Date: Mon, 18 Feb 2002 09:03:25 -0500 (EST)

Author: John Mocko

Subject: Explanation of flash of green from Meterology Today Text


The textbook "Meterology Today" by C. Donald Ahrens 6th Edition explains
the green flash this way;


"Occasionally a flash of green light...the green flash.... may be seen
near the upper rim of a rising or setting sun. When the sun is near the
horizon, its light must penetrate a thick section of the atmosphere. This
thick atmosphere refracts sunlight, with purple and blue light bending the
most, and red light the least. Because of this bending, more blue light
should appear along the top of the sun. But because the atmosphere
selectively scatters blue light, very little reaches us, and we see green
light instead.

Usually, the green light is too faint to see with the human eye. However,
under certain conditions, such as when the surface air is very hot or when
an upper-level inversion exits, the green light is magnified by the
atmosphere. When this happens, a momentary flash of green light appears,
often just before the sun disapperas from view.

The flash usually lasts about a second, although in polar regions it can
last longer. Here, the sun slowly changes in elevation and the flash may
exist for several minutes. Members of admiral Byrd's expedition in the
south polar region reported seeing the green flash for 35 minutes in
September as the sun rose slowly above the horizon, marking the end of the
long winter."


The textbook also shows a picture of the green flash and the green spot is
actually above the normal arc of the setting sun.

= <----- Green flash
- -
- -
- Sun -

John Mocko
Univ of Florida

On Sun, 17 Feb 2002 wrote:

> In a message dated 2/16/02 9:26:06 PM Eastern Standard Time,
> writes:
> << Hello Mary,
> So you live on the West Coast of Florida...if you don't mind me
> asking, where about? I live in Gainesville and work at the University of
> Florida. You are the first person on Tap-l I've seen who lives even
> remotely nearby. We teach a course at UF called "Light, Color, and
> Holography" and the textbook for the course explains the green flash if I
> remember correctly. Ask me to look it up on Monday morning when I return
> to work and I'll see if I can find it in the book and explain it.
> Sincerely,
> John Mocko
> Lecture Demos
> University of Florida >>
> John,
> I live in Tampa & have been active in AAPT for many years. I was a high
> school physics teacher before I retired. However, i am still active in
> physics. I would REALLY appreciate it if you could answer this question.
> Thanks,
> Mary Winn


John Mocko
Senior Teaching Laboratory Specialist (Lecture Demonstrations)
Department of Physics
University of Florida
Gainesville, Fl.