Green-rimmed cloud at sunrise
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.
Posted on March 21, 2017, in air mirages, green flash and distortions of sun and moon and tagged blue flash, green cloud rim, green flash, green rim. Bookmark the permalink. 2 Comments.
Hi, der grüne Rand ist doch sicher nur der Beugung durch den Tiefen Stand geschuldet. Je näher die Objekte am Horizont stehen, desto höher ist die atmosphärische dispersion.
atmospheric dispersion (due to refraction [Brechung], not diffraction [Beugung]) should indeed result in a green rim around the top of all remote light sources close to the horizon. This can be easily observed at the sun’s disk with a telescope (and appropriate filters of course) everytime you’ll find it at low elevation in the sky.
However, pronounced green or blue flashes require somewhat rare refractive index profiles in the air (e.g. due to temperature inversions) which drastically enhance the amount of dispersion. In my opinion, green cloud edges will become noticable only under such exceptional conditions. After all, a cloud edge is orders of magnitude less intense than the sun, and has to compete against a rather bright sky backround. This makes the green cloud rims such a rare sight.