Why does lightning occur?

Growing ice particles within a cloud interact with each other through collision, causing the particles to fracture and break apart. It is currently believed that smaller ice particles tend to acquire a positive charge, while the larger particles acquire a more negative charge. Under the influences of thunderstorm updrafts and gravity, these particles separate until the upper portion of the cloud acquires a net positive charge, and the lower portion of the cloud becomes negatively charged. This separation of charge produces electrical potential both within the cloud and between the cloud and ground. Eventually, the electrical resistance in the air between the charged regions breaks down and a flash begins. The resulting lightning strokes are an electrical discharge between the positive and negative regions of a thunderstorm.