September 2023

Iridescence – Clouds with Colourful Personalities

Have you ever looked up at a cloud near the Sun and noticed bands of pastel colours like those from oil on water? If so, you were probably looking at the optical effect known as cloud iridescence. The colours are also known as ‘irisation’, after Iris, the Greek goddess of the rainbow, but these colours have nothing to do with raindrops. They are caused by sunlight interacting with tiny droplets or ice crystals in clouds. As the sunlight shines through thin cloud, its waves bend around the tiny droplets or crystals in a process known as diffraction. Since the different wavelengths that make up sunlight bend by different amounts, it can be scattered into bands of iridescent colours like the ones spotted here by Paolo Bardelli (Member 45,963) over Sumirago, Lombardy, Italy.

For this colourful diffraction to happen, the droplets or ice crystals in the cloud must be small – very small. They need to be somewhere around 1–50 microns wide. How large is a micron? For comparison, the edge of a piece of paper is usually around 50–100 microns. Not only must the droplets or crystals be tiny to produce the iridescence hues, they also need to be all of a very uniform size since too much variation in their dimensions will mean the effect of each on the sunlight won’t combine. Finally, the cloud also needs to be thin enough for the light to shine through brightly, without being too diffused by the cloud. The most everyday place to spot iridescence is around the fraying edges of a Cumulus cloud, where its droplets are dissipating away. It is at its most dramatic, however, in thin Altocumulus clouds like the ones spotted by Paulo.

Look for iridescence when a fine layer of cloud passes in front of the Sun, but be careful to shield your eyes so they aren’t damaged by the dazzling rays. It is sensible to use sunglasses when you are iridescence spotting. If they have polarised lenses, they’ll also make the colours more apparent. You can try obscuring the Sun with your hand, a tree, or a lamppost. Or if you’re lucky, like Paolo, you might find a patch of Stratocumulus cloud helpfully provides the shade.

Iridescence spotted in Altocumulus over Sumirago, Italy by Paolo Bardelli (Member 45,963). View it in the Photo Gallery.

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