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Part 1 - Stepped Leader & First Return Stroke | Part 2 - Secondary Return Strokes
Cloud-to-ground lightning strikes often contain repeated discharges down the same path in rapid succession following the first return stroke. These secondary return strokes often make a lightning strike seem to 'pulse' or 'flicker' on and off.
 If you could make the clouds and the earth invisible - allowing you to see an entire cloud-to-ground lightning strike from top to bottom, you would see a full dendritic structure very similar to a tree. Above, branches extending deep into the cloud tap into areas of electrical charge. In the earth below, a network of 'roots' penetrates below ground. The photo at right shows part of a distant cloud-to-ground lightning strike's in-cloud structure as some of the dendritic branches peek out of the cloud. This 'lightning tree' makes up the first return stroke of the cloud-to-ground discharge.
There are usually additional areas of electrical charge higher up or farther horizontally inside the cloud. Once the initial lightning channel is established, it is like a long wire connecting the cloud to the ground. 'Earth ground' is now essentially brought up inside the cloud by this 'wire' after the first return stroke (See Stage 3 below). Now you have a scenario in which these pockets of charge deep in the cloud now are 'within reach' of 'earth ground'. Previously, these pockets were too far away to overcome the insulating air to reach the ground. Now, the initial conductive lightning channel has brought 'ground' close enough to the charge pockets for them to discharge to it.
And that's exactly what happens. These charge pockets 'spark over' to the now-established lightning channel and use it to reach the ground - creating a second return stroke and pulse of light (See Stage 4 below). The branches of the 'lightning tree' grow deeper into the cloud, making 'earth ground' accessable to more areas of charge, and more return strokes follow (See Stage 5 below).
This chain reaction continues until all charge pockets within reach of the grounded 'lightning tree' are tapped and discharged. A complete lightning discharge event can contain anywhere from a single return stroke to more than twenty, depending on the electrical structure of the storm. The numerous pulsing return strokes occur in rapid succession, making the lightning bolt appear to flicker on and off.
The sequence below demonstrates the multiple return stroke process of a cloud-to-ground lightning strike. Two animations of the sequence appears below, one in labeled slow-motion and the second in full speed.
- Stage 1: Storm electification results in pockets of charge in the cloud. For the sake of illustration, polarity has been ignored here.
- Stage 2: Main Charge Pocket discharges to ground via stepped leader connection. Dendritic network of branches 'taps' charge in main charge pocket.
- Stage 3. Established lightning channel brings 'earth ground' close to Charge Pocket #3.
- Stage 4. Charge Pocket #3 discharges to the established channel. Earth ground is now brought deeper into the cloud.
- Stage 5. Charge Pockets #2 and #4 now are close enough to 'earth ground' via the conductive lightning channel that they too can discharge to ground.
[ restart animation ]
A total of four return strokes occured with the 'sample lightning strike' above (First return stroke plus 3 secondary return strokes).
In this animation, the return strokes are identical in intensity. In actuality, however, different return strokes that make up a single lightning event will often vary in brightness and intensity. For instance, you might see: BRIGHT first return stroke, DIM second stroke, DIM third stroke, BRIGHT fourth stroke, DIM fifth stroke, MEDIUM sixth stroke, and so on. The brightness of each return stroke should indicate the size and strength of each charge area being tapped. In other words, larger and more intense 'pockets' discharging through the channel will result in a brighter, more intense return strokes. In some cases, secondary return strokes can be brighter than the first (branched) stroke!
The following animation shows what the above lightning discharge would look like in real time. Keep in mind that most of the upper structure of this discharge (the 'branched tree' sections) would be obscured from view in an actual thunderstorm. This demonstration here is using a fictional 'see-through storm' so the entire above-ground structure of the lightning is visible:
[ restart animation ]
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