hi again,
echo47, I believe that the v/m for sparks isn't linear. There are more factors than voltage (humidty, air pressure, pollutants) that can change the require potential for a 'cascade'. And of course, due to the vast difference in scale, you can't expect to simulate lightening with sparks under 10 feet.
I'm no expert, but as far as I know, the potential in a storm cloud reaches a massive figure. And, 'sparks' are not just air carrying electricity, first the air must be ionized, and its the potential that does this. Ionzing a lot of air around the cloud, and 'repeling' electrons on ground objects. A ground object (usually many) fire up 'streamers' which are long (6 feet?) channels of positively ionized air, caused by the massive potential above, but they have hardly any current. Like the sparks you get from a CRT screen (old TV) just bigger. The cloud also emits a streamer, but of course, because neither streamer in actually in contact with anything of different potential, little current flows...until.......a streamer from the ground (or ground object) makes contact with the cloud streamer.
This completes the circuit, and current flows (from ground to cloud), draining the cloud of its static electricity, a massive current surge super heats the air in the ionized channel, making it expand violently.
I'm sorry for calling lightening a 'big spark', but for the purposes of this topic (RF interference, wideband EM) its convenient to think of it as such.
anywya, regardless of why people are interested in lightening, theres been great advances in research in the past 10 years. And its all on google.
BruiedCode.