Physics Q & A - Physics Q & A - How does lightning happen?
Cheung Kai-chung (Translation by Tong Shiu-sing)  

The step leader
Fig. 1  The formation of the step leader.
 

What is lightning? How does lightning happen?

Scientists have not fully understood the mechanism of lightning. We can describe the process of lightning, but many phenomena of lightning have not been explained. According to observation, the upper part of a cloud carries positive charge, while the lower part carries negative charge, therefore the ground is induced with a large amount of positive charge. The potential at the bottom of the cloud is much lower than that of the ground, therefore negative charge will be accelerated towards the ground. When lightning begins, a "step leader" comes from the cloud to the ground. The step leader is not very bright, but it propagates at a high speed. First, it propagates for about 50 m, stops for a while, then proceeds in a different direction, and stops again. The process repeats many times, making a zigzag path filled with negative charge. High speed electrons ionize air, thus providing a conducting path for the stoke. When the step leader comes close to the ground, a strong electric field is created, which drives the positive charge on the ground to neutralize the negative charge in the path. The discharge releases an enormous amount of energy, and it is called the "returning stoke". The returning stoke is much brighter than the step leader, so what we see at lightning is a discharge which actually goes from the ground to the cloud! The returning stoke is the origin of the strong light, heat and sound in lightning.

The returning stroke
Fig. 2  The formation of the returning stroke.
 
But the story has not finished. Very shortly after the returning stoke disappears, another step leader comes from the cloud. This time the leader does not pause, it propagates straight to the ground, following the path of the first leader. The process of discharge repeats many times, even up to several tens times. Why is lightning so interesting? Perhaps nobody knows the answer.