Science for All | What causes rainbow clouds?
The Hindu
The Hindu’s weekly Science for All newsletter explains all things Science, without the jargon.
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.
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