What causes Rainbow Clouds (Cloud Iridescence)?

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.

UPSC 2023 countdown has begun! Get your personal guidance plan now! (Click here)

Get an IAS/IPS ranker as your 1: 1 personal mentor for UPSC 2024

Attend Now