How lightning is first triggered is still something climatologists are quite unsure about. Most lighting bolts occur internally and very few are between the cloud and the ground. Even for those that are external the mechanism is still in action within the cloud where direct observation isn’t possible. It is generally excepted that to creat a lightening strike a lightening seed is required. This is simply an ionised shard inside the cloud that magnifies the local electric field. This helps produce something called a lightening leader. These leaders are channels of ionised air that will conduct electricity and disperse from areas of high charge in the cloud. Positive leaders are forced upwards within the cloud while negative leaders are propelled towards the Earth. The negative ions get slowed down as they move down and so begin to collate at a pool in the air which then launches another leader, sometimes multiple. This is what leads to the distinctive jagged and tree like pattern of lightening. As the negative leader gets close to the ground, positive charges gather. Electric fields love to flow from sharp points and compared to the flat Earth a tree, lamppost or person seems pretty sharp. This leads to a positive channel getting flung out known as an upwards streamer. When the negative and positive ions connect the lightening strikes down the closed circuit to the Earth.
The problem is that the original leader formed within the cloud is hypothesised to be bidirectional containing both a positive and negative end. These origin leaders are completely enclosed within the cloud and so observation is very difficult and only a few have ever been seen. But recently, at the Lightening Observatory in Gainesville in Florida, a full lighting discharge was observed both optically and with electromagnetic radio frequency equipment. The bidirectional leader was seen to form horizontally before the negative end turned goundward. It was observed on the electromagnetic equipment that this particular leader demonstrated breakdown on the negative end as is commonly seen and the positive end performed a previously unobserved pulsating behaviour. This is a breakthrough achievement as it is the first observation of s lightening strike from beginning to end including return stroke (where charge on the ground uses the ionised path left by the previous bolt to fire lighting back at the cloud). Hopefully this data will either strengthen or disprove other hypotheses about lightening and develop our understanding of this strange and dangerous natural effect.