Rare ducted green flash, Libyan desert

Rare Ducted Green Flash in the Libyan Desert

The phenomenon known as a "Rare Ducted Green Flash" is a captivating atmospheric optics event that occurred in the Libyan Desert on the evening of March 28, 2006. The stunning sequence of photographs captured by John McKune near Waw an Namus showcases the unique transformation of the setting sun into a "flash" of vibrant green and blue hues. This occurrence took place just before a total solar eclipse, adding to the allure and rarity of the event.

As McKune observed the setting sun, he noticed it gradually assuming a peculiar "tin hat" shape, eventually shrinking into a thin bar on the horizon. This transformation was accompanied by the emergence of green and blue tips, which rapidly expanded and merged to form the final mesmerizing "flash." The entire spectacle unfolded against the backdrop of an exceptionally clean and dry atmosphere, making it challenging to gaze directly at the intensely bright sun even as it neared the horizon.

To capture this breathtaking phenomenon, McKune employed a combination of technical skills and precise equipment settings. He utilized a fixed shutter speed of 1/2000s and manually adjusted the aperture to achieve optimal exposure without overexposing or dimming the colors. McKune's camera lens was set to 400mm with a 1.4x extender, allowing him to zoom in on the sun's descent with remarkable clarity. Additionally, he used a shutter release attachment to burst at 3 frames per second, ensuring that no moment of the spectacle was missed.

Several crucial factors contributed to the occurrence of this rare ducted green flash in the Libyan Desert. The overall topography of the area played a significant role, as it was characterized by vast expanses of flat and dry terrain. With only gentle undulations and a shallow depression located approximately 6 kilometers westward, the landscape provided an ideal setting for the atmospheric conditions to develop. Furthermore, the evening was marked by exceptional calmness, with minimal wind disturbance affecting the air.

The key to understanding the formation of the green flash lies in the presence of a temperature inversion and the phenomenon of "ducting." Approximately 22 seconds before the flash, the sun exhibited a distinct "tin hat" shape, indicating the existence of a temperature inversion. In this scenario, rays of light become trapped between layers of the inversion and can travel significant distances before escaping. This ducting effect is responsible for the elongated shape and vibrant colors witnessed during the rare green flash event.

Andrew Young, an expert in atmospheric optics, expressed his delight upon seeing McKune's photographs. Young noted that the simulation models he had studied for years had finally been captured in reality. The exceptional cleanliness of the air in the remote Libyan Desert allowed for these elusive details to become clearly visible. With the sun seemingly setting on top of a duct below eye level, the apparent horizon was only a few hundred meters away, making the intricate features of the flash readily discernible.

The formation of this rare ducted green flash can be attributed to a radiative inversion that occurred during the final moments of the sunset. The extreme dryness of the desert contributed to the development of a sharp inversion, resulting in a thin and delicate duct formation. Although there was minimal wind, a slight movement of air near the ground allowed rays to pass between the bottom of the shallow duct and the desert surface. This delicate balance created an extraordinary visual spectacle that unfolded below eye level, defying conventional expectations.

In conclusion, the rare ducted green flash observed in the Libyan Desert on March 28, 2006, stands as a testament to the fascinating and awe-inspiring phenomena that can occur within our atmosphere. The captivating sequence of photographs captured by John McKune provides valuable insights into the complex interplay of atmospheric conditions, including temperature inversions and ducting. By shedding light on these intricate processes, researchers and enthusiasts alike gain a deeper understanding of the atmospheric optics that shape our visual experiences of the natural world.

John McKune took this sunset sequence near Waw an Namus in the Libyan Desert on 28th March 2006, the evening before the total solar eclipse there. He saw the setting sun first develop a "tin hat" shape and then shrink to a thin bar on the horizon. Green and blue tips developed which broadened and raced together to form the final "flash".

John McKune took this sunset sequence near Waw an Namus in the Libyan Desert on 28th March 2006, the evening before the total solar eclipse there. He saw the setting sun first develop a "tin hat" shape and then shrink to a thin bar on the horizon. Green and blue tips developed which broadened and raced together to form the final "flash".

"As the Sun began to set I would take a quick shot to check the histogram against my exposure settings. I used a fixed shutter speed of 1/2000s and was changing the aperture manually to try and get an image that was neither over-exposed nor too dim to register proper colors. The air was so clean and dry that it was difficult to look directly at the Sun even when it was only a sliver above the horizon. I have never seen the sky quite like this before. As the sun was going down, you could not look at it at all naked-eye; even to the very last moment it was too bright. I was using my 100-400 lens at 400 with a 1.4x extender. I also had my shutter release attached and was bursting at 3 frames per second as the upper limb began to flatten out. The Sun's disk was very clean and steady as it went down; no atmospheric turbulence was present at all. Just when it was about over and I had decided nothing was going to happen, two brilliant emerald green spots formed at the tips. These grew into bars which raced towards each other to form a single dazzling spot just before disappearing below the horizon."

Other crucial conditions for this particular flash: - "The entire area was extremely flat and dry. There were broad undulations of no more than a meter in height to the horizon." To the west, 6 km distant there was a shallow depression perhaps 1-2m lower - “That evening was very calm." - John’s camera was less than 2m above the desert surface.

The first clue to what happened is the “tin hat” shape of the sun 22s before the flash. The thin “brim” extensions along the apparent horizon signal that a temperature inversion was present and was sufficiently strong that ‘ducting’ took place. Ducting is when rays are trapped between layers of the inversion and can travel a considerable distance before eventually escaping.

Andrew Young says - "Well, I'm delighted to see this picture, as it shows stuff I've seen in my simulations for years but never caught in reality -- probably because our humidity .in California. is usually too high, and the air is too dirty when there's offshore flow. What's made this possible is the REALLY CLEAN air out on the middle of nowhere.

What you're seeing is the Sun setting on top of a duct below eye level; ..the top of the duct is no more than a few cm above the ground, and only a meter at most below the camera. And that, in turn, means that the apparent horizon is only a few hundred meters away, which makes these details -- normally invisible -- blatantly obvious.

The line is of course a highly compressed image of the upper part of the Sun.

Evidently, a radiative inversion had developed in the last minutes of the sunset, because of the extreme dryness of the desert. Because of the lack of wind, this inversion is very sharp, so the gradient is enough to make a duct -- but an extremely thin one. There must have

been a little air movement, so that a few cm of air next to the ground was mixed, allowing just enough room for rays to pass between the bottom of this very shallow duct and the ground itself. A very delicate balancing act is required to allow all this to happen below eye level!"

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  • "Rare ducted green flash, Libyan desert". Atmospheric Optics. Accessed on June 25, 2024. https://atoptics.co.uk/blog/rare-ducted-green-flash-libyan-desert/.

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