lightning rod question

[Tom Biasi]

"Don Kelly" <dhky@peeshaw.ca> wrote in message
news:ZMGdd.783596$M95.423353@pd7tw1no...

"Tom Biasi" <tombiasi@REMOVETHISoptonline.net> wrote in message
news:bNDdd.33560$YM4.10151285@news4.srv.hcvlny.cv.net...

"rayjking" <rayjking@bellsouth.net> wrote in message
news:57vdd.185347$as2.86216@bignews3.bellsouth.net...
Hi,

The ball is usefull in spreading heat from a direct strike. A #8 wire
can
carry any known strike but it will bern at the point of the strike
without
more thermal mass.

Ray
I didn't catch the original post but I thought that this may be a good
time
to mention that lightning rods are/were not designed for direct hits. The
purpose is to keep the accumulated charge below a safe level.
Tom

---------------
Actually this is extremely questionable. Design that I know of is based on
the probability of the stroke striking the rod rather than the protected
area. Granted, in some situations, particularly with tall structures, such
draining does occur (possibly increasing the chance of side
flashes-another
problem) but this is a bonus, not the basis for design.
I am aware of a purveyor of lightning protection that claimed that his
protection works on the basis of charge dissipation. I have seen no
evidence
that it does. I have no idea if he is still is in business.

Generally the source of the charge is several miles overhead -the rod
doesn't get seen by it. When a leader gets near a rod then it may be a
preferred target for the next step and if it is, then the main stroke
will
be to the rod-if not something else gets hit. Design is based on it being
such a target for higher current strokes (but not necessarily lower
current
strokes). This is true for protective systems for transmission lines as
well as structures. Catch the damaging strokes know that some of the
little
ones will get by.

References:
Moussa & Shrinivasta, "Shielding of Tall Structures Against Direct
Lightning
Strokes" Canadian Conference on Electrical and Computer Engineering,
Vancouver, BC, 1988
Energy Systems Journal, Vol. 11, N0.1 1991
EPRI Transmission Reference Book, 345KV and Above.
I believe that IEEE changed its standards to reflect the concepts involved
in these references.
Sorry that I do not have more recent references but I have been retired
for
some time.

I agree with Vermin on the lack of usefulness of the ball. There appears
to
be no rational basis for its use. Note that its thermal mass is generally
pretty small and by the time it starts to dissipate heat, any damage is
done.

--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

Thanks for the info, I'll see what else I can find. Probably the foremost

expert on lightning in this group is Mark Kinsler. I believe he did his
doctorate on the subject. I have not seen posts from him in a long while.
Tom

 

[William J. Beaty]

"Tom Biasi" <tombiasi@REMOVETHISoptonline.net> wrote in message news:<bNDdd.33560$YM4.10151285@news4.srv.hcvlny.cv.net>...

I didn't catch the original post but I thought that this may be a good time
to mention that lightning rods are/were not designed for direct hits. The
purpose is to keep the accumulated charge below a safe level.

Not true.

Even some reference books claim the above. They've fallen for
a type of physics myth; a "science urban legend."

Yes, in 1790 they assumed that a little bitty lightning rod could
discharge a miles-wide thunderstorm across the miles of space above
the rod. The experts reasoned that, after all, a needle could
discharge the main terminal of an electrostatic generator even
if the needle was many inches away. Researchers later figured out
how silly this was.

Check out Dr. Martin Uman's book "LIGHTNING" for some ACCURATE
lightning-rod discussion. (It's probably unwise to listen to
any of the emotional and dishonest "belief systems" espoused in
this thread, including my own! Go see what lightning physicists
actually say. Experiments and mathematical reasoning will win
over politics and "beliefs.")


Still reading?

Well then it's your own fault.

Why can't a lightning rod discharge the storm? The scale is wrong.
While a needle can discharge an electrostatic generator via ion-leakage
across a large gap, a lightning rod is not like a needle. A lightning
rod is tiny: like one fiber in a piece of felt if we obey the scaling.
If one fiber is slightly taller than the others, well, the high voltage
electrode doesn't care. If you erect a lightning rod, the storm
won't even notice.

