Iridescence in clouds is caused by diffraction of sunlight off cloud droplets of very uniform size. Such uniformity is characteristic of “young” clouds; older clouds have a wider distribution of sizes due to the coalescing of droplets. In cirrus clouds like these the diffracting agent may be tiny ice crystals or frozen droplets. Rainbows are similar in appearance but are caused by refraction of light.
Taken November 9, 1988, off Panama City, Florida
Nikon F3, 50mm, f5.6, 1/125 sec on Kodacolor VRG 100