Cloud iridescence occurs when sunlight is diffracted by uniformly sized water droplets or ice crystals most commonly in mid level Altocumulus or high level Cirrocumulus.

To the eye the colour and saturation of iridescence varies from extremely bright to soft pastel colours. By shielding the camera from direct sunlight, in this case by a grove of trees and exposing for the brightest areas of cloud near the sun this display proved to be one of the most spectacular events I've photographed although to many people it would have gone unnoticed. Looking on the ground-glass of my camera with the prism removed and the lens stopped down the colours were even more 'electric' than can be shown here.

Some iridescent cloud events are more vivid than others just as rainbows can vary in intensity. The colours can be best and more safely seen shielding the sun and using good quality sunglasses or looking at the reflection in a pond of water where the intensity is likewise reduced by about 90%.