Lightning occurs when violent movement within cumulonimbus clouds causes a build up of positive and negative charges. There are two types of lightning. Lightning does not always travel between the cloud and the ground. Where strong opposite charges exist, lightning can travel both between clouds, and even within the same cloud.
Thunder is the sound produced by the rapid heating of the air by the lightning.
The delay between the flash and the rumble of thunder is approximately 3 seconds per kilometre. Thunder is rarely heard more than 10 kilometres away, but lightning may be visible over a distance of 100 kilometres or more. A lightning stroke can extend over 10 kilometres. Lightning can therefore strike out of relatively clear overhead skies. The metal lightning conductor was invented by Benjamin Franklin in 1752.