Hey, my rainbow cloud has now appeared on the Cloud Appreciation Society website!
The photo gallery editor, Ian, sent me this link, which explains how this phenomenon occurs (equinox -tick; midday - tick; cirrus clouds - tick).
Just the thing while you're enjoying a long cup of coffee, or lunch, or just an idle ten minutes - click here and start the slideshow. There are shots of clouds (and a few rainbows) from all over the world - from the everyday clouds we all too often fail to marvel at, to the breathtakingly beautiful and the wildly freakish.
Also on the Cloud Appreciation Society site I found a link to these amazing photographs of clouds from above - taken from the Space Shuttle.