Cloud iridescence

Cloud iridescence is the appearance of pastel or vivid colors in a cloud, often in irregular patches or bands near the Sun. The effect is also called irisation. Unlike a typical rainbow, which is produced by refraction and reflection in raindrops at a particular viewing geometry, cloud iridescence is associated with diffraction of sunlight by small cloud particles. It can occur in several cloud types, including altocumulus, cirrocumulus, lenticular and cirrus clouds.

When light encounters tiny water droplets or ice crystals, it is diffracted—bent and spread around the particles. Different wavelengths of visible light are diffracted by different amounts, producing colored patterns. These colors are most distinct when the particles are similar in size, so their scattered light produces a more orderly pattern. The National Oceanic and Atmospheric Administration notes that iridescence is favored in thin clouds containing many droplets or crystals of about the same size.

Iridescence is often most visible along thin or forming parts of a cloud, where particles have similar sizes and the cloud does not scatter the light so extensively that the colors blend away. It commonly appears near the Sun, although observers should never look directly at the Sun to see it. It may be easier to spot when the Sun is safely screened by a building or another solid object. Cloud motion and changes in particle sizes can make the colors shift, fade or reappear.

Cloud iridescence and a corona around the Sun or Moon are related diffraction effects, and iridescent cloud colors can be understood as parts of a larger corona pattern. They are distinct from a halo, which is generally formed by refraction through ice crystals. Nacreous, or mother-of-pearl, clouds can also show striking iridescence, but they are a separate, rare type of high-altitude polar stratospheric cloud.