cloud iridescence
{{Short description|Optical phenomenon}}
File:Highly_iridising_altocumulus.jpg
File:Iridescent_cloud_at_sunset.jpg at sunset over Aberdeen, Scotland]]
File:Rainbow cloud 1.jpg, India]]
Cloud iridescence or irisation is a colorful optical phenomenon that occurs in a cloud and appears in the general proximity of the Sun or Moon. The colors resemble those seen in soap bubbles and oil on a water surface. It is a type of photometeor. This fairly common phenomenon is most often observed in altocumulus,{{cite journal |last=Gedzelman |first=Stanley David |title=In Praise of Altocumulus |journal=Weatherwise |date=1 June 1988 |volume=41 |issue=3 |pages=143–149 |doi=10.1080/00431672.1988.9930533}} cirrocumulus, lenticular,[http://www.answers.com/topic/irisation-meteorology Answers.com – Sci-Tech Dictionary: irisation] and cirrus clouds.{{cite APOD |date=25 November 2007 |title=An Iridescent Cloud Over Colorado |accessdate=}}{{cite web |url= http://www.atoptics.co.uk/droplets/irid1.htm |title=Iridescent Clouds |publisher= Atmospheric Optics}}{{cite journal |last=Sassen |first=Kenneth |title= Cirrus cloud iridescence: a rare case study |journal= Applied Optics |date=1 January 2003 |volume=42 |issue=3 |pages=486–491 |doi= 10.1364/AO.42.000486 |pmid=12570270 |bibcode= 2003ApOpt..42..486S}} They sometimes appear as bands parallel to the edge of the clouds. Iridescence is also seen in the much rarer polar stratospheric clouds, also called nacreous clouds.{{cite web |url= https://www.atoptics.co.uk/highsky/nacr1.htm |title= Nacreous Clouds |publisher=Atmospheric Optics}}
The colors are usually pastel, but can be very vivid or mingled together, sometimes similar to mother-of-pearl.[http://tierra.rediris.es/megacryometeors/photometeors.pdf PHOTOMETEORS, by Jesús Martínez-Frías] {{webarchive|url=https://web.archive.org/web/20091213022627/http://tierra.rediris.es/megacryometeors/photometeors.pdf |date=2009-12-13 }} 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 the sunlight with one's hand or hiding it behind a tree or building. Other aids are dark glasses, or observing the sky reflected in a convex mirror or in a pool of water.
Etymology
Irisations are named after the Greek goddess Iris, goddess of rainbows and messenger of Zeus and Hera to the mortals below.[https://books.google.com/books?id=MAiKY4xU6y8C&pg=PA233&dq=Cloud+iridescence+Irisation&lr= The Cloudspotter's Guide By Gavin Pretor-Pinney, p. 233]
Mechanism
Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon.{{Cite web |title=Cloud Iridescence {{!}} SKYbrary Aviation Safety |url=https://skybrary.aero/articles/cloud-iridescence |access-date=2022-08-19 |website=skybrary.aero}}
Irisation is caused by very uniform water droplets diffracting light (within 10 degrees from the Sun) and by first order interference effects[https://books.google.com/books?id=4Abp5FdhskAC&pg=PA133&dq=Cloud+iridescence+Irisation&lr= Color and Light in Nature By David K. Lynch, William Charles Livingston, p. 133] (beyond about 10 degrees from the Sun). It can extend up to 40 degrees from the Sun.[http://tierra.rediris.es/megacryometeors/photometeors.pdf PHOTOMETEORS, by Jesús Martínez-Frías] {{webarchive|url=https://web.archive.org/web/20091213022627/http://tierra.rediris.es/megacryometeors/photometeors.pdf |date=2009-12-13 }}
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 a corona, a bright circular disk around the Sun or Moon surrounded by one or more colored rings.{{cite web |url=http://www.atoptics.co.uk/droplets/corona.htm |title=Corona |publisher=Atmospheric Optics}}{{cite journal |last1=Shaw |first1=Joseph A. |last2=Pust |first2= Nathan |title= Icy wave-cloud lunar corona and cirrus iridescence |journal=Applied Optics |date=12 August 2011 |volume=50 |issue=28 |page=F6 |doi= 10.1364/AO.50.0000F6 |pmid=22016246 |bibcode= 2011ApOpt..50F...6S}}
Gallery
File:CircumHorizontalArc GQ 07312012 1938.jpg
File:Viet Nam.jpg|Ho Chi Minh City, Viet Nam. May 12th, 2024
File:IridescentCloudArran.jpg
File:Iridescent Cloud.JPG
File:Cloud iridescence 3.jpg
File:CloudOberGatlinburg.jpg
File:Cloud iridescence at Kokkola, Finland.jpg
File:IrisationCloudsPICT4884crop640x480.jpg|Irisation in clouds over Wellington NZ
File:Cloud Iridescence and Crepuscular Rays.jpg|Vibrant cloud iridescence captured before sunset
File:Iridescent cloud edge.jpg
File:At Teide Observatory 2019 027.jpg|Iridescent clouds seen in Tenerife
File:Cloud-iridescence-Parker-Canyon-AZ-Aug-6-2022.jpg|Parker Canyon, AZ. Aug 6th, 2022
See also
{{Portal|Weather}}
References
{{reflist}}
External links
{{Commons category|Iridescent clouds}}
{{Wiktionary|iridescent cloud}}
- [http://www.atoptics.co.uk/droplets/iridim0.htm Iridescent cloud gallery – Atmospheric Optics site]
- [http://chestofbooks.com/crafts/scientific-american/sup5/On-The-Cause-Of-Iridescence-In-Clouds.html On the Cause of Iridescence in Clouds] – Scientific American Supplement