[TowerTalk] Is lightning hitting the tower..or the yagi's

[K1TTT]

> 
> > "IF" there's a stormcloud going by, with "X" amount of electrical
> potential,
> > the only way it
> > can hit the Earth, is "IF" there's an equal/opposite charge in the
> Earth...I

the charge separation in the cloud is what attracts the charge to the ground
under it.  

> > think it's well known
> > that there are "feelers" also known by other names; tiny strands of
> > electrical charge that work
> > their way UP from the Earth, immediately prior to lightning strikes.
> 
> I think they are more commonly called "feeders."
> 

they are called 'leaders'.  

> I thought that the most common lightning strike starts from the cloud
> and goes down, but some might go opposite.
> 
> I've read that the feeder grows in spurts, by sending what amounts to
> a small cloud of charged particles downward.  It goes some small
> distance (I don't recall, something like 10-50 feet), then runs out of
> steam, then picks up again (perhaps after enough similarly charged
> particles have caught up with it), goes another short distance, then
> slows down again, etc. ... until it reaches the ground or another
> cloud.  This all happens in the blink of an eye.  These spurts are
> part of what give lightning its jagged appearance.
> 

In 'normal' lightning it is a 'negatively charged downward leader' that
comes out of the cloud.  It progresses in 50-100m jumps as the charge pushes
it down... Note, I said PUSHES, the ground influence on the leader direction
is relatively small, it is the gross field gradient between the earth and
cloud that causes the major movement groundward at about 1/3c.  There is
some steering on the leader as it gets closer to the ground by large objects
such as tall towers, large buildings, and high voltage power lines, that all
attract the downward leader a bit.

Normally the downward leader itself doesn't make direct contact with the
ground, it is met by an upward streamer that is initiated when enough charge
accumulates on an object attached to the ground.  The beginning of these
streamers is the crackling and popping you hear(both audibly and as rf 'rain
static') when a storm gets overhead.  The sound normally builds up until
there is a nearby stroke when they will stop until the field builds up
again.  when the field gradient gets high enough this streamer will bridge
the gap and that triggers what you see and hear as lightning.  The 'return
stroke' as it is called is the draining of charge out of the leader, which
is much faster than the leader jumping (though I forget the exact number now
its .8-.9c I believe).  

If there are more pockets of charge available in the cloud they can then
connect to the now discharged channel while it is still ionized and take
this easy route down, thus resulting the multiple return strokes that are
commonly seen.

Positive lightning is well known for causing lots of damage... there are
several reasons for this, but the biggest one is that it comes from the top
of the cloud.  Picture this, the cloud works like a big van de graf
generator with the positive charge accumulating at the top in the ball and
negative charge in the bottom.  The bottom of the cloud discharges
frequently to the ground over something like a 10,000' distance.  This
results in many small strokes.  The top of the cloud can be 30-50,000' above
that.  It will occasionally equalize by intra-cloud discharges, but when
conditions are right it will 'reach out' from the top of the cloud to strike
well outside the area where the negative lightning is hitting to discharge
all at once in a much larger stroke.  Another mental picture of why this
happens... the charge in the bottom of the cloud is normally negative, it
attracts positive charge, and repels negative charge so the area under the
cloud is positive, but around it is a ring of negative charge... just what
the positive top wants to get to.  These positive strokes are more damaging
mostly because they have a much bigger pool of charge to draw from.  Instead
of discharging in 5-10microseconds with peaks of 30ka, they last for
hundreds of microseconds with peaks of 200+ka as that charge drains over the
much longer path.  These are the ones that burn things up, blow up trees and
chimneys, and burn down power lines.  There have been reports of strokes
measured recently that may be 500ka or more, these had apparently been
thrown out by lightning detection systems in the past as being impossible,
but that is being reconsidered.



> Once that ionized path has been formed, then repeat strikes happen
> over the same path.  The second bolt is often a return strike from the
> ground up, and is much stronger (brighter) than the first one.  All of
> these subsequent strikes are almost instantaneous once the ionized
> path is formed.

Not quite instantaneous, they often stretch out for hundreds of milliseconds
as various pockets of charge in the cloud connect to the ionized channel.
it is also possible for subsequent strokes to have larger peak currents than
the first one.


> 
> > There's
> > a well-publicized
> > case of a woman that got hit, in the lower seats of a stadium...
> 
> A distant relative of mine was apparently struck by lightning, in his
> feet, standing in the doorway to his house.  (Although it's also been
> suggested the feet might have been where the lightning exited his
> body.  Either way, the house didn't protect him much.)
>

you don't have to get hit directly to be injured.  It is well known that the
voltage developed between your feet on the ground is enough to harm you when
a stroke hits a tree or the ground nearby.  Standing on a metal door
threshold, or next to a porch light, could have been enough to get a side
flashover from a stroke that hit the power line or something else nearby.



David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net