Re: Sunsets

[brian whatcott <inet@INTELLISYS.NET>]

At 17:45 8/29/00 -0700, Leigh wrote:
> > Here in British Columbia we often have sunsets that are not red.
> > These are the best ones to watch to see the green flash. In all
> > cases the air is present, but it doesn't suffice to produce red
> > sunsets.
                         /snip/
> >  the blue of
> > the sky which I agree is due principally to scattering from the
> > molecules of air. The question asked pertained to red sunsets...
>
> /snip/
> when the air is relatively free of
> particulates (which, you claim, occurs frequently where you live),
> the sun on the horizon is not only unreddened but also undimmed.
> I'm quite skeptical.  I'm also unclear on how the blue sky and
> the red sunset can be caused by different mechanisms.  Whatever
> it is that scatters the light, both the long and short wavelengths
> have to go somewhere.
>
> Dan


I see that it is time for a stamp-collector to step in.
Turning over the pages of the album, I see....

Elastic scattering - phase change, fx same, example Rayleigh

Rayleigh scattering, named for John Strutt who explored
scattering in his private lab. (Nobel 1904)
Assuming small particles, he derived an inverse fourth power law.
The 'particles' can & do represent thermodynamic aggregations,
affecting temperature, entropy etc.

Inelastic scattering - energy exchange between photon & particle,
so phase & fx changed. Examples -  Raman, Compton, Tyndall.

Raman: monochromatic light through a transparent medium produces
some higher fx (Anti Stokes) and lower fx (Stokes)
Inelastic collisions leave molecules with changed rotational
and vibratory energy levels. Scattering intensity is *low*.
Uses an intense source e.g laser for spectroscopy.

Compton Scattering: X Ray or Gamma ray photons lose energy by
collision with electrons.  (Nobel 1927)

However, Inverse-Compton: low energy photons gain energy when
scattered by much higher energy electrons.

Rayleigh vs. Tyndall: light scattered from particles smaller
than 1/20 lambda appears blue.
Tobacco smoke and skylight are prime examples.
May be considered as a diffraction effect, with light scattering
from small particles in inverse fourth power proportion to
incident wavelength. This can be associated with Rayleigh.

 Tyndall scattering where particles are the same order size
as the wavelength of the light is offered to explain white
scattering of light from clouds.

I missed Joe Thomson. And Coulomb.  Oh well.
Such pretty collectibles.


...but that's the trouble with stamp-collecting: I was hoping
for insights, for clear cubby holes, for clear reasons why large
 particle atmospheric contamination would provide the desired
red filter or enhanced blue scatter - and Tyndall just won't come
across with the goods.

(Oklahoma like Utah gets plenty of dust
 - they say it's either Kansas blowing south, or Texas blowing north
quite often in these parts, so the sunsets may be relied on for
technicolor effects.
 Meanwhile, the vis is unlimited; CAVOK as the birdmen might say.

In contrast, the humid, rainy UK is blessed with
visibility that can stretch all of 5 miles on an otherwise
reasonable day, and the sky is white - a pilot's invitation to
getting lost in the bad old days before GPS, before LORAN even.)



brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!