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> Priority: normal
> Date sent: Wed, 20 Sep 1995 20:34:25 +0200
> Send reply to: "Meteorological (atmospheric) optics"
<METEOPTIC_at_LISTSERV.FUNET.FI>
> From: Veikko M{kel{ <PVTMAKELA_at_KASKI.HELSINKI.FI>
> Organization: University of Helsinki
> Subject: Re: Red cloud in the horizon below the Sun
> To: Multiple recipients of list METEOPTIC
<METEOPTIC_at_LISTSERV.FUNET.FI>
> > I'm not sure but it appears the red colouration could have been
> > caused by Rayliegh scattering (the same thing that causes the Sun to
> > appear red when near the horizon). The cirrus was not only higher
> > but may also have been farther away than the lower cumulous clouds so
> > the optical path length through the atmosphere was longer for the
> > cirrus than the cumulous clouds. Therefore the light from the cirrus
> > would have experience more scattering than the light from the
> > cumulous clouds, and as a result, appear more reddened.
>
>
>
> That is perhaps correct. But I don't know is this good
> explanation to limited area on reddening just below the
> Sun. There were ciruus clouds in horizon, but red color
> only in about 30 degrees wide area around the horizon
> point below the Sun.
>
> Of course the Rayleigh scattering could explain this:
> Cirrus in higher altitude are not so far away from the
> observer and the scattering angle is big from clouds in
> horizon far from the Sun.
>
>
> -Veikko-
>
Veikko:
I saw the same thing on the weekend (Sept. 23) from the west facing
window of my apartment. I live in a 9th floor apartment with an
unobstructed view of the south and western horizon. About an hour
before sunset I could see a red coloured band of cirrus about 3 to 4
degrees thick, on the horizon. The band was about 30 to 40 degrees
wide and the centre of the band was below the Sun. At the same
altitude around the rest of the sky the colour was much less obvious.
It appears that the reason for the brightest red colour occuring
below the Sun is the optical thickness of the atmosphere. The reason
you see the bright red only below the Sun is that the light is of
higher intensity and therefore can penetrate the atmosphere to the
observer. The clouds at the other parts of the sky are the same
colour but not the same brightness and so they would not be visible.
What do you think?
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