If you’re in the right place at the right time, you can witness an elusive green glow on the sun as it crosses the horizon. A relatively rare optical phenomenon, this so-called green glow can only be observed if the atmospheric conditions, the surrounding landscape and sheer chance combine correctly.
Because they appear when the last streak of the sun appears to move away from the horizon, the green flares can only be observed during sunrise and sunset. They are often seen over large bodies of water or desert, as they require fresh air and an unobstructed view of the horizon. However, it can also be seen when viewed from a high place, such as an airplane, mountain, or tall building.
What is Green Flare?
If the sky is clear and the horizon line is clearly visible, it is the name given to the moment when the Sunlight turns a bright green color at sunrise or sunset. The reason we see sunlight in yellow when the sun is overhead is because the atmosphere filters out shades of blue while scattering the rays. These rays come at an oblique angle during the sunrise and sunset, that is, its exit from the horizon line and its disappearance on the horizon line. In this case, atmospheric lights cause us to see green. This is an event that happens for a very short time, and for it to be observed, suitable conditions must come together, as we mentioned above.
They can be observed most regularly in Cerro Paranal, Chile, as they consistently experience ideal green glow conditions. The 2,635-meter-high mountain in the Chilean Atacama Desert is also home to the European Southern Observatory’s Very Large Telescope. The conditions on the mountain are so favorable for the green glow that the observatory was even able to capture an image of a green glow on the Moon.
While the length of this Green flash depends on the rate at which the sun sets or rises, it usually lasted between one and three seconds, while the longest recorded observation took a full 35 minutes. This long sighting, witnessed by Admiral Byard’s 1929 expedition to the Little American base in Antarctica, was the result of the sun’s incredibly slow setting in the polar regions.
Although it is called “green flash”, there are actually four separate events that give a similar green flash effect.
I-Mir or lower mirage is most common and can be seen as the sun sets below the horizon. What appears to be a flattened oval occurs when there is warm air over the ocean and there is a strong temperature gradient near the water surface.
M-mir, or false mirage, is similar to the lower mirage, but occurs with an atmospheric temperature change with a bend of several degrees in the temperature gradient above the surface. It creates a pinched effect with a fine dot on the green area. This is best seen when viewed from a high level.
Probably the most impressive and longest lasting green glow, the subchannel glow can last up to 15 seconds. It creates a green hourglass effect caused by a much narrower elevation gradient as the observer is below the atmospheric change occurring near the surface.
The rarest is the green beam, which creates a beam of green light that lasts a few seconds upwards from the horizon. This event is not well documented and there are no known images of it occurring.
While different types of mirages are caused by slight atmospheric changes, the reason green glows can be observed has to do with the way light is refracted in Earth’s atmosphere. Depending on the wavelength of the light, blue, violet and green colors are refracted more than yellow, orange and red. When the sun is at the horizon, the yellow, orange, and red wavelengths that pass through are likely absorbed by the atmosphere, as it shines through a much denser atmospheric layer. The remaining blue and violet waves are dissipated leaving the strongest green wavelength to form the glow, but sometimes blue flashes can also occur.
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