Ice particle iridescence?
Normally, iridescence shows rather faint colours which can only be seen by covering the sun. On December 6, 2010, however, iridescence was visible in such a brightness and colourfulness in high level clouds that at first sight it rather looked like halo fragments than like iridescence. (1–2–3)
Manfred Nehonsky also observed extremely bright iridescence on high level foehn clouds over Upper Austria the same day. This iridescence looked like bright mother-of-pearl-clouds.
Another observation made the same day, but by mistake entitled as a halo, can be found here.
I think this is iridescence on globular ice particles as Paul J. Neiman and Joseph A. Shaw suggested in their article “Coronas and Iridescence in Mountain Wave Clouds Over Northeastern Colorado“.
Author: Claudia Hinz, Germany