Cloud iridescence

Cloud iridescence or irisation is the occurrence of colors in a cloud not dissimilar to those seen in oil films on puddles. It is fairly common. The colors are usually pastel and need searching for but sometimes they can be very vivid. Iridescence is most frequent near to the sun and the glare masks it. It is most easily seen by hiding the sun behind a tree or building. Other aids are dark glasses or observing the sky by its reflection in a convex mirror or in a pool of water.

Iridescence is a diffraction phenomenon. Small water droplets or even small ice crystals in clouds individually scatter light. Large ice crystals produce halos - the latter are refraction phenomena not iridescence. Iridescence should similarly be distinguished from the refraction in larger raindrops that gives a rainbow.

If parts of the clouds have droplets (or crystals) of similar size the 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. Newly forming clouds produce the brightest and most colorful iridescence because their droplets are of the same size. When a thin cloud has droplets of similar size over a large extent the iridescence takes on a structured form to give a corona - a central bright disk around the sun or moon surrounded by one or more colored rings.

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