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