What Causes Lightning Rods to Buzz?

[PatentLawyer]

I am familiar with the basic theory of how lightning rods work, at least as explained in the text "Electromagnetics" by Kraus (Ohio State).

I was once standing on the roof of a 14-story dormitory at the U. of Illinois, Champaign-Urbana. A thunderstorm was approaching.

The lightning rods on the roof all began to buzz.

I was taking a course in electromagnetic fields at the time. I related the buzzing to the prof, but he had no explanation.

Can anybody explain this ? I suppose the ambient 60 Hz field from the power company could be implicated somehow.

Thank you.

 

[Gordianus]

The buzz you hear is due to corona discharge. Google it.

 

[PatentLawyer]

I am sorry, but "corona discharge" does not answer the question of why buzzing (mechanical vibration) should occur.

When I said I was familiar with the basics as outlined in Kraus, why do you assume that I am unaware of "corona discharge" ?

Does anybody REALLY know the answer ?

 

[Meir Achuz]

There is intermittent discharge between the rod and the atmosphere.
The buzzing is the sound caused by ionization and heating of the atmosphere.

 

[GRDixon]

There is intermittent discharge between the rod and the atmosphere.
The buzzing is the sound caused by ionization and heating of the atmosphere.

I agree, although a few more words might add clarity. The intermittent discharges are so closely spaced in time that it is more like a waterfall than a single water drop. The myriad, tiny "thunderclaps" are so closely spaced as to produce the buzzing sound. An analogy would be the noise made by a modern gatling gun. Rather than hear the individual muzzle blasts one hears a loud buzz when the gun is fired. By the way, I was up on a ladder, staining log siding close to a lightning rod, when the sky darkened. I too heard the buzz and scooted down the ladder with the hairs on my neck standing erect (probably more due to primal fear than any associated electromagnetic effect).

 

[Gordianus]

I am sorry, but "corona discharge" does not answer the question of why buzzing (mechanical vibration) should occur.

When I said I was familiar with the basics as outlined in Kraus, why do you assume that I am unaware of "corona discharge" ?

Does anybody REALLY know the answer ?

As much as I never assumed you weren't unaware of corona discharge phenomena, I do assume you're quite aware what uppercase means in forums like this.
Having said that, I'd suggest further reading about corona discharges and you'll find they can be fairly powerful sound sources because of its pulsed behavior. Therefore, the ligthning rod can start vibrating at any of its resonance frequencies. That's the sound you hear.