Dan Robinson
I have finally got around to developing my 35mm slides from this chase season, hence the late post.
The image above is from a 35mm slide (Fuji Sensia 100, 20 seconds @ F8, 28mm lens) taken at the northern Oklahoma City tower farm during thunderstorms overnight on May 7, 2007. The image shows incomplete upward lightning leaders eminating from the tops of five of the towers in response to a simultaneous large 'anvil crawler' discharge directly overhead. I had shot video of this event and heard the short-duration, abrupty-ending thunder that indicated the presence of leaders, but the key video frames were overexposed due to the brightness of the parent anvil crawler. I was pretty happy to discover that the leaders showed up clearly on the 35mm slide!
As has been observed before, the initiation of these upward leaders is triggered by an in-cloud lightning discharge above tall towers or skyscrapers. The leaders normally indicate the beginning of a full ground to cloud lightning discharge, but as has been repeatedly observed, some of these leaders do not make it all of the way to the cloud discharge already in progress. In most cases observed, the leaders propagate all the way into the cloud, resulting in full return strokes as the charge areas and/or existing in-cloud lightning channels are contacted.
The full case study from this event, including zoomed crops and video of the discharge with distinct thunder from the leaders, is available here:
http://stormhighway.com/may72007b.shtml
There were also a couple of tower hits that night (products of 'complete' leaders!), only one of which I managed to get on slide film:
