Re: Lightning "cone of protection" myth debunked

 

zeez wrote:


  The cited presentation makes two points, one of which appears to me to
be valid (hazard from ground currents) and the other is that lightning can
strike within the "cone of protection".

  It does not explain under what circumstances lightning can strike within
the cone of protection, and I don't see support for a claim that this
concept is wrong in general.

  I tried looking around their site a bit for an explanation why the "cone
of protection" is wrong either in specific cases or generally or links to
evidence that it is, and I have yet to see any.

  I tried the search feature in their home page, and I could not find
anything other than the above link or things stating what the "cone of
protection" is.

  I would hope that the National Lightning Safety Institute would have
made support for such a claim easy to find.

 - Don Klipstein (don@misty.com)

 

On Sun, 20 Sep 2009 17:58:48 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:


The logic is pretty simple when you stop and consider how a strike
occurs.

A charged area in a cloud or the sky forms, and the ground under and
around that charged area has an opposite electrical polarity.  At a
certain point, a leader from the charge cloud extends closer to the
ground and numerous "false leaders" come UPWARDS from the ground
towards the area of increased potential, once it reaches a certain
level.  One or two of those false leaders connects, ionization of the
air and subsequent electric flow occurs, and the other leaders (which
may never have reached the point of being visible) are no longer
supported and retreat to the ground.

When "ground currents" are cited as a cause of death in a nearby but
not direct lightning strike, it isn't the current or voltage going
from one foot to the other, but the currents that extend from the
ground up the body that do the damage.  Ground currents is a misnomer.

The cone of protection, when I first saw mention of it in the 1950s
specifically referred to tall water towers, radio station antennas and
skyscrapers, all of which have highly developed grounding systems or
ground planes that quickly dissipate the charges, and it was a
statistical model and not an absolute model.  It did not include
trees, and IIRC mentioned 150' in height as a minimum for the effect
to be observed.

A lot of this is a matter of scale.  A five mile long lightning bolt
isn't going to be making last minute decisions.  It'll hit the easiest
path to ground, which may not be in a straight line, and may extend
along a line of rain rather than a dry area (or not).  The false
leader that gets closest to the main leader is likely to be the winner
in the lucky strike contest.  When a false leader is 30 feet high and
a building or tree is 25 feet high, you are in a world of hurt.  When
the building is the Empire State Building, you can be fairly sure that
the false leaders will be coming off the upper forty stories of the
building, where the potential is great, rather than threading up the
canyon between buildings whose walls are at ground potential.  Any
charge from the actual strike will head directly down a few stories
underground before spreading out much, so standing on the street
around the ESB is relatively safe.

That isn't a real _cone_ of protection, but more accurately a space
where the electrical potentials and differences can be mapped, and the
"safe" areas shown to have minimal gradients of charge.  If the
difference isn't great, no false leader and subsequent strike will
occur.  The "cone" model is a poor one because it doesn't reflect the
real knowledge and provides a false sense of safety.

 


  I am very much aware of "leaders" and "resultant ground leaders", and
how few to often one of each connect to result in visible lightning
strikes.


  Ground current through ground resistance close to "ground zero" results
in the voltage from one foot to another being a voltage that forces a
lethal current to flow from one foot to another.


  I did notice such exclusions in roughly 1980 to early-1980's versions of
the "ARRL Handbook" and the "ARRL Antenna Book".


  45 degrees deviation as allowed by "45 degree cone of protection"
appears to me cover this.


  A world-record-class thunderstorm "rainshaft" achieves 1 inch of
rainfall in a minute with raindrops of maximum of .25 inch wide, and a
slowish speed for such big raindrops to fall is 20 MPH, as in I think more
likely at least 25 MPH.  But for the following, I will use 20 MPH.

  Translate to metric with consistent units:

  .42 millimeter of rainfall per second.

  A .24-.25 inch diameter raindrop has "average thickness" of 3.2
    millimeters.

  This means "average impact rate" of once per 7.6 seconds of "largest
raindrops" even at world-record-rate of 1 inch of rainfall in a minute.

  With slowish-side descent rate of 20 MPH (8.94 meters/second), 7.6
meters means that even in a world-class heavy rainshaft, there is over
2000 times as much air as volume as raindrop volume.  And, the raindrops
have shape low for conduciveness to sparking.


  Except I sense that the electric field in the air around the Empire
State Building is reliably low even during a thunderstorm up to at least
the 80th.


  That I agree with.


  That I agree with.


  I hope you can provide cites for how the "cone of protection" breaks
down including mechanisms as to how it "breaks down" (my words), besides
known mechanisms of "ill effects within cone of protection" resulting from
lightning strikes that do not "directly violate" "cone of protection".

 - Don Klipstein (don@misty.com)

 

On Wed, 23 Sep 2009 01:24:09 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:


Yeah, I think  we explored this to my satisfaction.  Not sure on that
pic where the lightning is exactly.  My impression was that the main
bolt was a ways away, but others were up close.  I have no way of
knowing though.  Point is, the term "cone of protection" is a little
loose.  Maybe "cone of silence"?  :-)

 


don@manx.misty.com (Don Klipstein) wrote:


    I rather suspect the NLSI to be primarily about that " NLSI
Business Services" page. They lost me at "Fun With Lightning". As a
lifelong Florida resident, I cannot imagine using "fun" and "ligtning"
in the same sentence....