Lightning Questions: Answers to Elusive Mysteries

[Jeremy87]

I've been reading about how lightning occurs, but I have some questions that are never answered.

1. What exactly is the stepped leader? What in it gives off light before the connection is made and current starts flowing? Are there already some electrons flowing into this newly connected point of higher potential?

2. Why does the step leader split into multiple branches, and why do failed branches light up even more when the connection is made?

3. Why does the stepped leader pause briefly every few meters and then change direction? Also, is each segment completely straight?

4. Similar to #1, what is the "streamer" (the "leader" that starts from the ground and connects with the stepped leader)? Can it also create branches? Does it advance in steps?

5. I've read that positive lightning originates from the top of the cloud where there are positive charges. Where on the ground does it find a negative charge? Why does the lightning still go cloud-to-ground and not ground-to-cloud?

I might come back with more questions based on the answers I get.

 

[krd]

1. What exactly is the stepped leader?

A name for something happening. Lightening appears to progress in steps.

What in it gives off light before the connection is made and current starts flowing? Are there already some electrons flowing into this newly connected point of higher potential?

It's arc'ing - if you look up arclight. The air does not want to conduct electricity. But the potential difference is so great with the build up of charge in the ground and air - (by the way this is the same thing as static electricity sparks you'll see from rubbing your hair in a polyester jumper) - the PD is so great, the electrons of the molecules in the air feel the pull, and jump to higher energy levels, the greater the pull the higher they go - they go crazy. The pull can be so greater they leave the atoms completely and become free electrons - they release light without a current flowing because they've become so energised.

What you get is a plasma. And the plasma can conduct electricity.

The forks that have lit up are now conductors.

2. Why does the step leader split into multiple branches, and why do failed branches light up even more when the connection is made?

The branches are trying to make a circuit. Before the circuit is made the pull of the electrical difference is more spread out. When the circuit is made, those branches that haven't made a connection are still conducting plasma, so they get belt of power.

3. Why does the stepped leader pause briefly every few meters and then change direction? Also, is each segment completely straight?

Simply because it decides to go "maybe I should go the other way...maybe that would be easier". It's trying to turn the air into a conductor. And that isn't easy. It's looking for the easiest way up. It might jump a few inches or metres, then find the air is a little different and harder, and will try to go another way.

4. Similar to #1, what is the "streamer" (the "leader" that starts from the ground and connects with the stepped leader)? Can it also create branches? Does it advance in steps?

Streamers start from the ground...if you think about it, they are branches...several will start ...one may be lucky to climb high enough to be a full strike. The lightning comes down from the clouds and meets a streamer on the way up.

5. I've read that positive lightning originates from the top of the cloud where there are positive charges. Where on the ground does it find a negative charge? Why does the lightning still go cloud-to-ground and not ground-to-cloud?

There's something wrong with the formal understanding of lightening. When I was in college, one day we did an exercise on clouds creating lightning by the conventional understanding of what's happening. We couldn't get anything near the kind of power strikes have. My pet theory is lightning has nothing to do with clouds rubbing together and creating a charge, and may be more to do with charge building up in the ground because of geological torsion and pressure. I think that's what's happening. Because even though you just see a lightning storm now and again - it's happening non stop around the world as a whole. The problem with the idea though, is why the weather seems to effect the discharge as it does, if it is coming from the ground. There's probably a simply answer, I just don't know it.

I think it's more complicated than just clouds rubbing together - and the footage of those plumes of charged particles shooting into space after a lightning strike are really interesting and confusing too. I can't find a clip right now - but it does look amazing.

I might come back with more questions based on the answers I get.

It's the first law of physics. The in-conservation of questions. The more answers you get, the more questions you seem to have.

 

[Jeremy87]

A name for something happening. Lightening appears to progress in steps.
It's arc'ing - if you look up arclight. The air does not want to conduct electricity. But the potential difference is so great with the build up of charge in the ground and air - (by the way this is the same thing as static electricity sparks you'll see from rubbing your hair in a polyester jumper) - the PD is so great, the electrons of the molecules in the air feel the pull, and jump to higher energy levels, the greater the pull the higher they go - they go crazy. The pull can be so greater they leave the atoms completely and become free electrons - they release light without a current flowing because they've become so energised.

What you get is a plasma. And the plasma can conduct electricity.

The forks that have lit up are now conductors.

Well arc lamps send current through the initially ionized path. A lightning step doesn't really connect to anywhere so there's no continuous current, but maybe just ionizing the air does momentarily light it up slightly.

The branches are trying to make a circuit. Before the circuit is made the pull of the electrical difference is more spread out. When the circuit is made, those branches that haven't made a connection are still conducting plasma, so they get belt of power.

I've seen some slow-motion videos, and the incomplete branches do a bright flash when a connection is made. After that they fade away while the main connection keeps flashing from repeated strikes. That makes me believe the stepped leaders actually do have some low current going through to the new point of slightly higher potential, and when the connection is made, the electrons in the incomplete branches will all rush back to the main connection and momentarily light up these branches.

Simply because it decides to go "maybe I should go the other way...maybe that would be easier". It's trying to turn the air into a conductor. And that isn't easy. It's looking for the easiest way up. It might jump a few inches or metres, then find the air is a little different and harder, and will try to go another way.

But surely there must be a path present already, otherwise the lightning wouldn't even begin to form in the first place? I'm just wondering why it doesn't go all the way to the ground at once.
What would happen if there was a layer of infinite resistance above the ground? Would the leaders just reach the edge of this resistance because the potential is highest there, or would there be no leaders at all?

Streamers start from the ground...if you think about it, they are branches...several will start ...one may be lucky to climb high enough to be a full strike. The lightning comes down from the clouds and meets a streamer on the way up.

As I see it, streamers are "positive" stepped leaders, but since holes have a lower mobility than electrons, they won't start forming until the negative leaders are close enough.

 

[Dotini]

I'm currently reading Martin Uman's book, The Lightning Discharge. All the really excellent questions being discussed above are addressed by Uman in great depth in both theory and experimentation, with hundreds of journal references. An entire chapter is devoted to the stepped leader, another to the return stroke, etc. The central importance of electric fields to all aspects of lightning is thoroughly analyzed, all the way back to Maxwell's equations. You'll be both pleased and amazed with the knowledge to be gained here. I'd like to respectfully suggest that all PF forum members interested in lightning and associated subjects order this book (it's cheap).

Respectfully submitted,
Steve