The electrical discharges that make up lightning can be spectacular to watch. Inside thunderclouds, electrical charges are produced largely by collisions between ice crystals.

A huge electrical charge builds up as the cloud particles become electrified and the charged particles separate. The positive charge fixes near the top of the cloud and the negative charge at the base. A positive shadow charge builds up on the ground beneath. An electric current is then discharged in a blinding flash as lightning zig zags between ground and cloud. A weak spark known as the stepped leader reaches down from the cloud and a positive leader rises to meet it. The main discharge called the return stroke takes place from ground to cloud along the channel created by the two leaders. We also see intra-cloud lightning within the cloud and inter-cloud lightning between clouds. When the lightning is hidden or diffused by clouds we call it sheet lightning.

Air around the discharge channel is heated to about 30,000 C (55,000 F). The sound of thunder is made by heated air expanding very rapidly and causing soundwaves. Thunder rumbles because you get soundwaves coming from different parts of the storm. The speed of sound (760 mph, 1224 km/h) is very much less than the speed of light (186,000 mp/s, 300,000 km/s). So you see a lightning flash almost immediately it happens while the sound can take several seconds to reach an observer. If you count the gap between the lighting and the thunder, you can tell how far away the storm is. For every kilometre count 3 seconds, for every mile count 5 seconds. So if you count 15 seconds, the storm is 5 km or 3 m away from where you are.

A typical discharge delivers about 1.5 million volts. Much of this electrical energy is converted into heat energy and although these high temperatures only last a few millionths of a second, they are sufficient to vapourise the fluids in a tree trunk causing it to explode. A typical flash of fork lightning lasts for only about 0.2 seconds. Find out more about people who have survived lightning strikes, and how you can protect yourself from severe weather.