Tstorm avoidance

Roy Smith wrote:

Matt Whiting wrote:

It also looks at spectral spread using digital signal
processing. I won't go into the physics of it unless someone insists,
but the general principle is that a strong distant storm shows up as a
longer duration (broader) peak than a close weak one, ON AVERAGE.

I insist.


I'll take a shot at it. A lighting strike is essentially an
instantaneous electromagnetic pulse. What a mathematician would call a
"delta function". As such, it is composed of a wide spectrum of
frequencies (perhaps it's better to think of it as wavelengths).

Different wavelengths travel at different speeds. This is why, for
example, white light is broken up into a spectrum by a prism.

Here's an analogy which may help explain what's going on. Let's say you
have 10 cars starting out from the same spot, but each going at
different speeds (60, 61, 62, ... up to 69 MPH). If you stand right at
that spot, you see them all pass you at the same time. If you stand a
mile away, the fastest one will pass you first, then the next fastest,
and so on. They have spread out. If you stand 2 miles away, they will
have spread out even more by the time they get past you.

This is what's happening with a lightning strike. Right at the strike
point, you've got all these different wavelengths of electromagnetic
energy syncronized into one big spike. A mile away, the shorter
wavelengths have gotten to you a little before the longer wavelengths
(or is it the other way around?). Two miles away, the spread is even
greater. The further you go away from the source, the more the various
wavelengths have spread out, just like the cars moving at different
speeds. This is called spectral dispersion.

In the acoustic world, this is why a nearby lightning strike has a
thunder clap that sounds like one big "zzzaappp", while further away it
sounds like a rumble that goes on for a while.

So, the theory is that if you look at the width (in time) of a pulse and
compare it to its strength, you should get some idea of how much
spectral dispersion it has undergone, and thus be able to estimate how
far away it came from. It's not very precise, but it's good enough for
a strike finder.

To give any better explanation would require me to exercise neural
pathways which have lain dormant for many years, and are probably best
left that way.

Actually, Roy, I'm and EE and work with optical fiber so I understand
the principle of dispersion pretty well, be it spectral, PMD, etc. I
just wanted to hear the smart aleck explain it! :-)


Matt