What causes rainbow clouds?

Last week, pictures of an unusually-shaped rainbow cloud that appeared over China were widely shared

Last week, pictures of an unusually-shaped rainbow cloud that appeared over China were widely shared on social media. The cloud in question resembles a pileus cloud, and the phenomenon of bright colours appearing on a cloud is called cloud iridescence.

A pileus cloud is usually formed over a cumulus or cumulonimbus cloud. It is formed when the base cloud pushes a moist current of air upwards and the water vapour from the current condenses to somewhat resemble wave-like crests, or umbrellas.

In his book The Cloudspotter’s Guide, Gavin Pretor-Pinney classifies pileus as an “accessory cloud” that is “rather like a cloud haircut”.

A pileus cloud is transient in nature and lasts barely for a few minutes, making it difficult, and at the same time, exciting, to spot.

Cloud iridescence or Irisation is an optical phenomenon that mostly occurs in wave-like clouds, including pileus and Altocumulus lenticularis. Iridescence in clouds means the appearance of colours on clouds, which can either be in the form of parallel bands like in a rainbow, or mingled in patches.

In ancient Greek mythology, Iris is the goddess of rainbow. “Irisation”, the phenomenon of rainbow-like colours in clouds, is derived from her name.

Iridescence of clouds is a photometeor – an optical phenomenon produced by the reflection, refraction, diffraction or interference of sunlight.