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Iridescent Clouds imaged by Antonino Gumina, Milan, Italy. ©Antonino Gumina, shown with permission.

All very small cloud droplets and ice crystals scatter light waves to produce colours at particular angles from the sun.

Why then are clouds white or at least not full of iridescent colours?

In thick clouds most sunlight is scattered by many droplets. The angular information is lost and the colours mix to white.

Most mature clouds have a range of droplets sizes and, again, the colours are smeared out to white.

But when clouds are freshly formed and thin their droplets have similar sizes and many sun rays only interact with one drop. Then we see the iridescent colours that tiny drops and crystals always produce.