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Upward-moving lightning questions

Dan Robinson

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A few questions I thought I'd toss out to the forum regarding upward-moving lightning. I cross-posted this to the Lightning list but I'd be interested to hear discussion from Stormtrack members.

1.) Has upward-moving or 'ground-to-cloud' lightning ever been observed or documented occuring apart from man-made structures (towers, skyscrapers)? That is, is it safe to say that this type of lightning exists today largely as a result of man-made objects? I'm sure it probably *does* occur naturally in rare instances, but I have never personally observed or seen a photo of upward-moving lightning coming up from the earth itself (or a tree, mountain peak, etc).

2.) In a question closely related to #1, is there a known 'threshold height' for antennas/buildings to initiate upward-moving lightning? I'm estimating, based on observation, that towers need to be over 800 to 1000 feet (~300m) high (AGL) to be truly prolific initiators of upward discharges. The higher the tower rises above this mark, the more strokes will be initiated during an average storm. Of course, this might also depend on storm-specific conditions of cells passing over the tower.

3.) Dr. Orville offered the explanation a few months ago (on the Lightning List) about positive CG lightning being unbranched due to converging electrons at the leader front. Example:

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I have observed and photographed both branched and unbranched upward-moving strokes from towers - does this same converging/diverging principle apply to upward-moving lightning strokes initiated from ground-based points? That is, can it be assumed that a highly branched upward-moving channel is negative and an unbranched one is positive (basically a CG in reverse)?

To throw an even more complex twist to question #3 - many of us have seen the video of the jet airliner in Japan getting struck by lightning.

http://www.quartzcity.net/~chris/blogpicts...tningstrike.jpg

In the video, the plane clearly initiated both upward and downward leaders simultaneously, and both were branched.

4.) How does leader branching differ between upward and downward ground strokes? They are visually different - downward stepped leader branches terminate in mid-air, yet upward branching seems to universally continue up into the cloud to find a 'connection', none terminate in mid-air (as in the Japanese plane photo above)
 
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