Iridescent clouds: how diffraction separates colours

Clouds with colourful reflections can appear without rain. Their iridescence comes from the diffraction of light by tiny droplets or crystals, a mechanism distinct from those behind rainbows and halos.

Pink, green or blue colours can appear on a cloud without any rain or paint being involved. This phenomenon, known as iridescence, is caused by the way tiny water droplets or ice crystals diffract light. To recognise it, it must be distinguished from two other familiar spectacles in the sky: the rainbow and the halo.

Light separates near droplets

Sunlight appears white, but it contains several colours. In a cloud, when its rays encounter small water droplets or ice crystals, the light can be diffracted: it spreads out and bends near these particles. The different colours do not spread out in the same way. They then separate into hues that can sometimes be seen along the edges or in the thinner areas of the cloud.

Particle size matters. According to New Zealand’s weather service MetService and the US NOAA, iridescence appears more readily when the droplets or crystals are of a similar size, close to the wavelength of visible light. Wavelength is a property of light that varies according to its colour. When the particles are fairly similar in size, their effects can produce perceptible colours; if their sizes differ more, the colours tend to blend together.

The cloud must also let some light through. Thin or developing clouds are therefore particularly favourable: light encounters a collection of particles without the cloud’s thickness completely masking the effect. NOAA names altocumulus, cirrocumulus, lenticular clouds and cirrus among the types in which iridescence can occur. The cloud’s name alone, however, does not guarantee that the phenomenon will appear: transparency and the distribution of the particles also matter.

Iridescence, rainbows and halos: three mechanisms to distinguish

The colours of an iridescent cloud may evoke a rainbow, but the two phenomena do not form in the same way. In a rainbow, light is refracted — bent as it passes from one medium to another —, reflected inside raindrops, and then refracted again as it emerges. The geometry of these drops and the observer’s position relative to the Sun determine the visible arc.

Iridescence, by contrast, is linked to the diffraction of light by small particles in the cloud. It often forms patches of closely related colours along the cloud’s edges, rather than a regular arc. It does not require rain. The hues may be pastel or more pronounced, depending on the conditions and how the particles are distributed.

A solar halo is yet another phenomenon. It generally appears as a ring or arc of light around the Sun and is associated with the refraction of light in ice crystals. Iridescence may occur near the Sun, but its colours are often more irregular and linked to part of a cloud. A single photograph is not always enough to identify the phenomenon with certainty: its shape, position in the sky and the surrounding clouds provide clues.

Why does the spectacle not last?

Several conditions must come together: a sufficiently thin cloud, particles of suitable sizes and favourable geometry between the cloud, sunlight and the observer. Then the cloud moves and changes. Its transparency and the distribution of droplets change, so the colours can appear or fade quickly.

An event reported above the Bambouseraie de Générargues, in the Gard département, illustrates this convergence of conditions. ICI Gard Lozère reported images shared by an observer on Météo Gard and described the phenomenon as iridescence. This local report documents a particular event; the physical explanation, meanwhile, rests on the diffraction mechanisms described by meteorological services. The same colourful appearance should not automatically be attributed to iridescence without taking account of the phenomenon’s shape and context.

Comparing it with a soap bubble can help to picture colourful reflections, but it does not describe exactly how iridescent clouds work. In a bubble, the colours result from interference in a thin layer of liquid: some of the light is reflected at different interfaces in the layer, and the waves reinforce or cancel one another depending on their wavelength. In a cloud, the explanation put forward is diffraction by droplets or crystals. Two phenomena can therefore look similar to the eye without having the same optical cause.

Observe without looking directly at the Sun

As iridescence appears in a cloud lit by the Sun, it may occur in an area close to it. This is no reason to look directly at the Sun: when observing with the naked eye or taking photographs, avoid staring at it. It is better to focus on the coloured parts of the cloud and stop observing if the Sun enters your line of sight.

Iridescence thus reveals a subtle property of light: tiny particles, almost impossible to distinguish individually, can alter how its colours are distributed. It is neither a rainbow without rain nor a simple decorative reflection. It is a diffraction effect, made visible when the cloud’s structure and the geometry of the lighting are favourable.