Cloud iridescence

Cloud iridescence

Cloud iridescence or irisationis the occurrence of colors in a cloud not dissimilar to those seen in oil films on puddles. 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 od water.

Iridescence is a diffraction phenomenon. Small water droplets or even small ice crystals in clouds individually scatter light. If parts of the clouds have droplets (or crystals) of similar size the cumulative effect is seen as colors. The cloud must be opticall 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. They are formed from small water droplets of near uniform size. When the sun is properly positioned, mostly behind thick clouds, these thin clouds almost coherently diffract sunlight, and as different wavelengths are diffracted different amounts the effect is rainbow-like bands of color in the cloud.

Any cloud thin enough to be translucent can show iridescence, though the effect is especially clear with stratospheric nacreous clouds, which are concequentally known as "mother of pearl clouds"

See also

• Irisation

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External links

• Wisc.edu site with much explanation
• Astronomy Picture of the Day, over Boulder, Colorado
• Astronomy Picture of the Day, over France
• Answers.com on iridescent clouds