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What causes Rainbow Clouds (Cloud Iridescence)?

What causes Rainbow Clouds (Cloud Iridescence)?

note4studentFrom UPSC perspective , the follow things is are are important :prelims level : Cloud iridescenceMains level: NA Last week, pictures of

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note4student

From UPSC perspective , the follow things is are 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 is is ?

  • 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 form when the base cloud push a moist current of air upwards and the water vapour from the current condense to somewhat resemble wave – like crest , or umbrella .
  • In popular western culture , it is call 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 is is an optical phenomenon produce 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 fall on them .
  • The thinness is ensures of the cloud ensure 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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