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I think exhaust impurities do not have much to do
with the formation of circumhorizontal arc. The halo
is formed in the plate crystals with 90 deg
refracting angle, so it is often very colorful haloform.
And of cource, the bigger crystals, the better colors.
I think the contrails were formed on a little different level
than surrounding clouds (if the surrounding clouds were not
upper level clouds (Cs/Ci), then they can't make halos anyway.
But let's assume the surrounding clouds were upper level
clouds), so the meteorolocigal conditions have been also
different on that level, i.e. different vapor saturation and
temperature. So the conditions at the place where contrails
were formed were favorable for plate crystal formation but
the surrounding cloud level had no conditions for plate
crystal formation.
Exhaust impurities could have also something to do
with the plate crystal formation by causing heterogeneous
nucleation that could favor plate crystal formation.
If someone wants to read about airborne ice crystal
studies, then journals like "Journal of Atmospheric Sciences"
and "Journal of the Meteorological Society of Japan" are
full of information. Not to name any particular papers -
there are heaps of them.
Marko Riikonen
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