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   Iridescent Clouds  

  Iridescent clouds Salta Province, Western Argentina captured near sunset by Dallas Rhodes of Georgia USA. Iridescence in clouds most often occurs close to the sun. It is best seen when the sun is hidden. Here it behind the hill at right of centre.  Image ©Dallas D. Rhodes, shown with permission.
 

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. They occur most often in altocumulus, cirrocumulus and especially in lenticular clouds. 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.

Sometimes iridescence can be seen far from the sun but is most frequent near to it. As for coronas, search safely by hiding the sun behind a building and, even better, also viewing the reflection of the sky in water.

Very much rarer iridescence is that of nacreous or mother-of-pearl clouds. They can glow very brightly and are far higher than ordinary tropospheric clouds.
Iridescence is also seen in rocket exhaust trails.