The sky, sometimes, does a very simple and very unfair thing: it seems normal until a second before. A whitish cloud, perhaps frayed after the rain, remains there above the roofs with the look of someone who is just passing by. Then the edge takes color. First a light pink, then a pale green, a cold blue, a barely visible purple stripe. No perfect arc, no postcard scene. More of a soap stain suspended high above, fragile, almost embarrassed by its own beauty.
I am iridescent cloudsalso called rainbow clouds. The phenomenon has a more technical name, cloud iridescence, and arises when sunlight is diffused and separated by passing through tiny droplets of water or small ice crystals present in the cloud layer. NOAA links it to the diffraction of light in clouds such as altocumulus, cirrocumulus, lenticular and cirrus clouds, i.e. formations that are often thin enough to let the sun’s rays filter through without completely extinguishing them.
You need a thin cloud
The key word is diffraction. Light encounters very small particles and is bent, scattered, divided into colors. For the effect to be seen, those drops or crystals must be similar in size. If the cloud contains particles that are too different from each other, the colors mix poorly, lose sharpness, become a confused brightness. If, however, the bench is thin, semi-transparent, perhaps just formed, the rays pass through a few particles at a time and the nuances emerge with greater clarity.
The National Weather Service glossary describes iridescent clouds as bright spots, bands, or edges of color, often red and green, visible within about 30 degrees of the Sun. It associates them primarily with thin cirrostratus, cirrocumulus, and altocumulus. Translated into everyday life: you have to look close to the illuminated area of the sky, without staring at the Sun, looking for those light clouds that seem almost on the verge of falling apart.
After rain can help because it often leaves thinner layers, moist air and sudden openings in the sky. The rain itself, however, only serves as context. The real work is done by the light and the size of the suspended particles. It’s the same reason why iridescence resembles reflections on soap bubbles or oil slicks in water: very small surfaces and particles force light to show colors that usually remain hidden.
The Sun must be filtered
To see them well iridescent cloudsthe Sun must be in a favorable position. It often appears low on the horizon, partially covered by other clouds or shielded by a building, a hill, or a darker edge of the sky. Direct light is too strong and eats away details. The filtered light, on the other hand, lets out those subtle, almost liquid colors.
The Met Office explains a nearby phenomenon, that of coronas, again linked to diffraction: white light separates into colors when it bends around droplets in clouds; the smaller the drops, the larger the visible effect. When the size of the drops varies greatly, the pattern becomes more diffuse and irregular.
Here also lies the difference with other atmospheric shows that are often confused with each other. The classic rainbow arises from sunlight passing through raindrops and being separated into a neat band. Solar halos depend mainly on hexagonal ice crystals in cirrus clouds, with characteristic angles such as 22 degrees. Mother-of-pearl clouds, on the other hand, are very rare polar stratospheric clouds, observable especially before dawn or after sunset, when the Sun is just below the horizon. They have colors similar to iridescence, but belong to another altitude and another atmospheric scenario.
A rare phenomenon, even when it seems close
The iridescent clouds seem almost domestic, because they can appear over a street, a neighborhood, a still wet parking lot. In reality they require a precise fit: fine cloud, minute and fairly uniform particles, nearby but shielded Sun, observer in the right position. This is why they don’t last long. A current of air changes the cloud, the Sun rises or disappears, the drops grow, the colors fade.
It is best to observe them from the side, never looking at the Sun. The edges of the clouds are often the best point. There the bench becomes thinner, the light passes through with fewer obstacles and the color takes shape. Sometimes it’s just a veil. Other times it becomes an almost fluorescent, unreal patch, and it is natural to think of a photographic filter. Instead it’s atmospheric physics, only with a strong flair for special effects.
