What is the reason for lightning?

Lightning occurs because of electric discharge. Lightning is one of the most powerful natural forces on the planet. It is a natural discharge of the static electricity built up in storm clouds. Most lightning is formed as a part of thunderstorms. We know from observation of static electricity that static electrical discharges such as lightning are caused by separation of charges into positive and negative ions. Over time more of one charge builds until its natural attraction to the opposite charge causes it to migrate in an electrical discharge.