I received a very interesting question from an associate who lived in the
southwestern US for a number of year and it goes something like this:
When I lived in the Arizona many of the lightening storms would have lots
of what I would call branch lightening (cloud to cloud many forks). While
living in the eastern US, I rarely would see this phenomena except for
this year. For some unknown reason, this type of branch lightening has
been very pronounced to me here in the east this year. I did not know how
to answer this question, but was wondering if it has something to do with
the types of particulates that may or may not be present in the air or
some type of ionization that is causing this to be more prevalent this
year. Could you please advise me as to the answer, if any so that I can
share this with my associate. Thank you in advance for all your
information.
Replies:
Bruce,
Because there is usually little water vapor in the lower part of the
atmosphere
in the southwest, the air is less conductive and fewer cloud-to-ground strikes
occur than in more humid areas like the eastern US. This also means that
higher lightning energies (current) are required to be able to have a cloud-to-ground
stroke in the southwest. Therefore, cloud-to-cloud lightning strokes and
the branching between different cells in the thunderstorm (as you
describe) are
the preferential way for the cloud cells to dissipate their stored up static
charge. If it has been drier than normal where you friend lives (for instance
the southeast this year), he may have seen more cloud-to-cloud strokes than
cloud-to-ground strokes.
David Cook
Lightning researcher at Argonne National Laboratory
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