The formation of clouds involves friction between water droplets in the atmosphere. The friction charges the particles in the atmosphere. Among the positive and negative charges, the negative charges accumulate at the bottom of the cloud and the positive charges at the top.

As the accumulation of the charges increases, the cloud will induce positive charges on the ground nearby. As the number of charge increases, the negative charges on the cloud tend to make a path towards the ground, and it results in a narrow streak of electrical discharge, which we call lightning. The figure below shows the lightning.