Also, the scale is wrong for "charged wind" to transport any charge
upwards. If you hold a needle near a high voltage terminal, the
needle "emits" charged wind which travels at many cm per second,
and there is a significant electric current in the air; on the order
of microamps or tens of microamps. This easily shorts out an
electrostatic generator.

But if we scale things up and use a lightning rod and thunderstorm,
the "charged wind" coming from the tip of the rod STILL TRAVELS AT
CM/SEC SPEEDS AND THE CURRENT IS STILL MICROAMPS. To have a significant
effect on the charged storm cloud, this velocity would have to scale up
too. The lightning rod would have to "emit" an electric wind that
traveled at tens or hundreds of KM per hour, and the electric current
would have to be thousands of times higher than 10uA.



Note that the above is one of several "lightning rod controversies."
Emotions run high in such controversies, so you cannot trust what
either side tells you. You can't even trust many reference books!

 

[William J. Beaty]

NoSpam@daqarta.com (Bob Masta) wrote in message news:<4177a8df.925016@news.itd.umich.edu>...

I'm no expert on this, but I do recall my undergrad Electromagnetic
Fields instructor coming down on the draining side of this issue.

Interesting. I've also found that the "draining myth" has spread
to chindren's science textbooks.

His examples were opposite to yours: He pointed out that the
size of the ground wires on the typical barn or home lightning rod
was far too small to withstand a direct hit.

What did he think would happen if such a small wire supported such
currents?

I've seen kiloampere (kilojoule) pulses applied to 12 gauge wire.
It doesn't vaporise. Instead it gets warm. So the issue is: if
lightning was guided by, say, some 10-gauge solid copper wire,
would the wire become hot enough to start a fire? I would have
to be VERY hot: think of how hard it is to start a fire using a
piece of wood and a soldering iron.

If the wire is bent into tight curves, the mechanical (magnetic) forces
of kilo-amp currents can break the wire. But this would have little effect
on its function: the high voltage arc could easily jump across any break.

He noted that tall
buildings, which do sustain repeated hits, have massive ground
conductors to handle the current.

True. They want to guarantee that the wires never become so hot that
they could conceivably start a fire.

On the other hand, it sounds to me like he first made up his mind, and
then he started looking ONLY for arguments that supported his viewpoint.
That's "religious style" thinking. Did he present the opposing arguments?

It's much better to start out not knowing, and then to collect lots of
arguments and facts both pro and con.

 

[Tom MacIntyre]

On Thu, 21 Oct 2004 19:12:06 GMT, Rich Grise <rich@example.net> wrote:

On Thu, 21 Oct 2004 07:33:00 -0400, w_tom wrote:

Don't worry about the ball on the end of a lightning rod.

Hm. And here, I thought it was for catching ball lightning.

;-)
Rich

And I just thought of a whole new meaning for the term "ball and
chain".

Tom

 

[Sam Wormley]

Roger Johansson wrote:

St. Elmo's Fire is some kind of algea which glow in the dark, with the
same chemical processes as in fireflies. AFAIK

St. Elmo's fire is a plasma (i.e. a hot, ionized gas) that forms arounds
the tips of raised, pointed conductors during thunderstorms. It is known
as a corona discharge or point discharge to physicists.

 

[Don Kelly]

"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:4177a8df.925016@news.itd.umich.edu...

On Thu, 21 Oct 2004 04:19:05 GMT, "Don Kelly" <dhky@peeshaw.ca> wrote:

Actually this is extremely questionable. Design that I know of is based
on
the probability of the stroke striking the rod rather than the protected
area. Granted, in some situations, particularly with tall structures,
such
draining does occur (possibly increasing the chance of side
flashes-another
problem) but this is a bonus, not the basis for design.

