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.