Climate Change Impact on India and World – International Reports, Key Observations, etc.

What causes Rainbow Clouds (Cloud Iridescence)?


From UPSC perspective, the following things are important :

Prelims level: Cloud iridescence

Mains level: NA

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

What is the news?

  • The cloud in question resembles a pileus cloud.
  • Such 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 popular western culture, it is called 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.
  • It is 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.


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