Rainbow-colored patches in clouds are usually caused by cloud iridescence: sunlight interacting with tiny water droplets or ice crystals of similar size. Unlike a familiar large rainbow seen across the sky after rain, these pastel patches often form along the thin edges of clouds, with pink, green or pale blue bands that can shift as the cloud evolves.
What causes cloud iridescence?
Sunlight contains many colors. As light passes around or through sufficiently small, similar-sized cloud particles, the light waves interact. Diffraction and interference can separate colors into bands or small patches. The World Meteorological Organization notes that diffraction dominates very close to the Sun, while interference becomes more important at some larger viewing angles.
Why don’t all clouds have rainbow colors?
In ordinary thick clouds, a broad mixture of droplets and numerous layers tend to scatter colors together, giving the cloud a white or gray appearance. Iridescence is more likely when the cloud is thin and contains droplets or ice particles of fairly similar size. Young clouds or thin patches near their edges can show the effect especially clearly.
How is this different from a normal rainbow?
A classic rainbow appears when sunlight refracts and reflects inside raindrops, generally seen opposite the Sun. Cloud iridescence, by contrast, tends to appear closer to the Sun in irregular patches or color bands associated with thin cloud material. You are not looking at an arc formed in a distant rain shower.
What clouds can show iridescence?
NOAA describes iridescence in certain thin altocumulus, cirrocumulus, cirrus and lenticular clouds. It can occur in other suitable thin cloud forms too. The size and distribution of droplets matter more than a cloud’s name alone.
Is this the same as a nacreous cloud?
No. Nacreous clouds are rare, high-altitude polar stratospheric ice clouds often seen at twilight. Ordinary cloud iridescence is a separate optical appearance that can occur in much lower cloud layers and many locations. Both may look pearly, but their physical environments differ substantially.
How can I photograph rainbow clouds safely?
Use a building, tree or another solid object to block the direct solar disk while framing the cloud. Never stare at the bright Sun through a camera viewfinder or binoculars. A phone camera can capture the colored cloud edge without pointing directly into the Sun; adjust exposure to preserve subtle color.
Frequently asked questions
Does a rainbow cloud predict rain?
No. Iridescence indicates favorable lighting and small cloud-particle conditions, not a particular rain forecast.
Can it occur around the Moon?
Related diffraction effects can occur in moonlit clouds, although the color is often harder to see because moonlight is weaker.
Are rainbow clouds dangerous?
Cloud iridescence itself is not a hazard. Bright sunlight and any actual local thunderstorm are separate safety considerations.
Official meteorological sources
WMO International Cloud Atlas: Iridescence and NOAA: What Are Rainbow Clouds?.
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