Lightning is the result of the separation of charges within the cloud, as is illustrated by the picture on the left.
There are two main theories as to why this occurs.
- The first is that the separation is caused by cosmic particles crashing into the particles in the atmosphere which leads to the electrons from these particles being knocked free and so there become positive and negative ions which then separate to the top and bottom of the cloud.
- The second theory is that it is ice particles and rain droplets that collide within the cloud which then leads to the separation of charges.
Whichever the method, the result is this obvious separation of charge. At the base of the cloud all the negative charges start extending towards the ground and the positive charges that are attracted to the surface of the ground start extending towards the ground. Objects at ground level are now sending positive streamers to try to meet the downwards negative ones.
When the two streamers meet an upstroke occurs, that is a single weak upward discharge is released from the ground, which clears the path from ground to cloud. This process is illustrated in the diagram above.
A fluctuation in the brightness of the lightning strike is caused by fluctuation in the voltage that is flowing.
The flash which occurs is caused by the extreme temperatures that occur due to the currents involved and the thunder is a shock wave that is produced as the air around the strike is caused to expand at such a rate, also due to the extreme temperature.