Iridescence is the appearance of soft, shifting pastel colours — often greens, pinks and blues — in thin clouds close to the sun. It is a diffraction effect: sunlight bends around tiny, very uniformly-sized water droplets, and different wavelengths bend by different amounts, separating into colours. Where droplet size is patchy the colours scatter into irregular 'mother-of-pearl' bands; where droplets are uniform over a wide area they organise into the neat rings of a corona.
Worth knowing
Iridescence is most vivid in newly-formed clouds because their droplets share a common history and are unusually uniform in size — and uniformity is exactly what produces clean colour. A jumble of droplet sizes washes the colours out.
Note
Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.
