And the answer is . . ..
 

The last two Green Flash types are produced by the use of an alternate
horizon of sorts. Low lying very thick clouds of the altocumulus type do
nicely. Normally these are quite high but away from land the one's out
toward the horizon appear as a thick, dark colored, edge of disc and they
are, due to curvature of the earth, very close to the horizon. Something
like looking at a dinner plate from the side. Underneath all is very
clear. The conditions that produce the flash are duplicated as the sun
sinks past the cloud formation which has become a 'false horizon.' So by
watching the sun's top as it passes the cloud you see one flash and then a
bit later see another one as it passes the actual horizon. The same thing,
I was taught, can occur as the sun rises. However the conditions for a
morning flash sighting are even more exacting. I'm not sure of the
scientific explanation but it had to do with the rising rather than the
setting of the sun's motion and some other atmospheric condition stuff. At
that point skipping all those science courses took it's toll.

This last trip we were lucky to have a thirty year veteran in our ship's
captain who delighted in holding impromptu instruction on a variety of
subjects. Some more important one's were, where the best bar used to be and
currently is in certain ports but since we don't do that at sea . . .. back
to flashes ..

M.

And the answer is . . ..
 

Thanks for the information Michael.

Michael wrote
The last two Green Flash types are produced by the use of an alternate
horizon of sorts. Low lying very thick clouds of the altocumulus type do
nicely. Normally these are quite high but away from land the one's out
toward the horizon appear as a thick, dark colored, edge of disc and they
are, due to curvature of the earth, very close to the horizon. Something
like looking at a dinner plate from the side. Underneath all is very
clear. The conditions that produce the flash are duplicated as the sun
sinks past the cloud formation which has become a 'false horizon.' So by
watching the sun's top as it passes the cloud you see one flash and then a
bit later see another one as it passes the actual horizon. The same
thing,
I was taught, can occur as the sun rises. However the conditions for a
morning flash sighting are even more exacting. I'm not sure of the
scientific explanation but it had to do with the rising rather than the
setting of the sun's motion and some other atmospheric condition stuff. At
that point skipping all those science courses took it's toll.