CGs and tall structures

Dan Robinson

I found yet another example of a cloud-to-ground lightning strike not hitting the 'highest point'. This CG to the Sears Tower on October 2, 2006 hits in the cluster of secondary antennas well below the main two antenna masts.

I saw a still frame of this from another angle on the CBS 2 Chicago web site, but I can't locate it now.

Cloud-to-ground strikes to tall structures are more rare and behave differently than the more common upward-moving ground-to-cloud discharges that most of us have seen time and time again to tall towers. While a upward discharge starts (by leader initiation) from the top of the structure, a CG stepped leader starts in the cloud and moves downward - only jumping over and hitting a tall structure if it happens to descend near it or directly above it. In other words, unlike upward moving lightning, the skyscraper or tower doesn't play a role in the initiation of the CG, only in its final few steps before earth connection.

From photographic evidence, it appears that 25% to 40% of cloud-to-ground strikes to tall buildings and structures do not hit the highest point on the structure.

It is known that the stepped leader is 'blind' to objects on the ground until it gets very close to the ground or a grounded structure. In these cases, it appears that a horizontal propagation is made only after the stepped leader has already descended to an altitude lower than the top of the structure.

I have to bring up this excellent photo by Kevin Ambrose again as it illustrates this perfectly:

In the Sears Tower shot above, if the building did not exist, the CG would have still likely decended in the same spot - in other words, the building was inconsequential until the stepped leader got close enough to it. In the Washington Monument shot, that strike was clearly not 'aware' of the monument nor influenced by it until the final steps toward earth.

Pretty fascinating stuff.
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