As the atmosphere heats and cools, it expands and contracts causing changes in pressure and air movement. Water droplets inside clouds have a positive electrical charge at the top of the cloud and a negative charge at the bottom.
When the negative charges come near enough to attract a positive charge from another cloud or the Earth, electrical energy is released, forming a flash of light. There is often a loud bang called thunder, along with lightning.
Thunder is caused when the air is heated to a tremendous temperature and gives out explosive noise when it expands suddenly. As light travels faster, we see the lightening flash before we hear the thunder.
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.[4]
http://en.wikipedia.org/wiki/Lightning
