How exactly lightning forms is still open for debate, but scientists are certain there are two types of lightning: positive and negative. The most commonly accepted theory is that electrons are stripped off colliding ice particles caught in a thunderstorm’s updraft. Once these particles lose an electron, their net remaining charge becomes positive. These positively charged particles usually rise high into the upper-most part of the thunderstorm where they tend to collect.
Conversely, other particles in the cloud acquire an electron after the collision and as a result, their net remaining charge is negative. The negatively charged particles tend to collect at the base of the cloud.
How and why the charge separation occurs is not fully understood, but as more collisions occur, the positive charge is separated from the negative charge which in turn creates an electric field. When this electric field “short circuits”, a lightning bolt is generated.
Larger thunderstorms produce more lightning because there are more collisions caused by the stronger updrafts. Smaller storms don’t create the same conditions, therefore there are fewer lightning strikes.
Some storms have a lot of intra-cloud lightning strikes which never reaches the ground, while other storms have lots of cloud-to-ground strikes that do.
Although lightning comes in all different shapes and sizes, there are only two types of lightning; negative and positive discharges.
As positive charge builds in the upper part of the storm and negative charge builds in the lower part of the cloud, an electric field is created. As the charge continues to build, so does the attraction between the positively and negatively charged particles. The only thing stopping the electric current from flowing is air, which happens to be a very good insulator.
The air allows an immense amount of charge to build. Eventually, the insulating factor of the air breaks down due to ionization and a discharge will occur, whereby electrons will flow to balance the charge separation. This is a an example of a negative lighting strike.
If the discharge occurs inside the cloud it is called intra-cloud lightning, which accounts for about 75% of all lightning produced by a thunderstorm. If the discharge hits the ground, it’s called a cloud-to-ground strike, which on average accounts for another 20% (the remaining 5% is explained below).
There’s another type of lightning that is far more powerful, one in which positive charge flows instead of electrons. It’s called a positive lightning strike.
Only about 5% of lightning is positive and tends to occur during the peak of a severe thunderstorm or as the storm is decaying. Most positive strikes originate high in the anvil head of a thunderstorm where the positive charge tends to accumulate. However, instead of discharging with the negative charge at the base of the cloud, the ionization path travels outside the cloud and strikes the ground where there’s a pool of negative charge.
The scientific community doesn’t know for sure what causes a positive lightning strike, but a few things are for certain. Of the two types of lightning, they tend to be about 5 times more powerful and hotter than a negative strike, last about 10 times longer, strike several miles away from the storm (“bolt from the blue”), and produce huge amounts of ELF and VLF radio waves.
It’s commonly believed that positive strikes are responsible for most forest fires and extensive damage to electrical grids. They may also play a role in Sprites and Elves, which is another very interesting phenomenon produced by thunderstorms.