Is it possible for lightning to strike in the shape of a
circle? A continuous loop not touching earth or cloud, just
suspended in the sky?
I have seen lightning make at least half circles in the sky,
but usually where it is visible at the bottom of a cloud.
Certainly it is possible for lightning to emanate from one
part of a thunderstorm cloud and turn around and enter another
part of the cloud where the charge is the opposite sign;
that could possibly occur at approximately the same height
so as to make it look like a circle of lightning. However,
if what you saw did not emanate from and enter a cloud, it
probably was not lightning, as lightning energy flows from
one charge center in a thunderstorm to another charge center
in that thunderstorm or a nearby storm, or strikes the ground,
or strikes an elevated object such as a tall building.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
There is a form of lightning (ball lightning). The mechanism of its formation is
not well understood. The best reference, on this and other forms of lightning
discharges I have found, is "THE LIGHTNING DISCHARGE" by Martin A. Ulman.
The bottom line is that "lightning" is not well understood.
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Update: June 2012