Lightning is caused due to electrical discharge from cloud to cloud or from cloud to Earth.
It generally takes place, when clouds of opposite electric charges meet each other. Lightning is actually a rapid succession of discharges, each lasting from about 500 microseconds.
As each discharge takes place at a definite point along the line of the least electrical resistance, we have an optical illusion of a single flash of lightning travelling across the sky down to the Earth.
Below Lightning info from Wikipedia ( Read More):-
Lightning is an atmospheric discharge of electricity accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms.[1] In the atmospheric electrical discharge, a leader of a bolt of lightning can travel at speeds of 60,000 m/s (130,000 mph), and can reach temperatures approaching 30,000 °C (54,000 °F), hot enough to fuse silica sand into glass channels known as fulgurites which are normally hollow and can extend some distance into the ground.[2][3] There are some 16 million lightning storms in the world every year.
http://en.wikipedia.org/wiki/Lightning
