Clouds come in all shapes and sizes, and just when you think you’ve seen them all you come across this: cloud iridescence! That’s what Terry MacDonald saw June 4 high in the sky over Amherst, N.S. He wondered what it was.
Cloud iridescence is a fairly rare phenomenon usually seen in altocumulus or high cirrus-type clouds. The colours in the cloud are similar to those seen in oil films on puddles after a rain. The lovely display can also be described as cloud irisation; that term comes from Iris, the Greek personification of the rainbow.
Here’s what happens:
When parts of clouds are thin and have similar-sized droplets, diffraction can make them shine with stunning colours. These colours are usually found in random patches or bands. Iridescence occurs in an area of the cloud where all the droplets have a similar history and consequently have a similar size.
Because these pretty pastel patches are usually found in the vicinity of the sun, they’re often hard to see. It’s never a good idea to stare directly into the sun, so the best way to see an iridescent cloud is to place the sun behind a foreground object, like a building.
Look up, but always remember to protect your eyes!
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Cindy Day is the chief meteorologist for SaltWire Network.