A photograph of an iridescent pileus cloud aka a Rainbow Cloud was posted to the NASA site on September 6, 2022, as the “Astronomy Picture of the Day.”

Irisations are named after the mythological Greek goddess Iris. She was the goddess of rainbows, who also served Zeus as a messenger. 

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The featured image was taken in Pu'er, Yunnan Province, China. The rare image captured a very unusual phenomenon. Cloud iridescence or irisation appears as a colorful cloud in the general proximity of the Sun or Moon.

Cloud iridescence or irisation is an optical phenomenon. The colors resemble those seen in soap bubbles and oil on a water surface. They sometimes appear as parallel bands like a rainbow.

A pileus iridescent cloud is created from water droplets or ice that combine together to diffract different colors.

The colors usually appear as pastels. They can also be more vivid or muted. 

When appearing near the Sun, the effect can be drowned out by the Sun's glare. You may need to wear dark glasses to observe the sky event.

The iridescent colors that appear in pileus are diffracted sunlight. 

How a rainbow cloud is created

The formation of a pileus cloud capping a common cumulus cloud indicates that the lower cloud is expanding upward. 

These unique clouds contain warm air and water droplets or ice crystals. When the air boils it convects upwards. 

Layers of air above the cloud are also pushed upwards as the cumulus rises.

Rising air expands and adiabatically cools. This means it can cool in a curve similar to the arch of a rainbow or a circle.

 Pileus clouds don’t appear for long periods of time. The main cloud beneath them is rising through convection to absorb them.

The appearance of this cloud system means there is a high probability that a storm might develop.