|The stuff of UFOs - Lenticular cloud and iridescence imaged by Grace Dalley (more images) July 31, '09 Christchurch, New Zealand. ©Grace Dalley, shown with permission.
The cigar shaped lenticular looms ominously in the strongly backlit sky.
Lenticulars are famously iridescent. They are wave clouds where rising air enters one side, cools and droplets condense. The air pocket then starts to descend on the wave, heats and the drops evaporate - edge of cloud. Fresh drops with all the same history are same sized and give intense iridescence.
This sky is complex and much of the iridescence is external to the main cloud.