[TowerTalk] Lightning

[Bill Nelson]

>I suspect that there is a real reason that certain towers never get hit,
>and others get hit all the time. 

Certainly, but possibly not the reason that you may believe.

>Remember the story I told about my 70 ft tower at K4VDL topped by a 33'
>vertical that was never hit and a 12 foot mimosa tree 70 feet away
>converted to toothpicks by a strike. Do folks *really* want to ascribe
>that to random behavior? By some opinions here, the tower should have
>been 100 feet more attractive than the mimosa.

More attractive, certainly. But it was not the only target.

How may times had that mimosa tree been struck before?  What is the
ground conductivity like near the tree, as compared to near the tower?

>My elmer was the head engineer at WCTT (AM & low end of the band).
>Lessons were at the transmitter house out at a place lovingly called
>"the swamp". I think the tower was 450 foot or so quarter wave. Also
>think the swamp was 10% copper. Many summer nights watched the lightning
>gap at the bottom of the tower arc with an almost metronome regularity.
>Now and then lightning would hit the top of the tower and throw sparkles
>fifty feet from the gap balls. The gap balls got quite pitted and had to
>be replaced a couple times a summer. Lightning never took transmitter
>off the air while I was there.

>*HOW* do both stories fit under the same theory?

What other tall objects were near that tower?

>My own personal >*guess*< is that towers gain strike attractiveness in a
>manner proportional to their height. On the other hand, effective
>bleeding of charge in the region, by points, beams, whatever, creates a
>*fixed* deterence to strikes, the bled-off zone of ionization, whose
>extent is limited by laws of physics. This fixed deterence will balance
>an attractiveness which then equates to a certain height. Above this
>height, the attraction less deterence is still attractive, and strikes
>will jump the bled-off zone anyway.

There are megajoules of energy in a lightning strike. It is unlikely
that any tower (regardless of how many ionization points you put up)
is going to be able to bleed off the resulting energy. 

The lightning does not travel straight towards the earth, nor do the
leaders from the earth travel straight up towards the cloud. The jumps
are in steps (this the term "step leader"), anywhere from about 50
feet to about 150 ft.  Each step can be in any direction, but the
steps from the clouds are downwards - sometimes at right angles to
the previous step.  At this junction, another step will often form
(higher ionization) - and it will step towards the earth as well.
Now, this new step might become stronger than the previous step,
which might even die out completely.  

The steps are by the lowest impedance path towards ground, for
each step.  So the leader from the clouds might have been heading
toward the tower - then encountered a higher impedance (maybe due
to air density or even a area of lower ionization in the air). The
result could be a new leader forming away from the tower, but still
at several hundred feet. This new leader could encounter the upward
step leader from the tree - before it encountered the one from the
tower. 

>The WCTT tower was way over this height, and the K4VDL tower was below.
>I would speculate that a porcupine works FB under this height, and gets
>blown into lethal darts above this height (someone else's story). It
>would also say that lightning rods work fine on barns and houses, and
>get melted above the critical height.

The higher tower is just more likely to be have step leaders that will
intersect the step leaders from the cloud as they try to find ground.

For your shorter tower, it is only a couple of steps from tower height
to the ground.  Maybe the step leader from the clouds was well away 
from the tower in the direction of the tree.  There is nothing in
physics that says cloud leaders must form over the tallest conductor,
or head towards them.  Ever see a cloud to ground strike that was at
about a 45 degree angle to vertical?  I have.

>In the fireball story I told, where lightning came in the telephone
>line, he *did* have a small tower & a beam, which was not struck. He was
>on a mild knob with flat area around, and the knob took a lot of
>strikes. Why not his tower? This would fit with the theory above.

Maybe his tower did, and he didn't know it?  Maybe the lowest impedance
path to ground, near the tower, was not through the tower - but directly
to ground.

Maybe he has been lucky enough that the leaders from the clouds have
been a fair distance horizontally from the tower when nearing the earth.

>If the theory is true, I sure would like to know what the balance height
>is, or what it depends on. It's above 100 feet and below 450 feet. 

>I also would *guess* that ground conditions underneath the tower could
>raise or lower this height, and so could local geographic features, such
>as the ridge in one account. A local ridge would raise the critical
>height in an adjoining valley. Being on an isolated knob in otherwise
>flat territory would lower the critical height. 

It would certainly have an effect. But none of them ensure that any
given object will be stuck - or not struck. There are an infinite number
of paths that any given discharge can follow. The laws of probability
apply - there is no certainty.

Bill


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