Doc,
What goes on on the ground has less to do with the probability of a
strike than what goes on in the air.
Just before a strike as charges build and objects start emitting
streamers a leader from the charged cloud starts its way toward earth.
It moves in "steps" of around 150 feet or so at a time pausing slightly
with each step. It moves in these steps governed somewhat by the
conductivity and charges in the air in the area. The steps are not
straight down. They go at different angles, sometimes straight side
ways. If a step leader happens to get near enough to one of the
streamers coming off an object it makes connection. What follows is a
plasma trail that is very hot and has very low resistance. That conducts
the current between the object and cloud. A lightning strike.
Being that the step leaders move in the approximately 150 foot steps,
sometimes side ways, is the reason that tall buildings and tall towers
sometimes get hit on the side part way up rather than at the top.
This makes anything over 150 feet tall vulnerable to being hit anywhere
above the 150 foot mark just as easily as the top.
Charge can build rapidly. Some people install these brush affairs to try
and bleed charge off in hopes of deterring a strike. They don't work
because the earth can supply more charge much faster than it can be bled
off. But the sharp points on them may cause streamers to occur earlier
increasing the chance of a strike.
73
Gary K4FMX
doc wrote:
> Earlier today I was showing our college math teacher the "Physics for
> Criminal Justice Majors" course materials from 1974 when I attended
> Northeastern University.
>
> As we thumbed through the mimeographed pages (no textbook existed at
> the time) I observed that I had no idea what the formulas meant,
> even though some were written in my handwriting on the note pages!
>
> Although I passed the course I remain a physics-challenged Ham.
>
> That said, I think I may have a better sense of the issues now.
>
> The multiple-variable point is well-taken such that the best path
> is a moving target.
>
> As for the resistance/potential issue I was speculating as to
> best path without intending to isolate resistance as a decisive
> factor -- wondering aloud just what it is that at any moment makes
> one object versus others more prone to send out streamers and thus
> become at least momentarily more attractive.
>
> It is reasonable to postulate that conductivity-of-the-moment causes
> energy to move through an particular object (or assorted elements in
> conductive moisture in the air) more so than another?
>
> doc
>
>
>>It seems to me the answer lies in the unpredictability of the "best path."
>>
>>You have to look at that equation in four dimensions: co-ordinate geometry
>>set against time.
>>
>>Variations in humidity and wind currents mean that at any given second, the
>>"best path" may not be what it appears to be ** at that moment **.
>>
>>As well, if a charge builds up 10 feet from a tree, adding the ground
>>resistance to the tree's resistance may mean that moving through the ground
>>to the tree and into the air isn't the path of least resistance, just going
>>straight into the air may be.
>>
>
> > I'd like to know some physical reason why a change in resistance
> would > make the cloud to tree potential change. That's what doesn't make
> > sense to me.
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|