What Is The Green Flash Phenomenon?

What Is The Green Flash Phenomenon?
Image Credit: Juan José Manzano (Grupo de Observadores Astronómicos de Tenerife)

The image above shows a dramatic example of a green flash, as seen from the Canary Islands. While this particular atmospheric phenomenon was considered a myth for a long time, its exact causes have been known for an equally long time, and the only reason why the existence of green flash was doubted was that it only lasts for about two seconds or so.

Also known as “green rays”, the green flash is an atmospheric phenomenon that can be seen from any altitude, and that occurs either just as the top rim of the Sun descends below the horizon at sunset, or, just as the top of the Sun emerges from below the horizon during sunrise. It should be noted, though, that no part of the Sun actually turns green when the flash occurs; the flash is purely related to how sunlight is refracted and scattered through different layers of the atmosphere.

Typically, the green flash is best seen over a clear horizon such as the ocean, and in stable atmospheric conditions that allow more of the Sun’s light to reach an observer without any light being scattered, absorbed, or extinguished by atmospheric haze. However, why is the flash green, and not blue as might be expected, since blue light is the most refracted frequency, and should therefore be the last component of the Sun’s light to sink below the horizon?

The explanation is very simple. While the Sun is almost pure white when viewed from outside of Earth’s atmosphere as a result of the Sun radiating optical light in many frequencies, the Sun actually radiates more green light than any other color. By rights then, the Sun should appear green to our eyes, but the reason why we don’t see a green Sun (or other green stars, for that matter) is that all the optical light frequencies emitted by the Sun largely cancel each other out through absorption, thus resulting in an almost pure white Sun.

Nonetheless, when atmospheric conditions are right, in the sense that the atmosphere is strongly layered near Earth’s surface, the layers in the atmosphere act like a prism, but blue light is scattered preferentially out of an observer’s line of sight. Thus, because blue light is scattered to the point where we can’t see it, the green component of the Sun’s light becomes visible for a brief moment just as the top of the Sun sinks below the horizon.

Although the green flash is visible in most sunsets, the effect may be too brief to be noticed unless an observer actively looks out for it. However, when a mirage is present along with exceptionally strong layering of the atmosphere, the effect is greatly enhanced by the mirage, and under these conditions, the flash may be visible for up to two or even three seconds, depending on the local conditions. In rare cases, the flash may be seen as blue, but there is as yet no explanation as to why the momentary flash would be blue, instead of green.

The Green Rim

A closely related phenomenon is known as a “green rim”, in which most of the upper rim of the Sun or other astronomical object takes on a distinctly green hue, when the object descends below the horizon. The causes of a green rim are largely similar to those that cause the green flash, but most instances of the green rim are too faint for naked eye observation, although they often show up in photographs.

It should be noted  that the observed color of the upper rim of an astronomical object could vary from green to blue, to violet, depending both on the levels of atmospheric pollutants that are present in any given volume of atmospheric air, and how those pollutants are distributed throughout the various layers of the atmosphere. Regardless of the color of the object’s upper rim, the bottom rim is always red as the result of atmospheric scattering of red light.

However, unlike the green flash that is visible for only a second or two, a green rim can remain visible for extended periods under some conditions, and especially when a strong mirage effect is present in the atmosphere. Below is a description of a green flash event followed by a green rim event that remained visible for about 35 minutes, as recorded in 1934 by members of the Richard Evelyn Byrd party in Antarctica, from the Little America exploration base:

“There was a rush for the surface and as eyes turned southward, they saw a tiny but brilliant green spot where the last ray of the upper rim of the sun hung on the skyline. It lasted an appreciable length of time, several seconds at least, and no sooner disappeared than it flashed forth again. Altogether, it remained on the horizon with short interruptions for thirty-five minutes. When it disappeared momentarily, it seemed to have been shut off by a tiny spurt, an inequality in the skyline caused by the barrier surface. Even by moving the head up a few inches, it would disappear and reappear again and after it had finally disappeared from view, it could be recaptured by climbing up the first few steps of the antennae post.”

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