Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Bohren on sunsets



Dear Phys-L sunset lovers:

I finally asked Craig Bohren himself to help resolve the dispute
between me and Leigh. My question to him and his reply are
appended below. As you'll see, he basically sides with
Leigh, but he defines "red" much more carefully, and he does not
go so far as to claim that sunsets seen through clean air would
appear white. I haven't yet looked up the articles he mentions,
but if they're anything like his books, they should be excellent.

Dan

Dear Craig,

I'm wondering if you can help settle a dispute about the scattering
that produces blue sky and red sunsets. This dispute has arisen on
Phys-L, the physics teachers' email discussion list. The question
is, what is the relative importance of small particulates vs. clean
air in the scattering process? Do there ever exist conditions
when the dominant scattering is by air rather than particulates?
If so, is the sun noticeably red under these conditions, or is
there so little scattering that the sun is essentially white
(and too bright too look at), even on the horizon? Is it fair
to say that under most conditions, the dominant scattering is
by particles rather than by air molecules? Can you point me to
any references that would resolve these questions?

Thanks very much,

Dan Schroeder

Hi Dan,

You asked the right man about this.

Many years ago Alistair Fraser and I published a paper in The Physics
Teacher (Vol. 23, pp. 267-272, 1985) entitled "Colors of the sky". In this
paper we tried our best to demolish all the crap about sky colors. For what
it is worth, I have never in my life seen an explanation of the blue sky
and red sunsets that wasn't either misleading, partly wrong, or hopelessly
wrong.

Our paper was included in some kind of diskette of seminal expository
papers, perhaps published by the AAPT or APS or someone. I can't remember.

In very clear air, scattering by molecules can dominate. This is unusual
but not impossible. The blue of the sky has essentially nothing to do with
particles. That is, scrub the atmosphere completely clean and the sky will
still be blue. In fact, it will be a purer blue. In addition, the variation
of the color and brightness of the sky with zenith angle will be more
pronounced. You can observe this in airplanes flying at 10 km.

But particles are needed to give red sunsets. By red I mean
colorimetrically red, not orange or yellow. We show this in our PT paper.
In a purely molecular atmosphere, sunsets would be yellowish-orange or
orange but never red. In fact, I have observed and photographed this on
many occasions, even in Central PA. When we get some really clean air
masses, we don't have red sunsets or sunrises. The other extreme was after
the eruption of El Chichon in Mexico. We had some spectacular blood-red
sunsets.

To fully understand sky colors requires some knowledge of multiple
scattering, about which most physicists learn nothing. Not even mentioned
in optics textbooks. So in 1987 I published a primer on multiple scattering
in American Journal of Physics (Vol. 55, 524-533, 1987) entitled "Multiple
scattering of light and some of its observable consequences". The PT paper
is mostly non-mathematical whereas the AJP paper is more technical, but not
nearly as heavy with mathematics as treatises on multiple scattering.

I hope that this helps.

Cheers,

Craig