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

Roy Smith wrote
I'll take a shot at it.

It's a damn good shot, too. I don't disagree with anything you said
but will add a bit of clarification in a couple of places.

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).

Actually, a lightning strike generally consists of several discharges
- but each one is essentially a delta function. Usually a visible
bolt of lightning will show up on a stormscope (at least an older
model) as several dots on a radial line. A stormscope will also pick
up non-lightning static discharges (exchanges between vertically
moving parcels of air) - it's not strictly a lightning detector.
Those discharges show up as single dots.

More modern devices are smart enough to recognize that when several
very closely spaced peaks show up, all on the same azimuth, that was a
lightning strike - and will display a single point at some 'average'
distance.

Different wavelengths travel at different speeds.

That's the part that will confuse people. The speed of light is a
constant only in a vacuum. The atmosphere is not vacuum, so the speed
of light is slower, and varies by wavelength.

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.

On average, it works. For every individual point, the error can be
pretty large.

Michael

"PaulH" I have a fairly new IFR rating and am
wondering how much help to
expect from ATC on Tstorm avoidance. Will they suggest re-routing or
do you have to request it based on visual, FSS, or stormscope
location?

You can't count on any but at times you'll get a lot. It's your
responsibility and all that, as already stated.

But there's a big difference between storm flying in VMC and storm flying
IMC. I do the former a lot (SE US) and have done the latter a bit. I found
that I don't have the equipment to do the latter except for a wash and wear
pair of shorts.

Flying IFR and staying 99% in VMC is my strategy for T-storms. Works well
for me. No Stormscope but Cheap*******s helps a lot (do Google for info on
CB, THANKS AGAIN you guys).

Get as high as possible, stay out of the buildups, and pick your way around
using whatever you have plus advice from ATC (e.g. JAX Center I find to be
particularly skilled and responsive).

Anything that lets you validate what you see and what ATC advises is worth
its weight. Nexrad is fantastic and I've only experienced the 'bootleg' CB
version.

Don't let ATC vector you into anything you can't see through.

Night is almost out of the question but not always.

Taking a look works real well as long as you are able and willing to
turnaround and land somewhere you don't want to be.

IFR is better than VFR if you take the above approach.

VFR is the way to go if you decide to stay below cloud base and do it
visually there.

If you decide to play with the embedded stuff, I guess you can if you have
enough heavy expensive equipment. But if you have to ask....I thinks it's
better to draw the line and stay out.

Night is almost out of the question but not always.

Why? Wouldn't lightning help you pinpoint stuff, at least if you can see
somewhat?


VFR is the way to go if you decide to stay below cloud base and do it
visually there.


How do you stay away from it visually? Avoid rain and dark cloud bottoms?

Jose

--
(for Email, make the obvious changes in my address)

(Teacherjh) wrote
Night is almost out of the question but not always.
Why? Wouldn't lightning help you pinpoint stuff, at least if you can see
somewhat?

At night, you can often see lightning from over 100 miles away and
can't reliably tell how far it is. If there is a lot of activity, it
looks like the activity is all around you and close. You need more
equipment than the mark-1 eyeball to make it work.

Michael

"Teacherjh" wrote in message
...
Night is almost out of the question but not always.

Why? Wouldn't lightning help you pinpoint stuff, at least if you can see
somewhat?

No. But VMC is VMC. A slow moving dying line of storms can be
circumnavigated pretty easily in my limited experience.


VFR is the way to go if you decide to stay below cloud base and do it
visually there.


How do you stay away from it visually? Avoid rain and dark cloud bottoms?

Uh, yes. Put another way, fly in the daylight and an appropriate distance
from the storms.

I sense some sarcasm Jose. Make your point.

I sense some sarcasm Jose. Make your point.

No sarcasm. No point. Just asking a question. I've flown some of the stuff
too (but not much of it) and I"m always up to learn something from somebody
else's experience.

Jose

--
(for Email, make the obvious changes in my address)

I shudder to think Michael is in the same skies as myself the rest of us.




I think that was a rather rude statement.

Michael ) wrote:

At night, you can often see lightning from over 100 miles away and
can't reliably tell how far it is.

I learned this one night last summer while flying over Long Island en route
to Bedford, Massachusetts. Up ahead at our two o'clock were some very
impressive lightning strikes. It was very pretty except for the fact that
I thought we were heading right into it.

I called NY Approach and requested a 30-degree left deviation for weather.
The controller called me back and stated he had no weather on his scope for
70 miles or so, but approved my request anyhow. Before turning away we
were rewarded with the full moon rising above the cell. A definite Kodak
moment.

Later I learned that the single t-storm cell putting out all of that
impressive lightning was out over Cape Cod, easily 120 nm away. Whoops.

--
Peter












----== Posted via Newsfeed.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeed.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= 19 East/West-Coast Specialized Servers - Total Privacy via Encryption =---

Peter R. wrote:
Michael ) wrote:


At night, you can often see lightning from over 100 miles away and
can't reliably tell how far it is.


I learned this one night last summer while flying over Long Island en route
to Bedford, Massachusetts. Up ahead at our two o'clock were some very
impressive lightning strikes. It was very pretty except for the fact that
I thought we were heading right into it.

I called NY Approach and requested a 30-degree left deviation for weather.
The controller called me back and stated he had no weather on his scope for
70 miles or so, but approved my request anyhow. Before turning away we
were rewarded with the full moon rising above the cell. A definite Kodak
moment.

Later I learned that the single t-storm cell putting out all of that
impressive lightning was out over Cape Cod, easily 120 nm away. Whoops.

I once had some landing lights pointed at me at night and I thought collision
was imminent. The controller thought the lights must be on an aircraft several
miles away, and asked that pilot to blink his landing lights. Sure enough. Got
over my embarrassment and learned a lesson.

Dave