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The green flash is an atmospheric refractive phenomenon where the top edge of the Sun will momentarily turn green. It is seen rarely by the naked eye, primarily because it requires specific conditions to occur, but also because it requires the observer to know what to look for. Despite the name, there is no "flash"; the event only lasts from a fraction of a second to at the longest, a few seconds.
The basic cause for the green flash is that refraction bends the light of the Sun. The atmosphere acts like a weak prism, separating the light into different colors. Bluer light is bent more strongly than red light. However, the amount of refraction even at the horizon is quite small: only a few seconds of arc (one second of arc is 1/3600th of a degree). This effect is magnified by the atmosphere itself. Layering in the atmosphere causes an effect similar to a horizontal cylindrical lens: the separation of the color bands is exaggerated in the vertical direction, so that the separation can be up to several minutes of arc.
What conditions are required to see the green flash? The green flash is best observed when you have a clear view of the horizon uncluttered by foreground objects and pollution free. This usually means you need to see a distance of several miles "out", almost to the point where the curvature of the Earth defines the limit. This is primarily why stories of seeing the green flash frequently occur at the ocean. This is due to the additional amount of atmosphere one is looking through at the horizon when the Sun is setting. In addition - and equally important - is the fact that the line of sight is nearly parallel to the horizon.
Although Mount Wilson Observatory is near Los Angeles the green flash is frequently observed here, because the elevation of the mountain (5710 ft, 1740.4m) places it above the inversion layer. In addition, for approximately half the year the Sun sets over the ocean which is visible beyond the Los Angeles Basin. (The horizon is approximately 92 1/2 miles away.)
OK, then, why is it called a "green" flash and not a "blue" flash? Because contamination in the atmosphere scatters blue light removing it from the line of sight. More green light gets through and therefore is more clearly seen. In extraordinary conditions, a "blue" flash might be seen.
Actually, all celestial objects experience the same effect near the horizon; it is possible to see "green flashes" from the setting Moon, Venus, or bright stars like Sirius.
[h=2]How will I know when to expect the Green Flash?[/h]
To best observe the Green Flash, it is important to know what to look for. As with any observations of the Sun it is imperative to take proper precautions in order to prevent serious damage to your eyesight! Even though the Sun is near the horizon and attenuated from the increased atmospheric opacity, it is still very bright and concentrated observation with the naked eye or with optical instruments can be permanently harmful to eyesight. Observations should be made through heavily-smoked glass, welder's glass, or other neutral density filters.
As stated before, the atmospheric conditions which are most favorable for the Green Flash are a calm and stratified atmosphere. When the Sun is a few degrees above the horizon, it appears distorted and flattened in the vertical direction (although the azimuthal size hardly changes). The closer it gets to the horizon, the more flattened it gets. Soon, the edge of the Sun will become "notched" on either side. These notches appear to "ride" up the sides of the Sun from bottom to top; however, in reality, the notches are caused by the atmospheric layering and aren't moving - it's the Sun which is setting through them.
When the notches reach the top of the Sun they meet and "pinch" off the edge of the disk (a "floater"). This is the part that suddenly turns green: a green flash. The intensity of the color depends on many factors but appears to be somewhat sensitive to overall brightness. If the Sun is too bright, the color will pale and wash out; converesly, if it's too faint, the color will be almost impossible to see. As a result vivid photographs of the green flash are very difficult to obtain. The examples we have provided (courtesy of Lu Rarogiewicz) show clearly the distortions of the solar disk and frequently a hint of green near the floaters.
