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Marko Riikonen: "Exhaust impurities/circumhorizontal arc" (May 18, 12:28):
> Exhaust impurities could have also something to do
> with the plate crystal formation by causing heterogeneous
> nucleation that could favor plate crystal formation.
>
I'm not quite sure about this (ages since I studied the subject), but I
think freezing nuclei don't affect directly to the marcoscopic shape of
crystals, but only to the molecular structure of ice. And when we are
dealing with troposphere, practically the hexagonal structure is the
only possible one. Thus the factors affecting the macroscopic shape of
ice crystals are temperature and humidity.
However, impurities could affect ice crystals by enhancing the
freeze up of undercooled water droplets, so theoretically it is quite
possible that ice crystals are different inside and outside the exhaust
contrail, even if the meteorologic conditions are the same in the whole
layer. Also, there are some indirect effects, for example, growth of ice
crystals do change the relative humidity and thus may affect the chape
of new crystals. I'm not aware whether these indirect effects are
important.
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