Pileus iridescence

"Bird watcher David LaPuma was in the Everglades National Park on July 17, 2003, when he glanced up and noticed 'the top of a cumulus cloud looking strange. Something up there was refracting sunlight and creating a kaleidoscope of color that was absolutely stunning!' LaPuma captured this picture and five others using a Nikon digital camera and a Leica Televid 77 APO spotting scope.

"Atmospheric optics expert Les Cowley explains what David saw: 'His pictures show iridescence in pileus clouds just above cumulus clouds. During the day, the warm moist air of a cumulus cloud rises. Sometimes the upward movement of the cumulus cloud pushes a layer of moist air above it upwards, too. This causes the air layer to expand and cool. Water vapor suddenly condenses forming a misty veil-like layer of water droplets above the cumulus. This is the pileus cloud. Any cloud formed suddenly like this has all its droplets of similar size, which is an absolutely ideal condition for iridescence.'"

via Spaceweather.com