What is Lightning?
When you are photographing a thunderstorm, if your hair suddenly starts to stand on end and nearby metal objects start crackling and humming and there is an ozone smell in the air, run for your life, because lightning is about to strike very close to you.
Lightning is caused by electrical charges within clouds, and it is the interaction between the positive and negative charges and the resulting electric field that causes these strange effects just before a lightning strike.
A combination of air currents and moving ice and water particles in the cloud causes positive and negative charges to separate. Put simply, the positive charges gather near the top of the cloud and the negative charges gather near the base.
Once an appreciable negative charge has built up on the bottom of the cloud, a "shadow" is created on the ground below where a positive charge is induced. The conditions are now right for a lightning strike.
Once the charge on the cloud is big enough, a small path of negative charge makes its way towards the ground. Once it gets close, a path of positive charge leaves the ground and heads up to meet it.
This is the point where these strange effects take place.
Eventually, the two paths join forming a path of charged air connecting the ground and the cloud. This path forms a channel for the lightning to strike. When the lightning bolt strikes, the air along this path is heated so rapidly that the air expands faster than the speed of sound and a shock wave is generated. The cracking sound of thunder is caused by this faster than sound expansion.
You can work out the distance of the lightning strike by counting the time between the lightning flash and the thunderclap. (one thousand, two thousand........). Five seconds equals about 1.6 kilometres.