When the sun sets, a green flash can sometimes be seen due to the bending of sunlight. This phenomenon can only be observed with the naked eye when there is a mirage.
If you thought the Green Flash was a new version of The Flash in a green uniform or some distant cousin of the Green Hornet or the Green Lantern… then congratulations! You’re a sci-fi nerd like me!
However, that’s not the green flash I’m talking about. A green flash is a rare sight to witness when the sun is at the horizon i.e., either during sunrise or sunset.
As we all know, the sunset presents with the hues of yellow, orange and red, but occasionally, a green flash can be observed at the rim of the sun’s disc when it dips at the horizon. It looks something like this:
A green flash of light during sunset (Photo Credit GNU Free Documentation License/Wikimedia commons)
What causes the green flash phenomenon? Why does it only show as a green flash and not any other color?
What Causes The Green Flash?
As the sun dips over the horizon, a giant beam of green light rises from the sun for a few seconds and then dwindles off in the sky. The green beam of light might be interpreted as an extra-terrestrial affair by someone who isn’t familiar with this phenomenon. Some may even consider it to be an alien aircraft leaving our atmosphere.
In fact, a simple and common phenomenon—refraction—is the reason behind the green flash.
The green flash is a type of astronomical refraction of sunlight by the atmosphere. Astronomical refraction refers to an event where astronomical objects, such as the sun and moon appear, displaced from their actual position due to refraction.
The green flash is visible to the naked eye only when enhanced by the mirage effect. A mirage is an optical illusion caused by the inversion of the image by atmospheric refraction.
Refraction of white light by a prism (Photo Credit : ONYXprj/Shutterstock)
The atmosphere has uneven density; it decreases with an increase in latitude. This means, as you go higher up in the sky, the air will get lighter. Thus, the sunlight is traveling through a dense column of air when the sun is low in the sky during sunset or sunrise.
The atmosphere bends (refracts) the white light of the sun and separates it into various colors depending on its wavelength. Colors such as yellow, orange and red have long wavelengths, whereas blue and violet colors have shorter wavelengths.
Development of green flash (Photo Credit : Brocken Inaglory/Wikimedia commons)
During sunset, the sun is surrounded by shadow suns of different colors caused by the refraction of sunlight. Basically, the red sun settles first, followed by orange, yellow, green, blue and so on. The atmosphere absorbs light with long wavelengths, so the red, yellow and orange shadow of the sun is visible during sunset. Whereas, the shadow with short wavelengths is scattered by the earth’s atmosphere.
Only one shadow remains visible after the sun sets or rises—the green shadow. It can be seen for a brief moment when the sun is below or above the horizon.
Types Of Green Flashes
- Inferior-mirage flash: This occurs when the surface is warmer than the air above it. The green flash appears oval and flattened and lasts for 1-2 seconds. These green flashes can be seen from surfaces close to sea level.
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Inferior-mirage green flash (Photo Credit : Brocken Inaglory/Wikimedia commons)
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- Mock-mirage flash: This type occurs when the surface is cooler than the air above it and the inversion is below eye level. The green flash appears as a thin strip rising from the rim of the sun, lasting for 1-2 seconds. The chances of viewing this flash are higher when the eye is just above the inversion.
- Sub-duct flash: This flash is visible when the observer is below the strong atmospheric inversion layer. The green flash appears when the upper part of the hourglass-shaped sun turns green and lasts for around 15 seconds. It appears in the form of a green beam of light that shoots up from a green flash or appears immediately after sunset, lasting for less than a couple of seconds.
- Green Ray: This occurs when the air is hazy and a green flash is present, which allows for the production of columns of light.
Inferior-mirage green flash (Photo Credit : Brocken Inaglory/Wikimedia commons)
Why Only A ‘Green’ Flash?
If the blue and violet shadows of the sun are the last to disappear from the horizon, why can’t we see a blue flash or a violet flash? The answer is quite simple… lights with shorter wavelengths scatter more in the earth’s atmosphere when they collide with air molecules. This is why the sky appears blue. The scattered blue light spreads in all directions, giving the sky its blue color.
However, it is not impossible to observe a blue flash. If the sky is extremely clear with minimal light scattering, a blue or violet flash may be visible.
How Long Does A Green Flash Last?
During Admiral Byrd’s expedition to Antarctica on October 16, 1929, the green flash phenomenon was observed intermittently for 35 minutes. But how is it possible for it to last so long?
The duration of green flashes that are not enhanced by a mirage is said to be inversely proportional to the vertical speed of the sun at the horizon. The speed of the sun, in turn, depends on the latitude and time of year at which it is observed. In simple terms, if the sun sets quickly, the green flash will be short-lived, and vice versa.
Green flash (Photo Credit : Brocken Inaglory/Wikimedia commons)
This explains the 35-minute green flash observed in Antarctica. During the summer, the sun in the Polar Regions appears to move slowly along the horizon. As a result, the green flash can be seen for an extended period. Conversely, at lower latitudes like central Europe, the green flash lasts no more than 1.5 seconds.
Summary
If you want to see a green flash, you have a good chance of doing so in places with a clear view of the horizon and clean air, like the ocean shore. Flat grasslands and deserts are also suitable for observing the green flash.
Therefore, when you go for a sunset walk on the beach, be on the lookout for the green flash. However, be cautious and avoid looking directly at the sun if it is too bright!
