merrill@raja.DEC (Rick - Font Mgr. for Hardcopy Engineering) (07/26/85)
Once one lightining strike has ionized the air it leaves a path of lesser resistance than existed before. That alone should make successive strikes more probable until the available potential has diminished. Then air turbulence eliminates the ionized path and the max potential must rebuild. This does not explain why paths OTHER than the first one appear to be part of the cluster of strikes that follow the first one, a phenomena I have observed myself (from the ground). My suggestion is that there is leakage current moving through the air that preceeds the strike and that they are ALL about ready to produce stike paths. Which one goes first is therefore random and the burst is NOT "caused by" the first one. Rick Merrill 617-493-3751
dsi@unccvax.UUCP (Dataspan Inc) (07/27/85)
There was some gentleman on WBT (Charlotte, NC) last week talking with
Henry Bogan about some heavy duty research into why some areas are particularly
lightning prone. I only got bits and pieces of it (running errands) but
they're doing some relatively sophisticated photography and statistical
analysis. One thing the guest did mention was that they found lightning
paths were one 'helluva' lot longer than was originally postulated. Also,
with this ultrahighspeed photography and other techniques, it has been
shown that the cloud-to-ground burst is definitely 'ground - to -cloud'
and that ionisation occurs first (several hundreds of milliseconds).
This thing (called a 'stepped leader') precedes the actual event in
almost every case.
They are also studying an area roughly between Orlando and Cape Kennedy
which seems to be exceptionally lightning prone (he said about 250,000
discharges last year!)
Apparently, lightning isn't the only kind of current that flows
from the cloud - to - ground (or whatever). I've noticed that when
a thunderstorm is threatening, even in the absence of lightning, larger
broadcasting towers (in excess of 350 feet) will put on some spectacular
feats of arcing and popping around the 'johnnyballs', strain relief
insulators, and the base insulator. The tower I've in mind rarely, if
ever, actually gets hit. Is the tower actually reducing the potential
for strikes, or what?
Finally, on a related subject: It seems that when I was in junior
high school, there was a big to-do about tornado producing thunderstorms
having either 1) an exceptional absorption or 2) radiation around 3.2
mHz. There was a big plan to equip tornado-prone schools and other public
buildings with inexpensive receivers to monitor the effect. The articles
were in Popular Science, etc.
Was this really true?
David Anthony
DataSpan, Incethan@utastro.UUCP (Ethan Vishniac) (07/31/85)
> One thing the guest did mention was that they found lightning > paths were one 'helluva' lot longer than was originally postulated. Also, > with this ultrahighspeed photography and other techniques, it has been > shown that the cloud-to-ground burst is definitely 'ground - to -cloud' > and that ionisation occurs first (several hundreds of milliseconds). > This thing (called a 'stepped leader') precedes the actual event in > almost every case. > > David Anthony I saw a seminar on this a year ago. Evidently the ionisation path forms from the clouds to the ground in a series of short "steps". The path is like a biased random walk toward the ground. Then, as you say, the most energetic event occurs, a massive ground-to-cloud surge of current. I believe this surge starts once the ionisation path is some tens of meters from the ground. -- "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas
connolly@steinmetz.UUCP (C. Ian Connolly) (08/01/85)
> Apparently, lightning isn't the only kind of current that flows > from the cloud - to - ground (or whatever). I've noticed that when > a thunderstorm is threatening, even in the absence of lightning, larger > broadcasting towers (in excess of 350 feet) will put on some spectacular > feats of arcing and popping around the 'johnnyballs', strain relief > insulators, and the base insulator. The tower I've in mind rarely, if > ever, actually gets hit. Is the tower actually reducing the potential > for strikes, or what? Usually the atmosphere has a small potential - I forget the actual voltage per foot, but during thunderstorms, this potential can increase drastically. I believe a major part of Ben Franklin's famous experiment was merely in displaying this potential, i.e., a conductor stretched up a hundred or so feet into the atmosphere will give you quite a charge - even without lightning. I don't *think* it would inhibit lightning strikes, but I'm not sure. -- C. Ian Connolly, WA2IFI - USENET: ...edison!steinmetz!connolly , , ARPANET: connolly@ge-crd An rud a bhionn, bionn.
