CLOUD TO GROUND LIGHTNING
As the electric field strength around the cloud increases, the air begins to “break down” to neutralize the charge separation. This can only happen in extremely strong electric fields, tens of thousands of volts per centimeter (approximately 30 kV/cm). The air becomes ionized: it breaks down into positively charged ions and electrons. Ionized air (also called plasma) is much more conductive for electric current than non-ionized air.
A lightning bolt begins from the cloud down with the development of stepped leaders. As the air begins to break down, a negatively charged channel of ionized air emerges from the bottom of the cloud, glowing in purple, moving towards the positively charged ground. The stepped leader will generally follow the electric field (flux) lines, running from the bottom of the thunderstorm to the ground. In a completely homogeneous air and electric field, stepped leaders would develop vertically towards the ground, however, the field is not homogeneous. Variations caused by heterogeneities in charge distribution in the cloud and between the cloud and the ground cause the stepped leader to take a much more irregular path.
The stepped leader develops step-wise: in surges about 50 m long, with each surge producing a small flash of light. Leaders grow towards the ground, branching out along the way with new leaders forming in kinks, bends and turns. This process is extremely rapid, so do not be confused by the fact you have not seen it with your own eyes. Stepped leaders grow at a speed of 100 000 – 200 000 m/s (100-200 km/s); each step forms even faster (~7×10^5 m/s, over 10% the speed of light), but the stepped leader pauses before taking a new step. It takes a very high speed camera to capture this process. The upward leader takes several tens of milliseconds to travel from the point of origin in the cloud to (close to) the ground.
As stepped leaders approach the ground, the ground responds to the strong electric field by growing upward positive streamers (often called just ‘upward streamers’). They can form on any surface, but tend to be more prominent on sharp edges. Unlike stepped leaders, positive streamers do not grow: they reach upwards at a steady length as stepped leaders approach from above.
When the stepped leader going downward and any of the upward reaching positive streamers connect, a conductive path has opened between the cloud and the ground. Current flows through the channel, equalizing the charge difference between the cloud and the ground: this is the lightning bolt.
Note that the path the stepped leader traces from the cloud to the ground is not necessarily the shortest path – it actually never is. When the flash occurs, the charge from stepped leaders connected to the leader that reached the ground also provide current: charge within the leaders flows into the main lightning channel. Leaders undergo the same heating and light flash as the main channel.
From the point of contact between the stepped leader and the upward streamer, the current rapidly flows to the ground, progressively drawing charge from parts of the stepped leader ‘upstream’. This is the return stroke. The return stroke propagates upward along the main channel at 1/3 the speed of light (1×10^8 m/s).
The return stroke typically has a current of about 30 kA (30 000 Amperes). The rise to peak current is extremely rapid, it happens in a few microseconds. The air is heated to approximately 30 000 K – five times the surface temperature of the Sun. The rapid heating produces a shock wave, which rapidly transitions into an acoustic / sound wave – thunder.
This is a typical cloud-to-ground (negative) lightning discharge. It consists of:
1. Downward, negatively charged stepped leader
2. Short, upward, positively charged streamers
3. Upward return stroke
Most cloud-to-ground lightning discharges actually contain multiple strokes. When the downward stepped leader connects with the upward streamer the first return stroke travels up the main channel in approximately 100 microseconds. It is then followed by another cycle, very similar to the first. A dart leader propagates down the the existing main channel. The dart leader, unlike the stepped leader, does not branch, but only propagates down the main channel. As it nears the ground, a short upward discharge forms, and when it connects with the downward moving dart leader a new return stroke initiates and travels up the main channel.
A typical lightning flash contains numerous return strokes, sometimes several tens of closely spaced (in time) return strokes. This can give the lightning the appearance of flickering. Because the first return stroke drains all stepped leaders connected to it, only the first return stroke is branched. All subsequent return strokes form with the dart leader following the existing main channel and only the main channel conducts the charge.
Fun fact: the longest recorded lightning flash (cloud-to-ground) lasted ??. It was recorded
Fun fact: there is a similar process of air break down in extremely strong electric fields that everyone is familiar with. If you walk rugs or other types of synthetic (preproge??)