Just when you think you’ve seen all the different cloud types, you come across this: cloud iridescence!  That’s what Jim saw from his yard in Bathurst earlier this week.  He wondered what it was.

Cloud iridescence is a fairly uncommon phenomenon most often observed in altocumulus, cirrocumulus and cirrus clouds.  It’s the occurrence of colors in the cloud; you might say the colours are similar to those seen in oil films on puddles.  

When parts of clouds are thin and have similar size droplets, diffraction can make them shine with colours like a corona.  In fact, the colours are essentially corona fragments. The effect is called cloud iridescence or irisation, terms derived from Iris the Greek personification of the rainbow.

The usually delicate colours can be in almost random patches or bands at cloud edges. They are only organised into coronal rings when the droplet size is uniform right across the cloud. The bands and colours change or come and go as the cloud evolves.  Iridescence is seen mostly when part of a cloud is forming because then all the droplets have a similar history and consequently have a similar size.

The colors are usually pastel, but can be very vivid. Iridescence is generally produced near the sun, with the sun's glare masking it, so it is more easily seen by hiding the sun behind a tree or building.

Again, you don’t know what you’re missing when you don’t look up!

 

Chief Meteorologist

Cindy