Iridescent Clouds over Paris. Imaged by Jonathan Shock (images) June 28, '08. ©Jonathan Shock, shown with permission

Iridescence most often occurs close to the sun and is best seen with its blinding light shielded by a building or other obstacle. Another technique is to look in the reflection of the sky in water or to use very dark sunglasses. Never stare at or close to an unshielded sun.

Iridescence is the result of individual cloud water drops scattering light to form a diffraction pattern. Small drops make the largest patterns. When the drops are all the same we get ordered patterns, coronae, a fogbow and a glory. When drops vary in size across the clouds or fog the pattern and its colours are jumbled - iridescence.