Thunderstorm charging.. wasRe: [TowerTalk] Lighting

[Jim Lux]

As we steadily drift afield from towers and antennas..

One thing to consider in the whole "discharging the field with a sharp 
point" thing is that most of the charge in a thunderstorm is transferred 
within the cloud, not between cloud and earth.

The positive charge that appears on the ground under the thunderstorm is 
really the "image" of the bottom of the storm.  Think of the whole storm as 
a big vertical antenna with the positive end up and the negative end down 
(or, more accurately, like a big double ended Van deGraaff 
generator).  That creates an image in the conducting surface underneath 
oriented the same way.

There's actually not much charge transfer between earth and cloud, compared 
to the amount being moved internal to the storm.

Changing the "shape" of the ground under the thunderstorm will just result 
in rearranging the shape of the E field, but not materially affect the 
overall amount of charge in the cloud (we're talking hundreds of Coulombs, 
here, with pretty impressive overall voltage differentials).  A sharp point 
will result in corona, which will create a bunch of ions and space charge 
around the point, making it's apparent diameter (E field wise) bigger, but, 
will then reach equilibrium, with just enough current to keep the space 
charge cloud the right size to keep the E field below around 30 kV/m. The 
wind will blow the charged ions away, but it doesn't take much current to 
keep it going.


   There's also a LOT more within cloud discharges than cloud/ground. (A 
typical thunderstorm has within cloud discharges every second, or more 
often).  In fact, a big question is how enough charge DOES get moved to the 
upper atmosphere to account for the 2 pA/m2 average fair weather current 
flow down from the atmosphere to the earth.