I researched and talked about lightning today, here is what I learned. Caveat, I am not a meteorologist. I have embedded the slides, and I've roughly transcribed some notes to go along with them.
The most common kind of lightning is negative lightning, here's how it forms. A cloud is a big floating heap of ice or water particles that are constantly bashing into each other. There is some debate about this, but it seems that when two particles bump into each other, electrical charge is transferred from one to the other, leaving positively and negatively charged particles in the clouds.
Typically the larger particles end up negatively charged, and smaller, lighter particles end up positively charged. The larger particles tend to sit lower in the cloud than the smaller ones, so the cloud ends up being negatively charged closer to the ground.
Over time, this charge builds up, and even starts to cause the ground to become positively charged, as more positive charge is attracted towards the negatively charged cloud.
Sometimes, the electric field between the cloud and the ground will be high enough, such that streams of negative charge will head towards the ground, this is called a stepped ladder. If the eletric field is strong enough, streams of positive charge will even start to head up from the ground towards the clouds (typically from high, well earthed structures, like buildings/trees).
When these two streams meet, an electrical pathway is suddenly created between the ground and the cloud which can transfer all the excess charge. Because there is such a huge amount of charge, which gets transferred very quickly, a huge current flows between the ground and the clouds. This energy superheats a channel of air, which effectively explodes with light and heat - hence the lightning. This explosion also creates thunder.
When lightning strikes, the channel of air that exploded, becomes ionised - meaning it remains a good conductor of electricity - so if there is a lot of charge built up in the cloud, it encourages further lightning strikes in almost the same place. Thus proving that "lightning never strikes the same place twice" is nonsense, as it frequently does, in very quick succession, and sometimes more than twice!
The "diameter" of a lightning rod is about 1 centimetre - about the thickness of your finger. It looks so big because it is so bright!
A lightning strike happens at about 140,000 miles per hour, which is 3,255 times faster than an Edinburgh tram's top speed. And infinitely faster than any Edinburgh trams' current speed!
A lightning strike releases 500 Megajoules, the equivalent of 260 Big Macs, or enough energy to power a 100 watt (technically now illegal in Europe) for 2 months!
An average lightning strike is about 30,000°C - which is 3 times hotter than the sun, or 750 times hotter than your armpits!
When lightning strikes sand or soil, it is so hot, that it fuses the silica (a big component of sand) into glass rods in the ground called fulgurites. These can extend several metres into the ground!
A much less common type of lightning is "positive" lightning. Sometimes in particular cloud formations (anvil clouds for instance) a large amount of positive charge will build up high in the cloud. These charges can sometimes stream down to the ground in a huge arc to create lightning.
These arcs can sometimes touch down miles away from the cloud formation, meaning they can appear "out of the blue". Because they also have so far to travel, they typically end up being much more powerful too. An average positive lightning strike could power our 100 watt lightbulb for 95 years!
Positive lightning strikes can be much more dangerous than negative ones, because they are so much more powerful. When the specifications for airplane safety were created, we didn't really even know about positive lightning, so most planes aren't designed such that they would withstand a positive lightning strike :(. Indeed a number of planes are believed to have been brought down by being struck by positive lightning.
There are many other kinds of lightning, but one of the most dramatic (although it has a terribly undramatic name) is dry lightning. This occurs in clouds of dry particles - such as volcano or wildfire ash - creating some dramatic photograph opportunities.
Wildfire dry lightning is a cool example of positive feedback in nature. If a wildfire is big enough it may cause a dry lightning strike, which can set fire to more of the forest, which can create more lightning strikes and so on - scary stuff!
Getting struck by lightning, in the UK at least, is not very likely, so don't worry! In 2009, only one person died of being struck by lightning, meanwhile four people died from bee stings! So bees are way more dangerous than lightning, right?
Some people have even experimented with harnessing the energy of lightning strikes as a "green" energy source - although I don't think they have been very successful. It is ironic, however, that Amazon's North Virginia datacenter was down for many hours yesterday due to a power cut - caused by a lightning strike!
I gave this text as a talk on lightning as a lightning talk at the Scottish Ruby Conference fringe. It was fun to research and fun to give - and I think that's because the natural world is really pretty cool when you think about it.