The first stage of a lightning strike involves an initial discharge of low luminosity and intensity known as a downward leader. It forms at the cloud centre and moves down toward the ground in steps of several dozen meters at a time. At the same time, the electric charge in the atmosphere at ground level increases as the downward leader gets closer.

Any high point in the vicinity immediately gives rise to natural ionization in the form of a series of electrical discharges which are blue in color. This is the point effect or corona effect. As soon as the downward leader is close enough to the ground, the ionization due to the corona effect intensifies, especially near any high point, and eventually turns into an upward discharge: this discharge is the upward leader that develops toward the cloud.

When one of these upward leaders comes into contact with the downward leader, a conductive path is created allowing a powerful current to flow. This is what we call a lightning strike which is characterized by its bright flash and the deafening sound of thunder. The lightning strike may in fact be made up of a number of successive return strokes, only a few hundredths of a second apart, all following the same highly ionized and conductive path.