Lightning happens when the potential difference between the clouds and the grounds becomes too large [ Read : What causes Lightning ]. Once the voltage reaches a critical strength, the atmosphere can no longer act as an electrical insulator. Lightning strikes from sky to ground, as we all know but it also goes up from ground to sky. How this return lightning takes place, let's understand this....

A cloud-to-ground lightning strike starts with the creation of a channel known as stepped leader. First, a stepped leader is created at the base of the cloud which is a channel through which electrons in the cloud can travel to the ground. But while moving towards the ground, it searches for the most efficient(minimum electrical resistance) route possible. It does so by traveling 50-100 meters at a time then stopping for about 50 microseconds, then traveling another 50-100 meters. In this process it also branches out looking for the best route.

As the stepped leader gets close to the ground, a positively charged traveling spark known as streamers or upward leaders is initiated on some tall object (trees, towers etc) on the ground. The upward leaders moves upward and eventually connects with the stepped leader. Once the stepped leader and the upward leader have connected,usually between 30 to 100 metres above the ground, then electrons from the cloud can flow to the ground, and positive charges can flow from the ground to the cloud. This is known as return stroke and it is also what we see in the sky that is known as lightning. But this flow unlike the flow from up has a well defined shortest route now. This massive flow of electrical current occurring during the return stroke combined with the rate at which it occurs (measured in microseconds) rapidly superheats the completed leader channel, forming a highly electrically-conductive plasma channel.

The core temperature of the plasma during the return stroke may exceed 50,000 K, which makes it shine too bright.This whole process occurs so quickly (in less than one second!) that the lightning appears to travel from the cloud to the ground, when in fact, the opposite is true.