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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.

I assume your comment refers to the first part of the
paragraph. In the second part I'm talking here about
scattering *only* by the clean air
in the atmosphere on a good day. I have seen the sky even more
transparent than it was on Galiano last week (one can tell by
the look of distant mountains), but it was pretty good. We
could clearly see the Milky Way sitting on a lighted porch,
even with my old eyes. Vancouver has smog. I've only seen the
Milky Way well from my backyard once in twenty-one years, and
I know exactly where to look for it.

The white sky phenomenon does not depend on scattering by
large particles. When seen from above the same air looks
brown, a common sight when landing at LAX. When you see that
brown cloud from the air, check the sky after landing. A
fellow from Caltech named Hagen-Schmidt (?) studied this
stuff. He called it "smog". It is produced partly by
photochemical process and partly by particulates smaller
than a wavelength.

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.

Maybe you were; I was talking about a Sun which appears to
have a red color.

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.

For the numbers I've given I doubt that, but perhaps it is so.

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.

Well, yes, but we see lots of red sunsets over Vancouver Island,
which can't be more than a couple of degrees above a true
horizon. While it is true that reddening due to the air is
greater for a longer path length, it is also true that reddening
due to scattering from particulates is greater, and it is
relatively more important than the molecular scattering because
there are more absorption lengths of particulate scatterers. It
is the difference in the number of absorption lengths due to
each process that will tell us when one process is negligible
with respect to the other. If one process is slightly stronger
than the other, at greater path lengths the difference will be
amplified.

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.

I tried to caliper it on a small scale map. The distance is more
like 180 miles. Mt. Rainier is 14,410 feet high. We can see Mt.
Baker (10,000 feet high and 100 km distant) on any clear day,
even from sea level.

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.

Please reread my first posting on this topic. I discussed the
mechanism of the blue sky and blue mountains. Both phenomena
are due to wavelength dependent scattering.

Our difference of opinion seems to come down to my observation
that sometimes I see sunsets which are not red in relatively
clean air. A red sunset, while not uncommon, sometimes is not
seen, even when the atmosphere is present. I conclude that the
air alone cannot produce a red sunset.

Leigh