Cloud iridescence oririsation is acolorfuloptical phenomenon that occurs in acloud and appears in the general proximity of theSun orMoon. The colors resemble those seen insoap bubbles and oil on a water surface. It is a type ofphotometeor. This fairly common phenomenon is most often observed inaltocumulus,[1]cirrocumulus,lenticular,[2] andcirrus clouds.[3][4][5] They sometimes appear as bands parallel to the edge of the clouds. Iridescence is also seen in the much rarerpolar stratospheric clouds, also called nacreous clouds.[6]
The colors are usuallypastel, but can be veryvivid or mingled together, sometimes similar tomother-of-pearl.[7] When appearing near the Sun, the effect can be difficult to spot as it is drowned in the Sun's glare. This may be overcome by shielding thesunlight with one's hand or hiding it behind a tree or building. Other aids are dark glasses, or observing the sky reflected in aconvex mirror or in a pool of water.
Irisations are named after the Greek goddessIris, goddess ofrainbows and messenger ofZeus andHera to the mortals below.[8]
Iridescent clouds are adiffraction phenomenon caused by smallwater droplets or smallice crystals individuallyscattering light. Larger ice crystals do not produce iridescence, but can causehalos, a different phenomenon.[9]
Irisation is caused by very uniform water dropletsdiffracting light (within 10 degrees from theSun) and by first orderinterference effects[10] (beyond about 10 degrees from the Sun). It can extend up to 40 degrees from the Sun.[11]
If parts of clouds contain small water droplets or ice crystals of similar size, their cumulative effect is seen as colors. The cloud must be optically thin, so that most rays encounter only a single droplet. Iridescence is therefore mostly seen at cloud edges or in semi-transparent clouds, while newly forming clouds produce the brightest and most colorful iridescence. When the particles in a thin cloud are very similar in size over a large extent, the iridescence takes on the structured form of acorona, a bright circular disk around the Sun or Moon surrounded by one or more colored rings.[9][12][13]
