a blue flash is better than a green flash
Walter Nissen (dk058@cleveland.Freenet.Edu)
Tue, 10 Aug 1999 08:53:36 -0400 (EDT)
There is no such thing as a unitary, or uniform, "green flash" phenomenon,
such as you may have read about in less authoritative sources. Let us
stick to what we know. The atmosphere separates photospheric light like a
prism does. Thus, the violet sun rises first and the red sun last. At
sunset the opposite is true, the red sun setting first and the violet sun
last. Thus, in its first moments at sunrise and its last moments at
sunset, the color of the Sun will tend to vary (from its color when all of
the "colored suns" are seen simultaneously, as is the case nearly all
day). Superimposed on these color changes is the color change from the
long path taken simultaneously through the atmosphere. This long path
tends to scatter blue light more efficiently (thus the blue color of the
midday sky) making the image of the Sun, which is formed only from light
which survives the trip, redder in color. In particular, violet and blue
are frequently seen to be missing altogether. The long path also quite
commonly facilitates a wide variety of meteorological modifications of the
image.
My observations reveal a wide range of phenomena which seem well-explained
by these basic principles. In particular, the green flash is not
something you either see or don't see, like some sort of step-function. I
have seen the classically described phenomenon, an extremely bright flash
of green light, as "bright as flashbulbs". Much more frequently, I have
seen a bright green glow, lasting a greater or lesser length of time, and
assuming a variety of shapes and sizes. More frequently than that, I have
seen merely bright green specks, also of various durations and sizes. The
large number of these specks I have seen may be inflated by the
preponderance of my observations over a land horizon at sunrise through
binoculars, where it is easy to catch a "first bit" through leaves of
trees. Many, many times, there is no green, not really, just yellow, or
orange or red; many of these, of every hue. I generally ignore any dim
phenomena at sunrise, since I can't distinguish them from isolated
illuminated clouds. And it is only with experience that I can properly
anticipate the pace of events at sunset and make this distinction then.
On extreme occasions, the phenomenon is not green, but blue. I have seen
a number of bluish-green and greenish-blue phenomena, of various hues,
mostly very green. At the equinoctial sunrise, 1998 SEP 23, I saw a
BRIGHT BLUE FLASH. In detail: The Sun rose behind trees, which are far
away and not of any great consequence. Immediately prior to sunrise, the
whole area of the sky near the orient was especially luminous, so much so
that it raised the question "Why is the sky already so bright if the Sun
isn't up yet?". I saw a definite brightening of the pale orange at the
orient. 31.40 seconds after that, at 11h 18m 23.6s UTC (I was too excited
to have the patience to obtain the hundredths, personal equation = about
.25s not applied), I saw the "first bit"; it was bright green, rapidly
evolving, giving way to green and yellow spreading out horizontally, and
then it happened. A BRIGHT BLUE FLASH, as blue as could be, not more
green than violet, occurred 6.26s after the "first bit" and high, not
directly in front of the first bit; but a fraction of a degree higher, and
seemingly big, perhaps as big as the, as yet unrevealed, Sun. After the
blue flash, the color continued to evolve, moving left and right, until
the green ended 3.24s after the blue flash.
I spoke with an experienced and enthusiastic meteorologist, Andre Bernier,
who said there was nothing very unusual about the weather at sunrise. He
said there was a small inversion of 10 or 15 degrees F for about 200 feet
or 1000 feet at the surface. Apparently, the key to seeing blue phenomena
is a clear path, which can occur even if there are a few clouds as there
were.
As with my 9 observations of the corona of our Sun from within the umbral
shadow of the Moon, but to a lesser degree, the words above are rather
definitely insufficient to thoroughly describe the phenomena. There is
something very significant missing. You have to be there and see them
yourself to really understand. (And this remains certainly true even if
right now you are smugly telling yourself I am surely mistaken in this.
It's an observing thing).
Cheers.
Walter Nissen dk058@cleveland.freenet.edu
-81.8637, 41.3735, 256m elevation
---
This sky observing report is pretty far off-topic. But I know from direct
correspondence with a number of you that you would like to hear this from
me. So long as no one without such special knowledge emulates this
excursion, we should be ok. Please note, especially, that the purpose of
this message is to share desired information, and not, e.g., for some
selfish purpose. Even saying that, I apologize to those more focused on
satellites.