I'm no expert on this, but I do recall my undergrad Electromagnetic
Fields instructor coming down on the draining side of this issue.
His examples were opposite to yours: He pointed out that the
size of the ground wires on the typical barn or home lightning rod
was far too small to withstand a direct hit. He noted that tall
buildings, which do sustain repeated hits, have massive ground
conductors to handle the current.
-----------

The use of a #8 conductor is a bit iffy- even for home use. In addition the
placing of a rod at each end of a gable roof is also very iffy. Even in the
old days, a cone of protection was assumed. 30 degree cone good, 45 -not
bad. The problem is that this left most of the roof unprotected by the rod
system. Fortunately the probability of any house sized area being hit is
pretty low.
Another thing that was once written into codes was the use of odd sizes of
conductors- e.g.#5- only available through the purveyors if lightning rods.
However, from all that I have read on the subject and some design work on
the basis of known principles (i.e. Moussa's work and EPRI).
Look at your instructor's contention: say #8 was used - about 0.0006
ohms/ft- lets make it 0.01 ohms/ft to allow for skin effect and whatever.
Now consider a typical stroke- about 35kA peak- Assume that it peaks in
about 1 .5 microseconds and lasts for 100 microseconds for an average
current of less than 20kA for 100 microseconds. Average power =4x10^6
watts--WOW! but energy per foot of conductor is 400 Joules. Not all that
great. The point of the rod may be gone but the rest should be OK. What
might happen is that there is a poor contact somewhere and a high energy
dissipation at that contact- blowing it to ratshit.

As r w_tom indicated grounding and the soundness of connections is
important. Thank you, w_Tom
No point in diverting 35kA into a 10 ohm rod where the down conductor is 2
feet away from your backside on the john. EM theory comes in to play more
when the stroke has occured- but little more than consideration of
travelling waves and their reflections need be considered from a protection
point of view. . The effect of a reflection can seriously burn your butt.
--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

That was probably the only meaningful thing I recalled from that
class. All the rest was curls, dels, and other funny symbols.
(That was over 35 years ago. A lot of synapses could have
drained to ground since then!)


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

 

[Myxococcus xanthus]

billb@eskimo.com (William J. Beaty) wrote in message news:<2251b4e6.0410220942.32c3f461@posting.google.com>...

Why can't a lightning rod discharge the storm? The scale is wrong.
While a needle can discharge an electrostatic generator via ion-leakage
across a large gap, a lightning rod is not like a needle. A lightning
rod is tiny: like one fiber in a piece of felt if we obey the scaling.
If one fiber is slightly taller than the others, well, the high voltage
electrode doesn't care. If you erect a lightning rod, the storm
won't even notice.

Also, the scale is wrong for "charged wind" to transport any charge
upwards. If you hold a needle near a high voltage terminal, the
needle "emits" charged wind which travels at many cm per second,
and there is a significant electric current in the air; on the order
of microamps or tens of microamps. This easily shorts out an
electrostatic generator.

But if we scale things up and use a lightning rod and thunderstorm,
the "charged wind" coming from the tip of the rod STILL TRAVELS AT
CM/SEC SPEEDS AND THE CURRENT IS STILL MICROAMPS. To have a significant
effect on the charged storm cloud, this velocity would have to scale up
too. The lightning rod would have to "emit" an electric wind that
traveled at tens or hundreds of KM per hour, and the electric current
would have to be thousands of times higher than 10uA.

That's focusing on the cloud-discharging capabilities of -one- rod,
obviously miniscule.

Would be the -combined- effect of (1) thousands of lightning rods in a
moderate-sized community, plus (2) hundreds of miles of transmission
line, telephone line, supporting cable, etc. suspended between
hundreds of tall poles and tall towers, plus (3) tall lamp posts, tall
antennas, etc. have a significant cloud discharging effect?

Order-of-magnitude estimate on this? I'm wondering, since I've seen
several ground/tree strikes while out in rural areas, but only one
direct strike to a tree in the city, and I'm an urban dweller.

Myxococcus xanthus

 

[Richard Henry]

"Roger Johansson" <no-email@home.se> wrote in message
news:Xns958BEE4F8CDE86336@130.133.1.4...

Rich Grise <rich@example.net> wrote:

I would like some explanation of ball lighting. I have known of a ball
( corona ) in the cockpit of a sail boat.

Would that be St. Elmo's Fire?

