[TowerTalk] Atmospheric electricity and charging

[Jim Lux]

At 04:28 PM 7/4/2006, Keith Dutson wrote:
>I should have phased it differently to say there was insufficient potential
>to cause a strike rather than draw an arc.  At any rate, it is a THEORY.  I
>have not seen any other theory to explain this.
>
>73, Keith NM5G
><snip>



>On Behalf Of David Robbins K1TTT
>
>Cosmic rays super charge the cloud?!?!  Now that's a new one.  the last time
>I read a journal article about cosmic rays they were cited as a possible
>cause of the initial breakdown because they left a trail of ions.  there is

<snip>

>You also must remember, that the millions of volts often quoted as the
>potential of the stroke is not a measure of the breakdown voltage from the
>cloud to ground.

<snip>
 > -----Original Message-----
> > bounces at contesting.com] On Behalf Of Keith Dutson



> > >The measured energy of the strike is not the point here. The point is
> > >to
> > try and prevent the charge build up to where the strike occurs.
> >
> > Well, if you are going to prevent a strike, you basically have to
> > bleed off all of the strike energy.  I don't think that is possible.
> >
> > A cloud that is charged by the conventional rain drop method does not
> > have the potential to form a strike.  This charge has been measured
> > just prior to a strike and found to be far short of the potential
> > required to draw the arc.

Don't get mixed up between the "field" (volts/meter) under the cloud (10s 
of kV/meter), which is indeed lower than the breakdown for air (on the 
order of 3MV/meter *in a uniform field gap*) versus the charge stored in 
the cloud (on the order of tens of Coulombs).  In a nonuniform field, 
breakdown can occur at substantially lower overall fields (cf. Bazelyan and 
Raizer, "Spark Discharge", CRC press), to the point where it presents a 
significant problem in insulation for EHV power lines (that is, 1 MV can 
jump a lot farther than 30cm... more like many meters) and where a Tesla 
Coil with only 500kV or so on the top load can make 15 ft sparks.

It is not surprising that lightning can strike in relatively low fields, 
since the field only has to be high enough at the tip of the leader.  And, 
high altitude cosmic rays have been proposed as a possible source of the 
initial breakdown.  There's some very, very nifty work being done by a RF 
lightning visualization system near Huntsville AL that lets the development 
of the breakdown be seen.

As for charging mechanisms.. there's lots and lots theory out there (some 
backed up by experiment).. A lot seems to have to do with standard 
mechanisms of particle contact charging, and separation of charge by 
different sized particles. (this also occurs in dust storms and volcanic 
ash clouds, as well as the classic grain elevator explosion). Ice crystals 
are also known to play a huge role (there's no lightning when there's no 
freezing layer), because raindrops can't hold much charge before the 
electrostatic forces rip them apart.  There's also an interesting 
phenomenon where snowflakes and ice crystals become charged during the 
freezing process which is not very well understood.  It might have to do 
with the energy stored in the crystalline structure of the ice and a thin 
film of liquid water on the surface.



>  The latest theory is that random cosmic
> > rays strike the cloud causing a portion to be supercharged.  The
> > question is now how fast does this happen?  I certainly do not know
> > the answer, but if it is a matter of seconds, there is nothing that
> > can bleed off the charge and stop the strike.
> >


Not a heck of a lot of energy in any one cosmic ray.  Even at 1 GeV (which 
is a VERY high energy particle indeed), that's only about 16E-11 
Joule.  Considering a lightning stroke is on the order of 100kJ/meter of 
length, you need an awful lot of cosmic rays for that to be a significant 
source of atmospheric electricity.