I am talking about the flashing of light that often occurs during rain. We know that the cloud float in the sky containing water and ice. Warm air currents push them up while gravity of the earth pulls them down and due to this event they are compacted inside the cloud. This process forms static electricity in the cloud and later the electrical charges start moving separately inside it. As we know protons carry positive charges they acquire the top portion of the cloud and the electrons with negative charges accumulate at the downside portion of the cloud.
The electrical charges then attract towards the objects that have pointed top on the ground and after sharing their negative charges, they move towards positive charged area in the object.
When the precipitation process takes place in a cloud, it causes lighting in the sky. Each ice and water drop in the clouds carries a small amount of electrical charges, but when the number of ice crystals and water drops increase it causes huge variation between positive charges and negative charges within the cloud. Lightning happens to neutralize the huge difference between positive and negative charges
Source: https://www.skymetweather.com/content/weather-faqs/what-is-lightning/