[TowerTalk] Lighting

[Jim Lux]

At 10:07 AM 7/7/2004 -0400, Tom Rauch wrote:
> > While visiting Chuck, WA1EKV and looking at his 100 Ft
>tower on a hill with
> > a comb of sharp points on top and 3 large ground leads at
>the bottom, he
> > told me that he once heard hissing from the top of the
>tower during a time
> > of dark clouds. He then measured the current in one of the
>ground lines with
> > a clip-on DC ammeter and found 100 Amps....implying a
>sustained 300 Amps
> > total bleeding from the cloud to ground.
>
>I've measured current in my insulated base 318 foot tower
>several times when it has been hissing, and the current was
>in the milliamp range.
>
>As a matter of fact, we used RF chokes made of #18 nichrome
>wire to bleed AM BC towers to ground so LTU's didn't charge
>up and arc, and never had a choke heat or fail from
>lightning. If the corona current was anywhere  over a few
>amperes, the chokes would have been toasters.
>
>The stepped leaders at the start of a strike are estimated
>at 150-200 amperes. 300 amperes is lightning strike current,
>not corona current. Assuming 1 MegV of potential difference
>the dissipation in the path would be 300MegW.

Uman gives total charge in the stepped leader as an average of 5 Coulombs 
(3-20 range).  If the average velocity is 150 km/sec, and the average 
leader is 5km long, this works out to 5 Coulombs in about 35 milliseconds, 
or 150 Amps.(consistent with Tom's statement)

The return stroke, though, carries the 10-20kA (typical.. range is up to 
110kA)with a charge transfer of some 2.5 Coulombs. Which is a bit bigger 
than the 300 Amps Tom gives.  (excluding the "dark" continuing current, 
which actually carries most of the charge (some 25 Coulomb).

The energy in a typical lightning STROKE (not leader, not overall flash) is 
about 100 kJ/meter (based on temperature (from spectrographic techniques) 
and photographic measurements and the thermodynamics of the air).  Since 
the duration of the stroke is about 40 microseconds, I get a peak power of 
about 2.5 GW/m.  At that current of 10-20 kA, the voltage drop is 
remarkably low (say, 100 kV/meter = 5 ohms/meter), due to the fairly large 
diameter of the core (on the order of 1 cm) of highly ionized 
gas.  Interestingly, the field gradient along the stroke is higher than 
that in the overall surroundings (10-20 kV/m), but this is typical in the 
case of long sparks.

There's also a "dart leader" which precedes the subsequent strokes in a 
flash (typically 3-4 strokes in a flash), which is about a Coloumb of 
charge, and moves about 10-15 times faster that the stepped leader (because 
there's no stepping and pausing while charge builds up at the leading end 
of the spark)

http://home.earthlink.net/~jimlux/lfacts.htm has a summary of the data from 
Uman.


For those interested in a more detailed look at the physics of lightning 
and big sparks, particularly the initiation, I would suggest the book 
"Spark Discharge", by Bazelyan and Raizer, published by CRC Press in Boca 
Raton, FL.




>I'd recheck those currents!
>
>73 Tom
>
>
>_______________________________________________
>
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>
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