Streamers usually start at sharp points or tapered points of objects.
Corners of buildings etc. Charge tends to push towards and accumulate at
a small area. Same thing happens with corona discharge with very high
voltage circuits. Charge moves toward the sharp points.
When highly charged clouds move into the area with a positive charge
they start to pull negative charges from the earth towards them. The
negative charge starts to accumulate in buildings, towers and other
objects in the area. Even in humans if they are close by. It is not
uncommon for your hair to stand on end when lightning is imminent.
When charge has built very high is when streamers start to wander upward
as the cloud charge pulls on them.
But it is the step leader from the cloud that is the most determining
factor as to where the strike or attachment is going to occur. As
explained before it moves in random steps and if it happens to move or
step near a streamer the two are attracted and complete a path. The step
leader does not necessarily have to find a streamer to complete a path
though. It can move in any direction and make contact with any object.
Sometimes even with the ground directly. It doesn't always hit the
tallest object in an area for the same reason. Anything outside the
approximate 150 foot step can be struck on its next move.
73
Gary K4FMX
doc wrote:
> I really appreciate the description in simplified terms!
>
> I guess the only remaining question in my mind is the
> process by which the streamers are generated, that is,
> what variables cause streamers in one object versus
> another?
>
> doc
>
> Gary Schafer wrote:
>
>>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
|