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Re: Sunsets

There's the old rhyme--

Red sky in the morning, sailor take warning,
Red sky at night, sailor's delight.

Wind before dawn (stirring up the particulates) is an omen of an impending
storm, even if it is clear enough to see the sun. At night, if it has
cleared so as to see the sunset, the winds of the day have probably subsided
but left the particulates in the air to scatter the light. I think the
rhyme agrees with Leigh.

Rick

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Richard W. Tarara
Associate Professor of Physics
Department of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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----- Original Message -----
From: "Daniel Schroeder" <DSCHROEDER@CC.WEBER.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, August 29, 2000 7:10 PM
Subject: Re: Sunsets


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.

Dan Schroeder
dschroeder@cc.weber.edu
http://physics.weber.edu/schroeder/