St. Elmo's Fire is some kind of algea which glow in the dark, with the
same chemical processes as in fireflies. AFAIK

I have heard that ball lightning consists of superheated air, ionized or
plasma or something like that. The lightning has superheated the air and
some charge can maybe linger in the ball lightning. It is like water
droplets on a hot plate. They are isolated towards the hot plate by a
layer of steam. That is why the water droplets can survive so long.

In a dry climate the superheated air, or plasma, can stay together
because the air works as an isolator, somehow.

"I have heard...somehow"

That's what I like. Clear, concise references.

 

[TimR]

"Don Kelly" <dhky@peeshaw.ca> wrote in message news:<yaFed.9731$%k.8545@pd7tw2no>...

"Myxococcus xanthus" <mold-guardian@comcast.net> wrote in message
news:ce5e7813.0410230428.2e265365@posting.google.com...
billb@eskimo.com (William J. Beaty) wrote in message
news:<2251b4e6.0410220942.32c3f461@posting.google.com>...

Why can't a lightning rod discharge the storm? The scale is wrong.
While a needle can discharge an electrostatic generator via ion-leakage
across a large gap, a lightning rod is not like a needle. A lightning
rod is tiny: like one fiber in a piece of felt if we obey the scaling.
If one fiber is slightly taller than the others, well, the high voltage
electrode doesn't care. If you erect a lightning rod, the storm
won't even notice.

Also, the scale is wrong for "charged wind" to transport any charge
upwards. If you hold a needle near a high voltage terminal, the
needle "emits" charged wind which travels at many cm per second,
and there is a significant electric current in the air; on the order
of microamps or tens of microamps. This easily shorts out an
electrostatic generator.

But if we scale things up and use a lightning rod and thunderstorm,
the "charged wind" coming from the tip of the rod STILL TRAVELS AT
CM/SEC SPEEDS AND THE CURRENT IS STILL MICROAMPS. To have a significant
effect on the charged storm cloud, this velocity would have to scale up
too. The lightning rod would have to "emit" an electric wind that
traveled at tens or hundreds of KM per hour, and the electric current
would have to be thousands of times higher than 10uA.

That's focusing on the cloud-discharging capabilities of -one- rod,
obviously miniscule.

Would be the -combined- effect of (1) thousands of lightning rods in a
moderate-sized community, plus (2) hundreds of miles of transmission
line, telephone line, supporting cable, etc. suspended between
hundreds of tall poles and tall towers, plus (3) tall lamp posts, tall
antennas, etc. have a significant cloud discharging effect?

Order-of-magnitude estimate on this? I'm wondering, since I've seen
several ground/tree strikes while out in rural areas, but only one
direct strike to a tree in the city, and I'm an urban dweller.

Myxococcus xanthus
---------
Unlikely- A large conifer forest has thousands of tips each of which is
grounded( poorly but this isn't of imprtance until there is a strike) and
there are records of thousands of strokes in such a forest in one storm.
As for the towers, transmission lines, etc- these are targets. Tall
buildings will produce their own "lightning shadows" or protected
areas(which is what a lightning rod does). The effect of a strike will
depend on their grounding systems. Ever had a transformer fail during a
lightning storm in your urban area? Ever had the lights go out or flicker?

Back in engineering school one of the professors was famous for
studying lightning. I recall him saying #10 wire was perfectly
adequate, that we had to remember the wave form. A lightning stoke is
a damped sinusoid, it is not like trying to pass 30,000 Amperes DC
through a wire.

Something he never mentioned, but I have run into since, is Ufer
grounding. It certainly gives low ohmic measurements. Do we know how
well it works in real life, i.e. actual lightning strikes?

 

[Edward Green]

timothy42b@aol.com (TimR) wrote in message news:<87af0be7.0410240256.5ea16ad6@posting.google.com>...

Back in engineering school one of the professors was famous for
studying lightning. I recall him saying #10 wire was perfectly
adequate, that we had to remember the wave form. A lightning stoke is
a damped sinusoid, it is not like trying to pass 30,000 Amperes DC
through a wire.

That comment doesn't make much sense to me. What's waveform got to do
with it? What matters is power dissipation.

