I re-discovered the accompanying image searching through my slide collection recently. Still intrigued by what this image might represent, I picked up my copy of 'Kaleidoscope Sky' by Tim Herd, well known meteorologist and naturalist, and browsed throught the vast collection of most interesting images to see if I could identify this phenomenon. I suspected the image was displaying a phenomemon called 'iridescence'.
On this particular occasion I was traveling through Lundbreck in southern Alberta just at the edge of the Canadian Rocky Mountains. The sun had disappeared behind the horizon some time ago; the moon had risen above the mountains and could be seen behind rather 'colorful clouds'. I have seen this kind of thing only rarely.
To confirm my suspicion, I sent Tim an email which I have done on occasion when in doubt about a particular celestial phenomenon. Tim did confirm that I had indeed captured 'iridescence'. This is what Tim told me:
"Yes, that's iridescence. When light passes through very small water droplets, mist, or fog, or even a thin cloud, it diffracts and disperses in prismatic bands. (Diffraction is scattering of light by particles approximately the same size as the wavelength of light, which results in a pattern of constructive and destructive interference of the light waves - similar to the overlapping of water waves.) When the cloud cover is unbroken, a concentric series of small rings will form a corona or aureole. In fact, on the lower cloud in your photo the coloration appears as part of a ring. When clouds are not uniform or are scattered, the diffraction may spread fairly far and wide in small patches of pastels and is then called irisation or iridescence. Nice shot!". Tim Herd.
'Kaleidoscope Sky' is a wonderful little book and a great reference!