound in the form of thunder. When thunder occurs so far away that the sound cannot spread to the light of lightning or flash, you may see the thunder rather than hear it.
As shown in the figure, lightning is not evenly distributed on the earth.
About 70% of the lightning occurs in the tropics with the largest convection. This is caused by the increase and decrease of air quality and the difference of water concentration, and usually occurs at the boundary between them. Warm currents across dry land, such as the Gulf Stream, partly explain the increased frequency of lightning in the southeastern United States. Due to the influence of the small or lack of land area in a large part of the global ocean, which limits the differences of these variants in the atmosphere, the lightning frequency there is much lower than that in large terrain. Thunderstorms in the Arctic and Antarctica have limited coverage, resulting in the least lightning in the region.
Generally, cloud to ground (CG) lightning accounts for only 25% of all global lightning. This is the origin of most CG lightning because the base of thunderstorms is usually negatively charged. This area is usually at the altitude where freezing occurs in the cloud. Freezing and ice water collisions seem to be key parts of the initial charge development and separation process. In wind driven collisions, ice crystals tend to produce a positive charge, while the heavier ice and water, the muddy mixture (called graupel) produces a negative charge. The updraft in the storm cloud separates the lighter ice crystal from the heavier eddy current, which makes the top area of the cloud accumulate positive space charge, while the lower layer accumulate negative space charge.
Because the concentrated charge in the cloud must exceed the insulation characteristics of the air and increase proportionally to the distance between the cloud and the ground, the proportion of CG strike (relative to the cloud (CC) or the cloud (IC when the cloud is closer to the ground, discharge) will increase. In the tropics, the freezing level in the atmosphere is usually high, and in the tropics, only 10% of the lightning is CG. At an altitude of about 60 ° n in Norway, where the freezing altitude is low, 50% of the lightning is CG.
Lightning is usually generated by cumulonimbus clouds, whose bottom is usually 1-2 km (0.6-1.25 miles) from the ground, with a maximum of 15 km (9.3 miles).