Or I guess that's what you are suggesting: the pulse is short enough
that much higher instantaneous powers can be tolerated than at steady
state. Why didn't you just come out and say that!? ;-)

 

[Steve Harris sbharris@RO]

spamspamspam3@netzero.com (Edward Green) wrote in message news:<eca320d0.0410240616.72e8e767@posting.google.com>...

timothy42b@aol.com (TimR) wrote in message news:<87af0be7.0410240256.5ea16ad6@posting.google.com>...

Back in engineering school one of the professors was famous for
studying lightning. I recall him saying #10 wire was perfectly
adequate, that we had to remember the wave form. A lightning stoke is
a damped sinusoid, it is not like trying to pass 30,000 Amperes DC
through a wire.

That comment doesn't make much sense to me. What's waveform got to do
with it? What matters is power dissipation.

Or I guess that's what you are suggesting: the pulse is short enough
that much higher instantaneous powers can be tolerated than at steady
state. Why didn't you just come out and say that!? ;-)

COMMENT:

Yes. The waveform is "damped sinusoid." In this case it's the first
descriptive word that's important, not the second <g>.

SBH

 

[TimR]

Roger Johansson <no-email@home.se> wrote in message news:<Xns958D13BBA247886336@130.133.1.4>...

Rich Grise <rich@example.net> wrote:

What is "Ufer grounding"?


http://www.psihq.com/iread/ufergrnd.htm

The "Ufer" Ground

"The term "Ufer" grounding is named after a consultant working for the US
Army during World War II. The technique Mr. Ufer came up with was
necessary because the site needing grounding had no underground water
table and little rainfall. The desert site was a series of bomb storage
vaults in the area of Flagstaff, Arizona.

The principle of the Ufer ground is simple, it is very effective and
inexpensive to install during new construction. The Ufer ground takes
advantage of concrete?s properties to good advantage. Concrete absorbs
moisture quickly and looses moisture very slowly. The mineral properties
of concrete (lime and others) and their inherent pH means concrete has a
supply of ions to conduct current. The soil around concrete becomes
"doped" by the concrete, as a result, the pH of the soil rises and
reduces what would normally be 1000 ohm meter soil conditions (hard to
get a good ground). The moisture present, (concrete gives up moisture
very slowly), in combination with the "doped" soil, make a good conductor
for electrical energy or lightning currents.

Ufer techniques are used in building footers, concrete floors, radio and
television towers, tower guy wire anchors, light poles, etc. Copper wire
does not function well as a "Ufer" ground due to the pH factor of
concrete (+7pH is common). The use of steel reinforcement as a "Ufer"
ground works well and concrete does not chip or flake as has been found
with copper. The use of copper wire tied to the reinforcement rods
outside the concrete shows none of these problems. "

My question really has to do with whether the "good ground" is real or
not under high current conditions. Clearly you can get an extremely
low resistance measurement and pass your safety requirements. It
seems likely this ground would be fine for reducing static charge
effects and for electronic equipment grounding. I asked because I
wondered how well a Ufer ground can actually dissipate the current in
a lightning strike. If the soil outside the foundation continues to
be high resistance soil, it would seem the dissipation must occur
within the rebar.

I know I'm not being consistent claiming #10 wire is enough, then
wondering if a rebar array is enough. <grin> But is there any data?
Actual lightning hitting a Ufer grounded structure? It sounds like a
ground plane not a true ground, but I'm not an electrical engineer and
may not understand the difference.

 

[Roger Johansson]

timothy42b@aol.com (TimR) wrote:

I asked because I
wondered how well a Ufer ground can actually dissipate the current in
a lightning strike. If the soil outside the foundation continues to
be high resistance soil, it would seem the dissipation must occur
within the rebar.

You can combine this Ufer ground with a system of wires in a radial
arrangement out from the central Ufer ground, to spread out the charge.
Like 10 wires, 300 feet long, buried under ground, like spokes in a
wheel, so the charge is easier lead away from the Ufer ground point.

No grounding point is perfect.

You will still have to assume that the voltage of the grounding point
will be raised momentarily when lightning strikes.
How much, that depends on the strength of the lightning hit and the
quality of the grounding system, how fast it can dissipate the charge.

