Most of the time we see negative lightning, which originated both on the ground and in the lower, and more negatively charged, portion of the cloud.
Once the ionization process begins and plasma forms, a path is not created instantaneously. In fact, there are usually many separate paths of ionized air stemming from the cloud. These paths are typically referred to as stepped leaders.
The stepped leaders are mostly invisible, except to incredibly high speed cameras — they branch towards the ground unbelievably quickly — about 164 feet (50 meters) per microsecond!
They form branches because the air may not ionize equally in all directions. Dust or impurities in the air may cause the particles to stream more easily in one direction, giving a better chance that the stepped leader will reach the earth faster in that direction. Also, the shape of the electric field can greatly affect the ionization path. This shape depends on the location of the charged particles, which in this case are located at the bottom of the cloud and Earth's surface. If the cloud is parallel to Earth's surface, and the area is small enough that the curvature of Earth is negligible, the two charge locations will behave as two charged parallel plates. The lines of force (electric flux) generated by the charge separation will be perpendicular to the cloud and the earth.
Flux lines always radiate perpendicularly from the charge surface before moving toward the location of the opposite charge. Given that the lower surface of the cloud is not straight, the flux lines will not be uniform.
When one flux line reaches the ground first, it creates a conductive path that all other branches between the cloud and the earth will then follow. The path reaching the ground results in the strike — the sudden, massive, flow of electrical current moving from the cloud to the ground.
But while all this action is happening in the cloud, the surface of Earth is getting ready for the lightning strike as well. Next we'll look at positive streamers and what happens when these streamers meet stepped leaders.