Re: Sunsets

[Leigh Palmer <palmer@SFU.CA>]

> Leigh says scattering of blue-rich light is by particulates and
> aerosols; I say it's by the air molecules themselves.  Who is
> right?  Both, I think.  I'm not real sure how great the relative
> contribution of particulates is, especially because it's very hard
> to find air that is free of particulates.  (Particulates aren't
> just pollution; they include many natural materials, from plant
> matter to sea salt.)  When the relative humidity reaches about
> 70-80 percent, soluble particulates absorb water to become thick
> solutions or aerosols, and scatter more light because they've
> grown larger.  In any case, the dependence of the scattering rate
> on the fourth power of the frequency of the light holds only for
> scatterers that are significantly smaller than the wavelength.
> Larger particles (such as cloud droplets) can reflect light,
> and reflection doesn't depend strongly on wavelength.

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. Dan gives the conventional explanation for 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,
and the target audience is a biology teacher. I, too, love the
physics, but in this case one must also try to match impedance.

One final note I might add here. I visited the McMath Solar
Telescope at Kitt Peak one evening at sunset. The air was very
clear, but the image of the sun is too unstable to use for most
purposes at that time of day. An atmospheric chemist was using
the instrument to observe the atmosphere. He was doing
absorption spectroscopy to observe the very dilute molecular
species in the long path length available at sunset. He used
some very sophisticated techniques involving cells filled with
isotopically varying kinds of carbon dioxide, the details of
which now elude me.

Leigh