As water vapour move upward in the clouds, the falling temperature causes it to condense. In this process heats is generated, which pushes the water molecules further up. As they move to the temperatures below 0 °C, the water droplets change into tiny ice crystals. They continue to move up and up, gathering mass until they are so heavy that start to fall on earth. This leads to a cycle in which, simultaneously, smaller ice crystals moving up and larger ice crystals falling down.
Due to this, collisions occur and triggers the release of electrons (a process similar to the generation of sparks of electricity). The moving free electrons causes more collisions and forms more electrons, a chain reaction ensues. This process results in the situation in which the top layer of the cloud gets positively charged, while the middle layer is negatively charged the electrical potential difference between this 2 layers is huge (of the order of a billion to 10 billion volts). Very few times, a massive current, of the order of 100,000 of million amperes, starts to flow between the layers.
An enormous amount of heat is produced, and this leads to the heating of the air column between the two layers of the cloud. This heat gives the air column a reddish appearance during lightning. As the heated air column expands, it produces shock waves that result in thunder. While the Earth is a good conductor of electricity, it is electrically neutral.
However, in comparison to the middle layer of the cloud, it becomes positively charged. As a result, about 15%-20% of the current gets directed towards the Earth as well. It is this flow of current that results in damage to life and property on Earth. The temperature of a lightning strike can reach till 50,000 °F which is 5 times hotter than the sun's surface.
