Their dimensions are of the order of 10−3−10−2 m, and their lifetime is of the order of 10−4−10−3 s. The starting point in this scenario is the occurrence of electron avalanches in the vicinity of colliding hydrometeors, leading to the formation of ion production centers.
doi: 10.1364/AO.50.0000F6.We propose a scenario in which elevated ionic conductivity regions (EICRs) with dimensions of the order of 0.1–1 m are formed in the turbulent thundercloud environment. "Icy wave-cloud lunar corona and cirrus iridescence". ^ Color and Light in Nature By David K.^ The Cloudspotter's Guide By Gavin Pretor-Pinney,.^ PHOTOMETEORS, by Jesús Martínez-Frías Archived at the Wayback Machine."Cirrus cloud iridescence: a rare case study". ^ Gedzelman, Stanley David (1 June 1988).When the particles in a thin cloud are very similar in size over a large extent, the iridescence takes on the structured form of a corona, a bright circular disk around the Sun or Moon surrounded by one or more colored rings.
Iridescence is therefore mostly seen at cloud edges or in semi- transparent clouds, while newly forming clouds produce the brightest and most colorful iridescence.
The cloud must be optically thin, so that most rays encounter only a single droplet. If parts of clouds contain small water droplets or ice crystals of similar size, their cumulative effect is seen as colors. It can extend up to 40 degrees from the Sun. Irisation is caused by very uniform water droplets diffracting light (within 10 degrees from the Sun) and by first order interference effects (beyond about 10 degrees from the Sun). Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon. Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. Irisations are named after the Greek goddess Iris, goddess of rainbows and messenger of Zeus and Hera to the mortals below. Other aids are dark glasses, or observing the sky reflected in a convex mirror or in a pool of water. This may be overcome by shielding the sunlight with one's hand or hiding it behind a tree or building. When appearing near the Sun, the effect can be difficult to spot as it is drowned in the Sun's glare. The colors are usually pastel, but can be very vivid or mingled together, sometimes similar to mother-of-pearl. Iridescence is also seen in the much rarer polar stratospheric clouds, also called nacreous clouds. They sometimes appear as bands parallel to the edge of the clouds. This fairly common phenomenon is most often observed in altocumulus, cirrocumulus, lenticular, and cirrus clouds. The colors resemble those seen in soap bubbles and oil on a water surface. Cloud iridescence over County Kildare, IrelandĬloud iridescence or irisation is a colorful optical phenomenon that occurs in a cloud and appears in the general proximity of the Sun or Moon.
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