Cloud of the Day – Irisation
A close relative of corona is irisation. Both are caused by interference among light waves diffracted by cloud particles. Since different wavelengths of light are scattered at different angles, they interfere with themselves at different distances from the light source. This causes the colors to be separated out, creating the beautiful iridescent irisation. The differential scattering of wavelengths, shorter being more easily scattered, also leads to blue skies, as much of the short wavelength blue in sunlight is scattered, and red sunsets, as more long wavelength red light makes it through.
Irisation is the name of the meteorological phenomenon, and it’s caused by iridescence. Both words have their root in the Greek word iris, or rainbow, derived from the Greek god(dess) of the rainbow: Iris. Iridescence can be found in soap bubbles, bird feathers and seashells. In the case of irisation, it’s found in the clouds.
Since it occurs so close to the Sun, irisation is often lost in the glare. You can improve your chances of seeing it by wearing sunglasses, or by physically blocking out the Sun. It occurs in many different types of clouds, generally in the middle and high etages. Any time you see bright white clouds close to the Sun, you have a good chance of seeing it.
And one from Petr Hykš, as kindly offered in the comments below.
Irisation is not reliably predictive of weather.
Note: All pictures link to their larger originals.