So we always get back to the protection which holds the voltages in the
protected system together, so the voltage differences do not become too
big.

For that you need spark gaps and similar devices.



--
Roger J.

 

[William J. Beaty]

"Don Kelly" <dhky@peeshaw.ca> wrote in message news:<yaFed.9731$%k.8545@pd7tw2no>...

"Myxococcus xanthus" <mold-guardian@comcast.net> wrote in message
news:ce5e7813.0410230428.2e265365@posting.google.com...

That's focusing on the cloud-discharging capabilities of -one- rod,
obviously miniscule.

Not obvious, since some books (and teachers) still insist that lightning
rods protect your home by discharging the clouds.

Unlikely- A large conifer forest has thousands of tips each of which is
grounded( poorly but this isn't of imprtance until there is a strike) and

Yes, but we need to remember that the ions produced by things on
the ground would only reduce the net charge in the clouds if they
could *fill* the empty space between the cloud and the ground.

Trees and buildings might be producing charged air, which makes the
air more conductive, but this conductive layer of air would be like
a film of insignificant thickness... because we compare it to the
SEVERAL MILES of air between the cloud and the ground. I'm
imagining that, at most, the charged air might move upwards at a
foot per second. As the charged clouds arrive overhead and the
e-fields become strong, the sharp objects on the ground would only
have time to produce a layer of conductive air a few hundred feet
thick. The layer will also be blowing sideways, so we shouldn't
imagine that one lightning rod would make a cloud above itself.
Rather imagine a smoke stack with a plume travelling downwind.


Lightning rods aren't going to have a large effect on the storm.
A lightning bolt (a plasma streamer) is triggered up in the clouds,
and then grows longer, sometimes growing downwards. Suppose it
becomes several miles long and is approaching the ground. Could
some ionized air hovering over the buildings have any effect? Sure.
The movable charges will act as a resistive "coating" which makes the
lightning think that the Earth lacks buildings. The e-fields which
steer the growing plasma streamer would be altered by the conductive
air, so they would not respond to trees and buildings as much as if
there was no meters-thick layer of ions.

But is this what we want? The layer of charged air would keep the
lightning path random, so the streamer wouldn't be guided to a safe
attachment upon a nice thick copper ground-wire. Maybe it's a good
thing that any wind would blow away the ions, letting the streamer
"see" your lightning rod poking upwards.

there are records of thousands of strokes in such a forest in one storm.
As for the towers, transmission lines, etc- these are targets. Tall
buildings will produce their own "lightning shadows" or protected
areas(which is what a lightning rod does). The effect of a strike will
depend on their grounding systems. Ever had a transformer fail during a
lightning storm in your urban area? Ever had the lights go out or flicker?

Now the region BELOW a lightning rod... that's a different issue. If
an incoming plasma streamer approaches a protected building, then the
e-field around the building grows so intense that the lighting rod or
even the ground wires will launch their own plasma streamers up to
intercept the incoming streamer. A lighting rod acts like a "Scud
Missle Launcher" which shoots down any incoming lightning bolts, forcing
the incoming lightning to follow a trail leading back to the ground
wire. With luck, the *TOP* of the lightning rod will launch the
streamer, which prevents lightning strikes upon anything below the
tip of the rod. The farther away you stand from the vertical ground
wire, the more chance there is that the lightning rod wouldn't
emit a streamer that intercepts any lightning bolts aimed for your
head.

 

[Don Kelly]

"William J. Beaty" <billb@eskimo.com> wrote in message
news:2251b4e6.0410251813.3fce28f0@posting.google.com...

"Don Kelly" <dhky@peeshaw.ca> wrote in message
news:<yaFed.9731$%k.8545@pd7tw2no>...
"Myxococcus xanthus" <mold-guardian@comcast.net> wrote in message
news:ce5e7813.0410230428.2e265365@posting.google.com...

That's focusing on the cloud-discharging capabilities of -one- rod,
obviously miniscule.

Not obvious, since some books (and teachers) still insist that lightning
rods protect your home by discharging the clouds.



