A Rainbow With No Ends

On the bus ride back from my trip to Flanders (blogs to follow), I photographed a strange phenomenon that resembled a rainbow with no ends. 

I took the photo on the Autobahn A5 near Bühl on September 23, at 18h30. The low evening sun illuminated a small cirrus cloud and created this rare (?) spectacle. That's a case for ChatGPT.

Do you know anything more about this weather phenomenon?

This is a cloud iridescence in cirrus, caused by diffraction of low evening sunlight by tiny ice particles. With the Sun only about 8° above the horizon, sunlight came through the atmosphere at a very shallow angle, illuminating the thin high cloud from the side.

It is a relatively uncommon sight in ordinary mid-latitude cirrus. A published long-term study found that vivid iridescence in mid-latitude cirrus is quite rare because the very small ice particles needed for it do not persist for long.

And there is also a nice coincidence: September 23, 2026 was the autumnal equinox (at 02:05 CEST). So you photographed this essentially on the day when the Sun crossed the celestial equator.

You always get a bonus with ChatGPT.

Iridescent clouds in Liebenwalde on April 10, 2020

The phenomenon that sunlight is not refracted by water drops causing a rainbow, but diffracted by ice particles, was photographed by ©Andreas Möller, too.

And remember the spectacular pink in Freiburg's sky on December 9 last year.

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