On June 14th, 2014, I could observe green flamelets at the upper rim of an altocumulus cloud from Mt. Zugspitze (2963 m above sea level). The cloud was located left from the rising sun, and the phenomenon lasted from two minutes before the visible sunrise until shortly after it. At the moment of the astronomical sunrise the green flamelets at the cloud vanished. Additionally, green and blue rims appeared at the sun’s disk (see pictures 1 – 2).
I already observed a similar phenomenon a few seconds after sunset on September 24th, 2013, from Mt. Zugspitze. However, I could only take a single photograph of it. As there were seemingly no other reports about green cloud rims I decided to let the matter rest at that time. It was only after the second occurrence that I re-visited the case of the older observation.
When doing a new search for similar reports I encountered an observation from by Robert Wagner, January 7th, 2008, who also recorded green cloud rims during sunset on La Palma (2136 m above sea level).
No other documented observations could be found on the internet so far.
We cannot offer a complete explanation yet. It may be that the cloud edge, when illuminated from behind, acts as a separate light source and the green flamelets are then caused by the refractive dispersion of a weak mirage effect. This is consistent with the presence of blue and green rims at the sun, which indeed have been observed in all three cases. Furthermore, all observations were carried out from high mountains, from where the true geographic horizon already lies below zero elevation, and even the ordinary elevation shift due to refraction is already pretty high due to the long light path through the atmosphere.
More ideas and reports of similar observations are welcome in any case.
Author: Claudia Hinz
Edit 21th March, 2017:
I would like to add a video to this article, in which I was record the green rimmed clouds on the Mt. Fichtelberg/Ore Mountains on 20-12-2016.
Almost every times when on Mt. Wendelstein the sun is setting behind the main chain of the Alps in very clear air, I can watch the green and the blue flash. These phenomena were especially impressive on February 2, 2011, when the sun sat behind 2962 meters high Mt. Zugspitze, and on March 3 at sunset behind the 1801 meters high Benediktinerwand (series 2).
Author: Claudia Hinz, Brannenburg, Germany
This sequence of sunset images was taken from the Canadian prairies, looking ovcr the Rocky Mountains from a distance of about 100 km, in mid-December, 2007. The remaining solar limb shows distinct blue colour following green tints, but the sequence ends with a faint red band, which is probably caused by faint clouds on the horizon.
Author: Alan Clark, Canada
On December 26th I climbed the 1000m high Plettenberg to observe and photograph the setting sun with 1m focal length to look for the green flash. The transparent air and an inversion layer were promising, however a few clouds with their top at approximately the same altitude were disturbing. The upper limb of the sun turned out to be quite turbulent showing green rims and flashes, but also some blueish apparitions.
Why a blue flash?
A green rim of the setting or rising sun occurs due to differential refraction in the atmosphere. If conditions are extremely clear, also the blue light has a chance to get through, and there might be even a blueish rim. With a temperature inversion layer in the atmosphere, upper segments of the solar image might get separated from the rest of the solar disc. In the final moments of these elusive segments they do appear green and sometimes even blue (green or blue flash). However, it is not clear to me why in my observations both happens. Most last moments are green, whereas the third frame shows a blue color and at the same time other turbulent segments with a green color. Should not every segment turn from green to blue, at least in the very last visible moment?
Maybe something more than just clear air plays a role for a blue flash visibility!?
It is the first time I see green and blue flashes simultaneously in one image. I strongly encourage other observers to record video data to show these effects in higher time resolution.
Author: Till Credner