Re: OT. Lightning rod ???

Posted by keb on June 17, 2006 at 17:04:44 from (207.94.27.6):

In Reply to: OT. Lightning rod ??? posted by I Bleed Green on June 16, 2006 at 19:17:28:

All right guys, enough of this nonsense about lightning rods repelling lightning or discharging the earth/air before a strike can accumulate. That premise, along with the porqupine balls, fuzz balls, & other smoke & mirrors, has been disproved any number of times. Unfortunately, people who make that stuff tend to expouse a lot of junk science.

I designed lighting protection for aircraft in a previous job, so I know a little bit about how lightning propagates and what does and does not work.

Lightning propagates in steps on the order of 50 meters long. Essentiallly what happens is enough charge accumulates to break down the air, just like a corona discharge off a van de graff generator that we've all seen in the old science fiction movies. The steps will tend to move in the general direction of the opposite charge, but as you can see from watching a lightning stroke the path is anything but straight. Once a steped leader has established a path to the next discharge, further charge propagates down this path & the when voltage at the end of the path has reached a sufficient value, another leader (or multiple leaders) will form.

Things like lightning rods have no effect on lightning propagation until the end of a leader comes within a few hundred meters of a conductive point. When the electrostatic field around the leader becomes strong enough, leaders (corona discharges) will begin to form off of things like trees, structures, lighting rods, power poles, etc., etc. Whichever one of these leaders makes contact first with the downward propagating leader determines thes path of the discharge.

Note that we don't see the leaders (special cameras are used). There isn't a lot of current flow until the path is completed. Once the path is completed, then there's a big discharge, which is what we see & is what creates thunder by superheating the air around the channel.

Lightning can also originate on the ground & propagate upward, although strikes originating in the coulds are much more common.

The "voltage" in a lighting strike only serves to establish the initial discharge path. Its the large current pulse that occurs once the path is initiated that does the damage. A severe stroke can have a peak current on the order of 200,000 amperes.

If you're interested, theres a book called "Lighting" by Martin Uman (a professor who's done a lot of studies of lighting). I got my copy through Amazon, & I assume its still available. The best all around reference I've seen is the FAA Lightning Protection Handbook for Aircraft. I don't think its available for download on the web, but you can get a copy from the Government Printing Office.

Anyway, as far as installing lighting rods for looks only, they won't do a thing either to attract or avoid lightning. Lightning is a very complex phenomenon, and the lightning community is still learning. One thing that has been well established, by NASA among others, is that there is no known scheme to dissipate or repel lighting. Anyone that says otherwise is blowing smoke or depending on bad science.

Believe me, if there was a way to repel lightning, the aircraft inducstry would have adopted it a long time ago.

Keith

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