pmk@prometheus.UUCP (Paul M Koloc) (08/06/85)
> > with this ultra-high-speed photography and other techniques, it has been > > shown that the cloud-to-ground burst is definitely 'ground - to -cloud' > > and that ionization occurs first (several hundreds of milliseconds). > > This thing (called a 'stepped leader') precedes the actual event in > > almost every case. > > David Anthony > > I saw a seminar on this a year ago. Evidently the ionization path forms > from the clouds to the ground in a series of short "steps". The path > is like a biased random walk toward the ground. > -- > Ethan Vishniac Charge is lowered from the cloud toward the ground by the step leader process. As it does the electrons spray off from the leading tip radially outward, generating a "comet like" discharge observable only with fast image intensifiers. The electrons decelerate enough after expanding a few tens of meters from the step leader channel to attach to O2 molecules where they are essentially stored in a "charge cylinder" for the remainder of the high speed lightning process. The direction of the steps may be determined by ionization tracks due to cosmic rays so they are random. Sometimes the step process "forks". Now when the cloud to ground electron transfers reach several tens of meters from the ground, the potential is high enough to generate a "bright" anode streamer or return stroke. The discharge largely consists of discharging the "charge cylinder". That means as the channel reaches a "forked branch" the discharge then tracks through the fork as well as continues toward the cloud. That's why lightning seems to have branches that just "end in the air" and never reach ground (except as noted above). The upward flying radial discharge of the charge cylinder to the return stroke tip generates energetic electrons which can "trap" components of the earth's field in the channel. That's why lightning in Florida is quite different than the stuff in New York. Lightning in Florida can do all kinds of loop the loops and follow wild paths. In New York the component of the earth's field has much greater declination so lightning there may be more stablized and follow somewhat shorter (straighter sectioned zig-zag paths) to ground. - - NOTE: MAIL PATH MAY DIFFER FROM HEADER - - +-------------------------------------------------------+--------+ | Paul M. Koloc, President: (301) 445-1075 | FUSION | | Prometheus II Ltd., College Park, MD 20740-0222 | this | | pmk@prometheus.UUCP; ..seismo!prometheus!pmk.UUCP | decade | +-------------------------------------------------------+--------+
msb@lsuc.UUCP (Mark Brader) (08/10/85)
(For net.video readers: The subject is misspelled because this is a followup to a net.physics article where it was spelled that way.) I happened to videotape a TV news show a couple of weeks ago that included a view of a lightning strike (in Wyoming). It was interesting to play it back in slow motion. The bolt was more or less vertical. In the first frame where it appeared, it occupied only the bottom 1/5 of the frame; in the next frame, it occupied the whole height of the frame. In both of these frames there was a bright aura splaying out from the bolt; the second frame was close to washed out. (Incidentally, this passed by too fast to notice at normal speed. It was much more dramatic in slow motion.) I presume the aura was simply reflected light from the cloud and rain. Anyway, in the first of the two frames, both bolt and aura were sharply cut off by a horizontal boundary. I take this to mean that the light rose from zero to full intensity in a time comparable to that between two scan lines -- i.e. about 1/15,000 second -- if not shorter. (I must admit that I didn't look closely at the boundary, and if it was spread over a few scan lines I might not have noticed. That's why I said "comparable to" rather than definitely "less than".) The frame after the washed-out frame had no lightning in it, though there was a powerful afterimage (artifact of the TV camera, I assume) that faded in about 1.5 seconds. Therefore the total time of the bolt was no more than 1/30 second. It could have been much less, of course; once the image is in the camera it is going to persist for a little while. I'm sure there's no new science in this message, but I found it impressive that I could make observations on the millisecond level using apparatus found in my home! (And it only took me weeks to realize that I had done so, and meanwhile the tape has been reused...) Mark Brader
ABOULANGER@BBNG.ARPA (08/12/85)
From: Albert Boulanger <ABOULANGER@BBNG.ARPA> Charge is lowered from the cloud toward the ground by the step leader process. As it does the electrons spray off from the leading tip radially outward, generating a "comet like" discharge observable only with fast image intensifiers. The electrons decelerate enough after expanding a few tens of meters from the step leader channel to attach to O2 molecules where they are essentially stored in a "charge cylinder" for the remainder of the high speed lightning process. The direction of the steps may be determined by ionization tracks due to cosmic rays so they are random. Sometimes the step process "forks". Now when the cloud to ground electron transfers reach several tens of meters from the ground, the potential is high enough to generate a "bright" anode streamer or return stroke. The discharge largely consists of discharging the "charge cylinder". That means as the channel reaches a "forked branch" the discharge then tracks through the fork as well as continues toward the cloud. That's why lightning seems to have branches that just "end in the air" and never reach ground (except as noted above). The analogy that Martin Uman, a lightning researcher, gives is that this charge cylinder is like a long forking sand bag; when it reaches ground it unzips. The wave of sand falling out is what causes what you see as lightning. There was a pulsed nitrogen laser project in SciAm a few years back that worked on a similar principle. I found the design simple, clever, and elegant. By the way, lightning seems to be a fractal curve. There have been some papers about this. Albert Boulanger -------
brown@nicmad.UUCP (08/15/85)
In article <747@lsuc.UUCP> msb@lsuc.UUCP (Mark Brader) writes: >(For net.video readers: The subject is misspelled because this is a >followup to a net.physics article where it was spelled that way.) > >The bolt was more or less vertical. In the first frame where it >appeared, it occupied only the bottom 1/5 of the frame; in the next >frame, it occupied the whole height of the frame. In both of these >frames there was a bright aura splaying out from the bolt; the second >frame was close to washed out. (Incidentally, this passed by too fast >to notice at normal speed. It was much more dramatic in slow motion.) >I presume the aura was simply reflected light from the cloud and rain. I hope you aren't misled by the structure of television scanning. Now, if the lightning bolt was shot on video tape, then what I have to say will be appropriate. But if the bolt was shot on film the what you saw in slow-motion makes sense. Now, every 1/60th of a second, one tv field is scaned. But, because it takes time to scan from the top to the bottom, part of the field could be done before it picks up something that heppens real fast, ie, the bolt you saw, using slow-mo, could have been to the top of the field after the time it started and the point at which the camera 'sees' it. Because of the time it takes to get back to the top of the field, the camera can and will miss things that are happening. This is the alias effect. Examples of this can be seen when TV camera shots of lasers and strobes are done. So, you can't pay too much attention to the fact that you saw only part of a bolt on a frame. The best way to see lightning is with a high speed film camera. Then the resolution, in the time domain, is much finer. TV and normal film cameras have too course a time resolution. -- Mr. Video {seismo!uwvax!|!decvax|!ihnp4}!nicmad!brown