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I'd read that the current theory of how lightning rods protect a house, antenna tower, or other structure is by preventing lightning from striking -- that is, the sharply pointed rod lets charge dissipate from the structure, so that it doesn't build up a potential opposite that of the cloud and attract strikes.

I've also read (in older references, mostly) that lightning rods attract lightning and actually increase the risk of a strike, by offering a higher path for the stepped leader (and hence the main bolt) to ground.

I can easily see how either situation could be argued -- put up something to attract lightning and route the strike harmlessly to ground, or put up something that makes the strike less likely. It's been well known for a couple hundred years that lightning rods do appear to protect structures where they're correctly installed. What's the up to date theory on how?

Mike Waters
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Zeiss Ikon
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    This question doesn't seem to be about radio or the technology of radio, and furthermore it already exists on physics.stackexchange.com. – rclocher3 Aug 29 '19 at 18:32
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    @rclocher3 I'm not a member on Physics.SE, and those questions don't come up here when typing a question. Seems to me lightning protection is about radio -- if I don't have an antenna up, I don't have a radio that works, and if I'm in a lightning risk area, an antenna is a potential problem. If I need to edit to be more explicit on that side, please say so. – Zeiss Ikon Aug 29 '19 at 18:52
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    I suppose that you are referring to my answer here? – Mike Waters Aug 29 '19 at 18:59
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    @MikeWaters That's the argument I'm referring to, though I didn't recall you'd answered anything related recently. – Zeiss Ikon Aug 29 '19 at 19:02
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    @rclocher3 Thank you for linking to that answer on PhysicsSE! I am inclined to agree with Zeiss. I haven't read that entire linked article, but perhaps something there could be copied and pasted into in an answer with these guidlines in mind. IMHO, https://ham.stackexchange.com/help/duplicates refers to duplicates within the same SE site. This particular question is one on the rare ones in that it is on-topic on more than one site. I think it's a great on-topic question that deserves an answer. – Mike Waters Aug 29 '19 at 19:06

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These ideas have been around for centuries as lightning dissipation theory and lightning diversion theory.

Notably, Benjamin Franklin was an early proponent of lightning dissipation theory. He noticed on a small scale that a static charge could be dissipated without a spark by bringing a grounded needle near a charged object. He correctly reasoned lightning was static electricity on a larger scale. He then hypothesized it could be discharged in the same way by putting pointy, grounded lightning rods at the tops of buildings.

This experiment works in the lab because the sharp point of the needle creates a high local potential gradient that facilitates corona discharge. And it was empirically shown that structure fires were reduced when Franklin's lightning rods were installed. So does that mean lightning dissipation theory is correct?

No. Structures with pointed rods are still frequently struck by lightning, but they don't burn down. While Franklin's idea works well at small scales, the difference with lightning is the static charge is many orders of magnitude greater. Trying to discharge a thundercloud with a lightning rod, or even hundreds of them, is like trying to put out a forest fire with a garden hose.

Fortunately, a lightning rod intended to dissipate lightning also works well to divert it. As the rod is solidly grounded, the strike current prefers to follow the grounding conductors and as such does not create arcing within the structure. It's the high temperature of the arc that starts fires. No arc, no fire.

Phil Frost - W8II
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As with most argued questions like this, there is some truth to both sides.

Lightning rods can dissipate small charges if they have sharp tips, but when that doesn't work, their real purpose is to conduct the lightning around your house instead of through it.

I think the current standard (recently changed?) for lightning rods is round tips.

Lightning rods are one of three parts of a good protection system. The other two parts are ground rods (connected to the lightning rods) and surge protectors at each device connected to an external wire. (If you have a direct strike on a wire, the lightning rods only reduce the spike voltage there to a manageable level, not elminiate it entirely. Lightning doesn't take the shortest path -- it takes ALL paths, but follows the voltage divider rule with some paths getting more.)

mrog
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user10489
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    +1, but more lightning strikes occur at round tips than sharp points. In some circles, this was somehow misconstrued as somehow being a good thing. – Mike Waters Sep 06 '19 at 11:59
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    I don't know the reasoning behind switching from sharp tips to round tips. For all I know, ti could be because it's safer to fall on a round tip than a sharp tip and have nothing to do with lightning at all. Either way, I don't think it changes the function or purpose of the rods in a huge way. – user10489 Sep 07 '19 at 13:23
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    The principle at work here is field emission. Basically, the sharper the point, the easier it is for electrons to escape from it at a lower voltage in a corona discharge. I plan on answering this in detail when I get caught up with work here. – Mike Waters Sep 07 '19 at 17:07
  • I know the theory of sharp vs. round tips, but ironically, I don't think it's relevant to the question and the industry standard choice either ignores the theory or is using some counter intuitive theory beyond the obvious field emission / corona reasons. – user10489 Sep 08 '19 at 13:33
  • What do you mean "If you have a direct strike on a wire, the lightning rods only reduce the spike voltage there...? " The lightning rods (air terminals themselves) do nothing at all until they're struck directly (or suffer a collapsed leader), they don't help with lightning coming in via the power line, for example. – tomnexus Sep 08 '19 at 16:45
  • Well, there should be surge protection between the wires and the ground the lightning rods use... – user10489 Sep 08 '19 at 17:14
  • Contrary to the intuition, sharpest point makes poor discharge performance. I also want to add that discharge from pointy point only works for discharging electron (only works againts positively charged cloud). So lightning rod very possibly will get strike when dealing with negative-charged clouds. For that, we have active type lightning pole (in Sweden?) using radioisotope but not widely accepted. – RainerJ Sep 19 '20 at 05:50
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One of the best demonstrations, that I've seen anyway, of the effect of pointed "air terminals" begins at 17:25 of this rather old video...

https://www.youtube.com/watch?v=2sSqzLPMb4s

The whole video is well worth the time as the demonstrations are very compelling, but your answer awaits at 18:04.

JSH
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    Please give a description of the phenomena seen in the video and what conclusions should be drawn. While links in answers are good, an answer should not rely on links to provide a sufficient answer to the question. – Kevin Reid AG6YO Sep 04 '19 at 00:54
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    Links rot. YouTube takes down videos for a multitude of reasons, or occasionally they vanish for no discernible reason. Further, I seldom have time to watch 20 minutes into a video on any subject -- got a "watch later" list a mile long. – Zeiss Ikon Sep 04 '19 at 11:05
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    Nice training video, but this just isn't how lightning works. Read the linked article at physics.se. What's happening in the video is that the toy voltage generator isn't able to sustain a high voltage when it has a few sharp discharge points on it. It's such a big cheat that the makers of the video must have known about it, but thought it was worth including to reinforce the need for conductors. Lightning is very very different. For one, there is effectively unlimited charge available in the ground and the leader, so it can't be dissipated. – tomnexus Sep 04 '19 at 11:24