Lightning is an electric discharge within cumulonimbus clouds (the most common type of lightning); between two clouds; or between the clouds and the ground. A strike can be several kilometers long. Cumulonimbus clouds are negatively charged at the bottom and positively charged at the top due to air movements within the cloud. The negative charge at the base induces a positive charge on the ground beneath it. People struck by lightning have felt the hairs on their hands, legs, back and head stand up and a prickly feeling due to this charge. When the potential difference between a pair of clouds or a cloud and the ground is high enough (about 10,000 v per cm), the air ionises and a leader stroke moves down to the ground in a series of steps. A second leader can move up from the ground to intercept the first leader at the last moment. These create an ionised path, and a return stroke rushes up from the ground to the cloud along this path at close to the speed of light, releasing a huge amount of energy, the bright flash of light and the sound of thunder. There are often multiple return strokes, as can be seen from photos. Thunder is heard because the lightning stroke heats the air to tremendous temperatures extremely quickly - to about 20,000o C. This air expands rapidly and fires out a shock wave which we hear as thunder.
This topic is very popular on the Internet, and there are a lot of lightning photography sites, as well as general information sites. As a friend has survived being hit by lightning twice (twice??!! he obviously upset the sky gods or something), I thought I'd include a bit on what happened to him, as well as some links.
On one occasion as he walked into a post office, lightning hit the sign above his head, then him. Obviously it earthed some of its power through the building, but he was thrown into the post office and hit the far wall, much to the astonishment of the other customers as he literally flew by them. He survived withot any damage being done (to him that is, the sign was history!).
The second time was really funny, if you can call being hit by lightning funny.
Martin's a meteorologist and was doing research on storms and electronic devices designed to point to the likeliest place to be hit by lightning - well its direction from an observer, to be more precise. He was in contact by radio with a several sets of researchers all armed with the same devices, all a mile or two apart, and they could tell him the direction that their instruments thought was the likeliest one. He would then draw the lines on a map to find the location at which they all intersected. At one point he called up to tell them his instrument didn't seem to be working properly as the direction kept changing all the time, so he asked them what readings they had. As he drew the lines on the map he realize they all pointed at him! And was then hit almost immediately. Luckily he had got into his van to use the radio equipment and was not standing outside!
Martin was lucky. Very, very lucky. Lightning is beautiful, spectacular, awesome. But lightning kills hundreds of people every year. We have all heard the advice about not sheltering under a tree during a storm. Trees get struck much more often than people, and if you're under the tree...
According to Nasa scientists, the followng are the six most common dangerous activities that lead to people being hit by lightning, in order of frequency: -
If caught in the open during a strike and the hair on your head or neck begins to stand on end (yes, this really happens) go inside the nearest building. If no shelter is available, crouch down immediately in the lowest possible spot and roll up in a ball with feet on the ground. (DO NOT LIE DOWN.)
A Lightning Primer (that's an introduction, not a lightning maker) can be found at this NASA site. It is a good introduction to the subject, written by one of their senior scientists. I was going to give a lot more information on the background theory of lightning, but this site covers it so well, i made it my first link.
Lightning and Atmospheric Electricity is the home page to the above site, giving access to a bit more information. It is an excellent educational site on this exciting topic.
How Stuff Works - Lightning is this well-known site's set of pages on how lightning works.
Lightning by Stormguy is part of a larger site on several aspects of severe weather. The images here are pretty impressive, and there are lots of them. Stormguy (Dave Crowley) is a professional photographer, and sells mounted prints of these images, which could make a great present to a friend who shares an interest in weather and lightning. He also has a tornado and cloud section as well, the cloud pages having been recommended on my general weather page.
Strike One is yet another Aussie site dedicated to lightning photography and storm chasing (I should get their tourist board to sponsor me at this rate!).
Lightning strike on a plane shows as a sequence from a film on a plane taking off the moment when it gets hit by lightning. Wow! This you got to see! (Not the Dunstable glider incident, that's below.)
The Dunstable Glider Lightning Strike This page was set up by the passenger in the glider on this famous incident, when one of the most powerful lighting strikes in the UK hit a glider. The glider was destroyed, but the pilots parachuted to safety.
Scientific American on aircraft strikes explains what happens when lightning and aircraft get together.
Scientific American explains why lightning has a jaggered shape rather than taking a simple straight line path to Earth.
Bob's Ball Lightning Page covers a topic that has fascinated people for centuries. Ball lightning, also known as St. Elmo's Fire, is covered well here with links to related sites on the subject at several points. It does say that some people have been skeptical of its existence, but it is generally accepted by most scientists as a genuine phenomenon.
The Ball Lightning page also covers this topic well and provides many links for the viewer to follow up.