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Re: Sunsets

Here's a reply to Leigh's latest...

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).

Leigh, you now seem to be talking about scattering by larger
particles or aerosols, which does not depend strongly on
wavelength. This kind of scattering is common, of course,
but it isn't going to make sunsets red.

If by "reddened" you mean that more blue light has been
scattered out of it than red light, then you are correct.

Good! I see that we were partly just arguing semantics.

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.

Maybe not if you look directly at the sun, but if you look
at clouds or mountains that are illuminated by the sun,
you can certainly detect the "warmer" color.

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.

By "true horizon", I merely meant that the sunlight should be
traveling through a lot of air before it reaches you. If the
sun "sets" behind an obstacle such as a mountain, the amount
of air that the light travels through will be much less than
if it sets over the sea, for instance.

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.)

That boggles the mind. An expert once told me that 100 miles
is more or less an absolute limit due to atmospheric scattering.
When I told him that I *regularly* see that far on clear days,
he softened his position somewhat. But I'm not sure he would
believe 200.

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.

No it isn't what you were saying earlier, but I'm glad it's what
you're saying now.

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.

When the sun is 10-20 degrees up, you wouldn't expect that much
reddening from the atmosphere in the first place, so no such
exotic mechanism is needed to counteract the reddening. Cloud
droplets are much larger than visible wavelengths, so they
scatter all wavelengths about equally.

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

I certainly agree. Also, if the blue sky were due to an absorption
resonance, the sky would have a very narrow spectrum, not a broad one.

I think I'll do a bit of research (library and/or contacting experts)
to try to settle the dust-vs.-air question. If I learning anything,
I'll post it.

dan