All the discussions about where lightning will strike remind me of
the tornado warnings and watches I listened to while living in Denver
(which, for those who don't know, is the western terminus for tornadoes
heading east toward Kansas and north through Weld County [the new
airport {DIA} is smack dab right in the middle of this convergence zone]).
The warnings would be on the order of "east of I-25 [the north-south interstate
bisecting Denver] until 9 PM." I always wondered if the tornado had a map and
a watch!
Lightning, while it does sek the lowest impedance path to earth, also travels in
150-foot increments (the step leader) and meets up with the streamer coming
from the various points on earth (tower, tree, chimnet, etc.). So it takes the
lo-Z path within the confines of this hemisphere.
This step leader and its associated 150' radius "rolling ball" is also used to
determine
the "cone of protection." It is not a straight line coming off of the top ofthe
tower at
a 45-degree angle but rather the circumference of this ball. However, for
figuring out
areas and whether or not a house (etc.) is within this protected area, the
45-degree
angle methodology will probably be "close enough." Even within this cone, you
are
only in a 96% safety area because the step leader is not precisely 150' each
and
every time.
As always, I conclude with the Lee Trevino #1 Iron golf club quote. For those
unfamiliar with this, basically the best lightning protection is to mount a #1
Iron
golf club atop your tower. As Lee once siad, "Not even God can hit a #1 Iron."
73,
Bob AA0CY
----------
From: Bill Nelson[SMTP:billn@peak.org]
Sent: Thursday, June 04, 1998 3:04 PM
To: towertalk@contesting.com
Subject: [TowerTalk] lighting
>Well, I'm not a lightning expert, and all the talk about lightning
>reminds me of a yiddish proverb. But the theory I've heard and understand
>is as follows:
>A grounded conductor aloft assumes the same potential as the ground it is
>connected to. It therefore becomes difficult for it to build up a charge
>and make it a potential target.
Unfortunately, the charge in the cloud generates a like (but opposite
polarity) charge in the ground below it. This charge will travel through
any conductor that is connected to the earth. So a grounded conductor
will build up to the same potential as the earth below it.
Now, that does not mean that any discharge will be from that conductor.
It could easily be from some other conductor under the cloud - such as
a tree or power pole/line.
>An ungrounded conductor aloft assumes whatever potential it wants. If it
>is bombarded with rain, it picks up the charge of the rain. Wind friction
>can also generate a sizable static charge. It may pick up enough of a
>charge to become a target.
Maybe. But any charge that builds up is going to be minute compared to
the charge that is in the earth under the cloud.
>Of course, lightning is dumb, and it doesn't understand theory - it
>strikes where and when it wants....
Within limits, that is certainly true. However, it WILL take the
lowest impedance path to ground.
Bill
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