The process of electric discharge between clouds and the earth or between different clouds causes lightning. During the development of a thunderstorm, the water droplets move downwards while the air currents move upward. Due to these strong movements, the separation of charges takes place. The positive charges collect near the upper edges of the clouds and the negative charges collect near the lower edges. There is accumulation of positive charges near the ground as well. When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound. We are able to see these streaks as lightening. The process is called an electric discharge.