How is lightning formed

What is lightning?
Lightning is electricity produced during a thunderstorm. Each bolt of lightning can contain up to 100 billion volts of electricity.

How is lightning formed?
During a thunderstorm, particles of rain, ice or snow will collide. The collisions will create an electric charge and after a while, the thundercloud is filled with electrical charges. The charges will separate. The negative charges, electrons, will concentrate in the lower part of the thundercloud while the positive charges, protons, will form at the top of the thundercloud.

Since like charges repel, some of the negative charges on the ground are pushed down away from the surface of the ground, leaving a net positive charge on the ground.

Then as opposite charges attract,  the positive charges on the ground and the negative charges at the base of the thundercloud are pulled towards each other. Since the negative charges (electrons) are many times smaller than the positive charges, they move more easily. When the negative charges move down, an invisible stroke known as the stepped leader is formed.

When the negative and positive charges on the stepped leader connect, there is a conductive path from the cloud to the ground. Once the stepped leader reaches one of the positively charged streams from the ground, the positive charges we get are what we call a lightning, a bright stream in the sky that has several branches (see figure 3). The positive charges going up to the cloud are actually where we see the bright lightning. This is also know as the return stroke. When the return stroke reaches the cloud, if there are still more negative charges left, the lightning will continue with negative charges being taken to the ground and another return stroke taking positive charges to the cloud.