Unlikely- A large conifer forest has thousands of tips each of which is
grounded( poorly but this isn't of imprtance until there is a strike)
and

---------------
What you clipped may be of more importance than what you included.
" .....and
there are records of thousands of strokes in such a forest in one storm.
As for the towers, transmission lines, etc- these are targets. Tall
buildings will produce their own "lightning shadows" or protected
areas(which is what a lightning rod does). The effect of a strike will
depend on their grounding systems. "

This contraindicates the "discharge" effect.

Also ,in a previous comment:
"Generally the source of the charge is several miles overhead -the rod
doesn't get seen by it. When a leader gets near a rod then it may be a
preferred target for the next step and if it is, then the main stroke will
be to the rod-if not something else gets hit. Design is based on it being
such a target for higher current strokes (but not necessarily lower current
strokes). This is true for protective systems for transmission lines as
well as structures. Catch the damaging strokes know that some of the little
ones will get by."

In other words, the above is in agreement with what you have said quite
well.

Yes, but we need to remember that the ions produced by things on
the ground would only reduce the net charge in the clouds if they
could *fill* the empty space between the cloud and the ground.

Trees and buildings might be producing charged air, which makes the
air more conductive, but this conductive layer of air would be like
a film of insignificant thickness... because we compare it to the
SEVERAL MILES of air between the cloud and the ground. I'm
imagining that, at most, the charged air might move upwards at a
foot per second. As the charged clouds arrive overhead and the
e-fields become strong, the sharp objects on the ground would only
have time to produce a layer of conductive air a few hundred feet
thick. The layer will also be blowing sideways, so we shouldn't
imagine that one lightning rod would make a cloud above itself.
Rather imagine a smoke stack with a plume travelling downwind.


Lightning rods aren't going to have a large effect on the storm.
A lightning bolt (a plasma streamer) is triggered up in the clouds,
and then grows longer, sometimes growing downwards. Suppose it
becomes several miles long and is approaching the ground. Could
some ionized air hovering over the buildings have any effect? Sure.
The movable charges will act as a resistive "coating" which makes the
lightning think that the Earth lacks buildings. The e-fields which
steer the growing plasma streamer would be altered by the conductive
air, so they would not respond to trees and buildings as much as if
there was no meters-thick layer of ions.

But is this what we want? The layer of charged air would keep the
lightning path random, so the streamer wouldn't be guided to a safe
attachment upon a nice thick copper ground-wire. Maybe it's a good
thing that any wind would blow away the ions, letting the streamer
"see" your lightning rod poking upwards.


there are records of thousands of strokes in such a forest in one storm.
As for the towers, transmission lines, etc- these are targets. Tall
buildings will produce their own "lightning shadows" or protected
areas(which is what a lightning rod does). The effect of a strike will
depend on their grounding systems. Ever had a transformer fail during a
lightning storm in your urban area? Ever had the lights go out or
flicker?

Now the region BELOW a lightning rod... that's a different issue. If
an incoming plasma streamer approaches a protected building, then the
e-field around the building grows so intense that the lighting rod or
even the ground wires will launch their own plasma streamers up to
intercept the incoming streamer. A lighting rod acts like a "Scud
Missle Launcher" which shoots down any incoming lightning bolts, forcing
the incoming lightning to follow a trail leading back to the ground
wire. With luck, the *TOP* of the lightning rod will launch the
streamer, which prevents lightning strikes upon anything below the
tip of the rod. The farther away you stand from the vertical ground
wire, the more chance there is that the lightning rod wouldn't
emit a streamer that intercepts any lightning bolts aimed for your
head.

Note that design doesn't count on this streamer and the protection zone is
determined by the downcoming leader's strike distance which is related
(empirically) to the main stroke current . The protected range is given by
the "rolling ball approach. Basically a ball of radius equal to strike
distance and just touching ground (or the local ground plane) and the rod
tip. The shadow of protection is outside this radius for currents equal to
or greater than the design current. Weaker strokes can get by. It's a saw
off- you get what you pay for (and hope that the statistics are reasonably
correct).
--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer