Re: Sunsets

[Daniel Schroeder <DSCHROEDER@CC.WEBER.EDU>]

> My apologies to Dan and any biology teachers I insulted.
> The insults were unintentional

Apology accepted.

> When one observes a red sun at sunset, the color is due to
> preferential scattering of shorter wavelength light from
> the Sun by particles in the atmosphere. The scattering due
> to air molecules which account for the blue color of the
> clear sky is much smaller.

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?  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?

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.

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.

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.
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.

Cheers,

Dan