Why in news?

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.

What is a pileus cloud?

• 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.

What is cloud iridescence?

• 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.

What is a photometeor?

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

What causes cloud iridescence?

• In pileus clouds, small water droplets or ice crystals, usually of a similar size, diffract the sunlight falling on them. The thinness of the cloud ensures more exposure to sunlight for each water droplet or ice crystal.
• To ensure its wave crest-like appearance, water droplets or ice crystals in these clouds are always moving – droplets form at one side of the cloud and evaporate from the other end – and hence these clouds remain small and thin since the droplets have no way of combining and growing in size.

• In its International Cloud Atlas, the World Meteorological Organisation says that iridescence or Irisation is caused by diffraction within 10 degrees from the sun. Beyond ten degrees and up till about 40 degrees, interference of light is the main cause of iridescence.