Dubliners have been treated to an amazing sight the last two mornings: rainbow clouds, just before dawn. Their delicate beauty captures the imagination, but how do they form?
Clouds are made up of tiny droplets of water, or in the winter, ice. So clouds can spread sunlight into different colours, the same way you’d see different colours in a rainbow, when light passes through water droplets in the air. This happens due to an optical process called diffraction: when light passes from air into a medium like water, it bends because the properties of water are different than the properties of air for an electromagnetic wave like light which is passing through. Different colours of light bend different amounts, giving us the splaying of colours as if from a prism. And to see this effect, you’re best off looking in pre-dawn or twilight, when the sun is just below the horizon and lighting up clouds but not yet the ground.
The diffraction of light to make rainbows in clouds is most common in winter or very cold conditions, with clouds so thin and frozen they can almost act like a lens for sunlight. Light then diffracts through the frozen crystals, creating colourful interference fringes. These conditions are more easily met by clouds which are very high up in the stratosphere, because the stratosphere is very dry and rarely supports clouds, but the clouds which do form there are likely to be thin and wispy. Polar stratospheric clouds occur in the far north and south of the globe, where ice crystals in clouds are common, and are also called ‘nacreous clouds’ for their resemblance to mother-of-pearl. The chemical composition of the cloud can affect which colours it shows, but even clouds of pure ice will show diffraction if they are high and thin enough.
Another even rarer winter sky phenomenon to keep an eye out for is ‘sun dogs’. Diffraction in ice crystals in clouds can bend the light from the sun so much that it appears to be in multiple places at once!
If you’d like to read more on light and diffraction, check out our archive post on why the sky is blue.