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Alister Ling: "noctilucent clouds" (May 16, 23:36):
> Hi gang,
>
> Being a resident of the 54N parallel, I am in a favored viewing position
> for noctilucent clouds (NLC). I presume with a Finnish background to
this
> list, that this subject is within bounds?
>
> 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.
>
> Any ideas about this?
>
> Alister.
>
I haven't seen the picture in question myself, but I am pretty sure you
are right, they should be rocket exhaust. Actually, NLC clouds do not
behave like cirrus, because cirrus clouds scatter light rather
neutrally, but NLC's scatter short wavelenghts much stronger. This is
partially the reason why NLC's are blue and cirrus clouds are white.
Reason for this difference is (partly, at least) in the size of ice
crystals in these clouds, for cirrus the crystals are usually much
larger than the wavelenght of visible light (we couldn't see haloes if
the crystals weren't at this size range), and for NLC the crystals are
an order of magnitude smaller than the wavelength. To put it
"scientifically", cirrus crystals are in geometric optics range and NLC
particles are in the Rayleigh scattering range. There are likely also
differences in crystal shapes etc., but that is umimportant for this
subject.
Here at the observatory in the Univ. of Helsinki, We have tried to
figure
out any way how rayleigh particles of ice could cause colours accociated
to rocket exhausts and we haven't been able to find any. Rayleigh
scattering doesn't behave that way and water is not strongly absorbing
either. So, the unusual colours must be due to inpurities and larger
scatterers, perhaps even undercooled water droplets. I have studied a lot
of NLC literature and I have never seen any good explanation for wild
colours accociated to rocket exhausts, even though the phenomenom is
widely known (Should be, at least) and laos these exhaust clouds are
quite closely connected to NLC's.
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