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Re: Sunsets



Leigh, you still haven't explained what happens to the blue
light scattered by these particulates. Mustn't it contribute
to someone else's blue sky?

It produces a brighter sky which is "less blue" in a sense.
I lived in LA for many years and did not really know what a
blue sky was; the sky was a pale blue most of the time, and
sometimes it was even white (which we called "hazy"). When
the sky is really transparent (as it was on Galiano last
week) it is "deep blue"; it is a more saturated blue and it
is darker (less bright).

And in particulate-free air,
if there is a blue sky, doesn't this imply that the sun
must be reddened somewhat, especially to someone viewing it
on the horizon?

If by "reddened" you mean that more blue light has been
scattered out of it than red light, then you are correct.
That is what we mean when we speak of reddening by
interstellar dust. The result of reddening of this sort
does not produce a red Sun, however. If twenty percent of
the blue light is scattered out and only two percent of
the red light is scattered out, the Sun is reddened in the
astronomical sense, but it is not perceived to be red.
Color (of light) depends upon its intensity, and slightly
attenuated sunlight is too intense to be perceived as
having appreciably saturated color. A red Sun at sunset
may sometimes be viewed directly because it is greatly
attenuated.

You asked whether I've ever observed a sunset that wasn't red.
I can't remember ever seeing the sun on the true horizon when
it wasn't noticeably red, or at least orange. Of course,
I've often observed sunsets obstructed by mountains or other
obstacles, and then the sun is much brighter and less red when
it disappears. I've also been in places (always with very
high humidity) where the horizon was unobstructed but the sun
was invisible at very low angles, apparently due to scattering
(or absorption or reflection--I'm not sure) by aerosols.

When the air is very transparent the Sun is too bright to look
at as it sets. When it is too bright to look at it has no
perceptible color. I don't understand the "true horizon" here.
Red sunsets occur on regular horizons. We see them often over
Vancouver Island.

I've lived in Utah for seven years, and I must say, we have
some of the best, reddest sunsets I've ever seen. Depending on
the season, I can often see the sun out over Great Salt Lake
until it is very close to the true horizon. There ain't much
pollution out there, though there may be plenty of dust. Except
when we get bad inversions, visibility around here tends to be
excellent: from the summits of local peaks, I can often make
out mountains up to 100 miles away. From these observations
I've always concluded that our air tends to be relatively free
of particles large enough to scatter strongly.

I've seen many sunsets from Kitt Peak. They are often red, and
I am told that what I am seeing is the plume from LA! For some
time it was actually Pinatubo, but the LA plume is often there.
On rare occasions we can see Rainier (!) from Simon Fraser, a
distance of something like 200 miles. (SFU is on a 1200-foot
high hill.)

To get a third opinion on sunsets, I checked one of my favorite
books, Clouds in a Glass of Beer by Craig Bohren. He mentions
both molecules and particulates as scatterers, without saying
which tends to be more important. He implies that the mechanism
of sunset reddening is always the same as that of sky blueing.

I believe that's what I've been saying. It is the conventional
wisdom.

Finally, on page 93, he claims that particles with sizes
comparable to the wavelength of visible light (neither much
larger nor much smaller) can, in some cases, scatter red light
more than blue. This could be one possible explanation of
your observation of nonred sunsets, though I wouldn't expect
this to happen very frequently, and such sunsets would be
even dimmer than the red ones.

That is not uncommon here. When the Sun is observed through just
the right thickness of cloud, the cloud acts like a neutral
density filter. On such occasions large sunspots can sometimes
be seen. The Sun usually appears to be white in this case. I see
this phenomenon most often in the morning while driving to
school, through fog, with the Sun 10-20 degrees up.

I have seen the green flash twice in sunsets over Vancouver Island
and both times the horizon was clear; the Sun was too bright to
look at directly until the instant of setting. If I am without
an instrument when this happens I often look at the Sun through
a small hole made by crossed finger openings, pulling my hands
away from my eyes when the apparent limb of the Sun reaches the
horizon.

Leigh

Cliff's question was mostly for Dan; I have nothing to add.

Hugh, the ozone conjecture is spurious. The strong plane
polarization of light from a deep blue sky is entirely
consistent with the scattering model.