Aledgedly, the green flash is that last tiny bit of the sun seen just before the sun dips over horizon into the sea at sunset. It is a light wave shift due to the passing through the atmosphere that is similar to seeing one edge of a rainbow.
When there''s no clouds to the north of Puerto Rico, we see "it" infrequently from our deck and when we hang off the drop until sunset fishing. The horizon has to be perfectly clear. I''ve seen the flash on numerous occassions, but I couldn''t swear to the fact that it''s green.
There are some great articles online about the "mechanics" behind the green flash. I have seen them a few times on hazy days and clear ones. Here is a link to a picture of one. I did not take the photo, but rather got it from another site.
I have seen it as well. Three times.
Only in the West Indies near Bequia on very clear cloudless sunsets. I have shot it with a video camera and it does not show up. (30 Frames per second)
It is as they call it a flash, very very fast.
I have sailed in the South Pacifc over 90 days at different times of the year with perfect weather and have never seen one there.
As for the photo that Jollymonjeff is talking about that''s got some green to it.
But it is not a green flash.
The flash comes at the very fraction of a second the top of the sun goes below the horizon
The green in the green flash is supposedly due to the inversion of the light rays.
As the celestial body decends through the event horizon, the refracted light rays are apparently inverted by the reflection off the sea surface and the atmosphere, from the orange/red end of the spectrum to the green/blue end.
I''ve lost count of the times I''ve seen a green flash just after sunset. Some were bearly visable ; others brightly lasting for 2-3 seconds. If one can start low in the boat & move up as it is seen, this view is prolonged. Many doubt it until they see their 1st one. A clear horizon is needed; they are more common in the tropics.
Do a web search on Google for "green flash". The Mt Wilson site has several good pics of a green flash and several sites have good explanations. However, I like the beer bottle or Margarita Glass explanation better.