Lightning is a powerful sudden flow of electricity (an electrostatic discharge) accompanied by thunder that occurs during an electric storm. The discharge will travel between the electrically charged regions within a thundercloud , or between a cloud and a cloud, or between a cloud and the surface of a planet. The charged regions within the atmosphere temporarily equalize themselves through a lightning flash, commonly referred to as a strike if it hits an object on the ground. There are three primary types of lightning; from a cloud to itself (intra-cloud or IC); from one cloud to another cloud (CC) and between a cloud and the ground (CG). Although lightning is always accompanied by the sound of thunder, distant lightning may be seen but may be too far away for the thunder to be heard.
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source: Wikipedia