New Delhi, Aug. 29 — Residents of China’s Haikou city were left stunned when they spotted a unique phenomenon in the sky above them: a “rainbow”, but in the shape of a cloud. However, the phenomenon was not a rainbow but, in fact, a cloud.

Pileus cloud

What was seen in the skies of Haikou on August 21 is called a “Pileus cloud,” also known as a “cap cloud” and “scarf cloud.

According to The Weather Network, a pileus is formed when the rapidly rising air in the updraft of a towering cumulus cloud (also called a cumulonimbus) pushes against the cooler air above it.

This causes condensation of the moisture right along the top of the updraft, leading to the formation of the Pileus.

Rainbow formation

When the sunlight is at the right angle, diffraction of light takes place between the droplets and ice crystals in the cloud, giving the Pileus its colours, which resemble those in a rainbow.

However, such clouds generally have a very short life. This is because these get ‘eaten up’ by the cloud on top of which they appear; the lower cloud rises up through convection to absorb Pileus.

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