According to Live Science, the green flash is 'a phenomenon in which [the very top part of] the sun can be observed suddenly and briefly changing [to a green] color.' The change in color only lasts a brief moment, just as the sun is dipping over the horizon, thus the reason for using the term "flash." And why the green color? Refraction in the atmosphere bends the light of the sun, filtering out colors like red and green and allowing green to show.
HA! Here I was thinking that I was going to see this flash of green light coming from the sun as it was passing over the horizon!!! And it only took me a couple of years of chasing sunsets in Costa Rica to finally witness a green flash - - which is more like a green glow than a flash. And now I know what to look for, I've been lucky to see many of green flashes.
So what is the best way to see the green flash?
Seeing the green flash is all about the right conditions. For a green flash to occur, there needs to be clear skies (no clouds and in an area free of pollution), very good visibility, and a good sunset view location. A perfect place to watch the green flash is over the ocean or body of water - - like watching the sunset in Tamarindo Costa Rica, especially during dry season here when there are no clouds in the sky!