Frise
Bandeau Lafoudre 
 
Atmospheric discharges
Cloud-to-cloud, or inter-cloud lightning and intra-cloud lightning within a single cloud, cover more than fifty percent of all atmospheric discharges. Most of the advanced research, however, has involved cloud to ground lightning.  This is mainly due to the physical practicalities of being able to measure and record the deaths, injuries, forest fires, telecommunications and transport interruptions etc. generated by cloud to ground lightning strikes, as well as its optical characteristics. The figures opposite illustrate the four different types of lightning strikes.

These are then divided into four sub-groups, depending on the direction of the leader stroke which causes the electrical discharge or lightning bolt, upwards or downwards, and whether the leader is positively or negatively charged.

In moderate climates 90% of lightning strikes are in the first category, with a downwards negatively charged leader stroke. The second category covers downwards positively charged leader strokes,   which account for the remaining 10%  of all cloud-to-ground lightning discharges. Categories 3 and 4 are generated by upwards leaders and are quite infrequent, occurring mostly in mountainous regions or with very high structures.

Descending negatively charged leader lightning
The lightning bolt is created inside a storm cloud. The most probable hypothesis as to how this occurs is via a bi-directional discharge with a positive channel and a negative channel inside the cloud, probably between the positive and negatively charged areas. The positive discharge moves horizontally,  forking inside the cloud. The negative discharge moves towards the ground. The leader is generally a "stepped leader", proceeding downwards in a number of quick jumps or steps, each being 10 to 30 metres long. It contains a negative charge of around 10 C and moves at between  0.15 and 1 metre per microsecond.
Atmospheric discharges
Atmospheric discharges

As the leader approaches the ground with a potential charge of  -10 MV, it provokes an increase in the earth’s electric field and generates one or more upward moving leaders, or positive streamers. They form the discharge path. The upward moving positive streamers and the downward moving leader, join up a few tens of metres above ground level.

The stepped leader discharges when a ground based positive streamer meets it and neutralises its charge. This is called the return stroke.  It takes place at around one third of the speed of light. The first return strike has a ground level current spiking at around 30 kA and an upwards speed of a few microseconds. During this phase, the channel temperature can rise to 30000°K, which compresses the surrounding clear air and creates a supersonic shock wave which decays into an acoustic wave that is heard as thunder.

After the return strike phase, the lightning bolt disappears. If, however, there is still a charge in the storm cloud, another leader will fill the channel at a speed of 3x106 m/s with a charge of around 1 C and a current peak of 1 kA. The leader meets another streamer and creates another return strike. The process continues and can involve up to 15 return strikes. The last return strike is usually very powerful with a current of around 100 A which drains the residual charge in the cloud.