Lightning is always found with thunder - the two go together.
Thunderstorms form when particularly warm, moist air rises into cold air above land or sea.
As the humid air rises to a great height, water vapour condenses, forming large cumulonimbus clouds. Water droplets and ice crystals in the clouds bump together and break up as they rub against each other in strong currents of air. This action builds up positive electrical charges at the top of the clouds and negative charges at the base. When the charge at the base of the cloud gets to a certain strength, electrical energy is released and passes through the air to another point with the opposite charge, such as the ground. This release of energy is called the leader stroke and forms a path of charged air for the main stroke to travel along.