Inside a storm cloud there are very powerful convection currents. The top of such a cloud may be more than 3000 m above the base and so inside the cloud there will be crystals of ice as well as droplets of water. Friction between the crystals of ice and droplets of water as they are churned around inside the cloud may cause static charges to build up and this may lead to potential differences of tens or even hundreds of millions of volts both within the cloud and also between the cloud and other clouds and between the cloud and the Earth.
At some point the potential difference becomes high enough for a small current to find a path through the air - because of the air movements set up by the convection currents this path will almost certainly not be straight. As this current passes the air along the path becomes ionized and this lowers its resistance. Then the main discharge takes place along this path. The current may be millions of amperes - more than the total output of every power station in Britain - but it lasts for less than a thousandth of a second. During this time the air in the path will be heated to a very high temperature. The flash of lightning is the glow given out by the air as it is heated. As the air is heated it expands very rapidly, much faster than sound, and this creates a shock wave, like the one created by a supersonic aircraft. This shock wave is the thunder.
The speed of sound at sea level on a warm day is about 330 metres per second (about 1200 kilometres an hour) compared to about 300 million metres a second for light, so we shall see the flash before we hear the thunder.
If there is a gap of 3 seconds between seeing the lightning and hearing the thunder the storm is about one kilometre away. If the storm is very close we shall see the flash and hear the lightning as a very loud bang. If it is slightly further away we shall probably still see the flash but the thunder will not be quite so loud and we shall also hear the echoes as the shock wave is reflected off other clouds, hills etc. If the storm is several kilometres away we may see only the reflection of the lightning in the sky as a sheet of light (sheet lightning) and hear the thunder only as a distant rumbling. Until the storm is very close most people are more aware of the thunder than the lightning, so we usually talk about thunderstorms.
Thunder can be dangerous because, like the bang from a supersonic aircraft, it may break windows, start avalanches or startle old ladies and make them drop the tea tray; the torrential rain, hailstones the size of golf balls (in the Tropics, footballs) and winds of more than 150 km/hour often associated with thunderstorms can do enormous damage and cause widespread disruption. But this Web Page is only about lightning.
Aircraft are designed not to be affected by ordinary lightning, and it is not usually dangerous to aircraft. But pilots still try to avoid flying near or through storms because of the very strong winds and air turbulence associated with them.
Mega-lightning is a rare type of lightning that goes from the tops of clouds upwards for thousands of metres into space. It consists of electrical discharges called sprites and elves and is very dangerous to very high flying aircraft and spacecraft, but it has only recently been discovered and very little is yet known about it, so it is not considered further on this Web Page.
Lightning is usually really only dangerous when it hits the Earth. The effect on a person of being struck by lightning is the same as receiving an electric shock, a very, very large electric shock. Every year there are more than two hundred and fifty million lightning strikes to the Earth but only about a thousand people are killed by lightning every year. There are a number of sensible precautions you can take if you find yourself out of doors in a thunderstorm, the most important of which is not to take shelter under a single tall tree. To link to a web site giving simple safety rules click here 
If lightning hits a tree a very large current passes through it. This heats it up and any moisture such as the sap in the trunk is instantly turned into steam. The very sudden change in volume of the water as it turns into steam will often split the trunk in half. In dry weather it may also catch fire. Here is a photograph of the remains of a tree which has been struck by lightning.
Glass is made by heating sand with certain other substances. These all melt together and as they cool they form glass. In certain parts of the world the surface soil is very dry and sandy but also contains all of the other ingredients needed to make a very impure form of glass. When lightning strikes this soil the very high temperature produced melts these ingredients together. Surface tension pulls the liquid mixture into an approximately spherical shape before it cools and solidifies. The subsoil is also heated, and any water in it is turned into steam. The steam produced propels the newly formed glassy sphere several metres into the air. This is a thunderbolt. Thunderbolts can do minor damage to buildings and break car windscreens. But you need to be within a few metres of where lightning strikes to be hit by one, and if you are in such a position being hit by a thunderbolt is probably the least of your worries!
The soil conditions needed to produce real thunderbolts are very rare indeed in Britain but they are very common in Greece. So the Ancient Greeks would often see thunderbolts jumping out of the ground where lightning had struck, and thought, entirely reasonably, that the lightning was the thunderbolt being thrown down from the heavens, like a bolt shot from a crossbow, and bouncing off the ground - hence in fact the word thunderbolt.
Today we often use the word thunderbolt very loosely, but originally it meant something real which you could touch and pick up - once it was cool enough of course! And you do still get real thunderbolts in Greece.
As an aside, I have tried many times to find a photograph of a real thunderbolt on the Web so I can provide a link, but so far without success. If you are reading this Page and can help me please could you e-mail me. Thanks.
If lightning hits power cables or telephone wires it may travel along them into your home. Here it may damage electrical equipment, particularly computers, modems and burglar alarms, and you may also get an electric shock if you touch anything metal. You can buy special anti-surge extension sockets and it is a good idea to use one with your computer. It is also sensible not to use electrical equipment which you hold in your hand such as hair-driers or telephones, and get out of the bath or shower, during a thunderstorm.
If lightning hits a building it may touch metal parts such as water and gas pipes and wiring, making them live and damaging electrical and other equipment attached to them. So it is a good idea to stay away from electrical equipment and metal objects such as radiators. Lightning may also set fire to some of the wooden parts of the building such as roof rafters. But when lightning strikes a building, by far the biggest danger is structural damage.
When substances are heated they usually expand. Stone, brick and concrete and many other similar building materials are very poor conductors of heat. This means that when a building made mainly of one of these materials is struck by lightning the material in the direct path of the electricity is heated very quickly to a very high temperature and so expands very quickly. But the rest of the building is not heated and does not expand. This uneven expansion sets up enormous stresses right through the building from top to bottom, and the surrounding cooler material may be pushed outwards. Then when the heated materials cool again and contract huge cracks may open up. Chimneys and towers, even whole walls, may be brought crashing down.
It is possible to protect buildings by installing lightning conductors. Unfortunately the science behind lightning conductors is not usually taught at school, so there are lots of wrong ideas about what they are and how they work. But the science is not difficult and if you want to have a go at understanding it please click here
