How exactly lightning forms is still open for debate. However, meteorologists are certain there are two types of lightning: positive and negative.
The most commonly accepted theory for lightning genesis is that ice crystals lose and electron when they collide in the upper part of the cloud. Once the ice particles lose an electron, their net remaining charge becomes positive, and they they tend to collect at the higher altitudes within the storm. Conversely, the ice crystals that gain an electron tend to collect at the base of the cloud. This separation of charge creates an electric field.
How and why the charge separation occurs is not fully understood, but as more collisions occur, electric fields gains in strength. When this electric field “short circuits”, electrons rush from one part of the cloud to the other creating negative lightning. If positive charge flows, it’s creates positive lightning.
Lightning comes in many different shapes and sizes. 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. Larger thunderstorms produce more lightning because there are more collisions caused by stronger updrafts, while smaller storms don’t create the same conditions, resulting in fewer lightning strikes.
As positive charge builds in the upper part of the storm, negative charge builds in the lower part. This creates 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.
Eventually, the insulating factor of the air breaks down due to ionization. When this happens, a discharge will occur, whereby electrons 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. This makes up about 75% of all lightning produced by a thunderstorm. If the discharge hits the ground, it’s called a cloud-to-ground strike. This makes up about 20% of the lightning. 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. It tend 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. They tend to be about 5 times more powerful and hotter than a negative strike. They also last about 10 times longer and strike several miles away from the storm (“bolt from the blue”). Lastly, they produce huge amounts of Extremely Low Frequency (ELF) and Very Low Frequency (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.