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> In a recent Sky & Telescope, there was an incredible shot of NLC. I have
> never seen such colors. Those colors seem to be more associated with
> rocket contrails than NLC. In fact, until now, I had never seen an image
> of NLC having sucg color. Being comprised of tiny ice crystals and
> behaving like cirrus, I still find the color rather odd. Meanwhile,
> rocket exhaust could survive at high altitudes as supercolled droplets
> for some time, and pictures of those *typically* show strong iridescence
> associated with droplets.
I checked out the S+T June 95 issue and there is a photograph
taken by Pekka Parviainen. The text tells that the strongly
coloured clouds are noctilucent clouds. That's incorrect.
This case (which also the photographer knows) is rather surely
found out as a military missile experiment in Kola Peninsula
or Arctic Sea area. I have been lucky to see and photograph
the same display, which was at the end of July 1988.
I have observed NLCs several years since early 1980s and I
haven't never seen strongly colors in noctilucent clouds.
They are always white or bluish, perhaps some yellowish or
reddish tint in low altitude. In some cases I have observed
amorfous (class V) NLC forms, there have been a pale "cream"
colour.
In some rocket exhausts observed by Finnish observers there
have been colors like above mentioned S+T photograph, but these
have been much more pale than in this July 1988 case. I have
ever seen such strong coloured clouds like in this display.
Perhaps some ordinary iridescent clouds have a little like
this rocket exhaust.
I agree the possibility of uniform spherical droplets. The
mechanism of formation these colours could be same as in
irisdescence, coronae or mother-of-pearl clouds. In these
there are spherical particles (mainly droplets) and color
formation to due to diffraction and interference. In NLCs
the particles are clusters of water molecules around
condensation nucleii. I think the difference between the
size of NLCs and rocket exhaust clouds could be the clue
of lacking of colors in NLCs.
-Veikko-
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