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From: msb@lsuc.UUCP (Mark Brader)
Newsgroups: net.physics,net.video
Subject: Re: Re: lightening bursts
Message-ID: <747@lsuc.UUCP>
Date: Sat, 10-Aug-85 13:19:28 EDT
Article-I.D.: lsuc.747
Posted: Sat Aug 10 13:19:28 1985
Date-Received: Sat, 10-Aug-85 13:31:52 EDT
References: <3305@decwrl.UUCP> <251@unccvax.UUCP> <454@utastro.UUCP> <159@prometheus.UUCP>
Reply-To: msb@lsuc.UUCP (Mark Brader)
Organization: Law Society of Upper Canada, Toronto
Lines: 36
Summary: Fast as lightning

(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