Editing Cloud (section)

== Mesospheric classification and distribution ==
[[File:Helkivad ööpilved Kuresoo kohal.jpg|thumb|right|[[Noctilucent cloud]] over Estonia]]

[[Noctilucent cloud]]s are the highest in the atmosphere and are found near the top of the mesosphere at about {{convert|80|to|85|km|abbr=on}} or roughly ten times the altitude of tropospheric high clouds.<ref name="autogenerated1">{{Cite book |last1=Michael Gadsden |url=http://www.iugg.org/IAGA/iaga_pages/pdf/ONC_Sep06.pdf |title=Observing Noctilucent Clouds |last2=Pekka Parviainen |date=September 2006 |publisher=International Association of Geomagnetism & Aeronomy |page=9 |access-date=31 January 2011 |archive-date=31 October 2008 |archive-url=https://web.archive.org/web/20081031003710/http://www.iugg.org/IAGA/iaga_pages/pdf/ONC_Sep06.pdf |url-status=dead }}</ref>  They are given this Latin derived name because of their illumination well after sunset and before sunrise. They typically have a bluish or silvery white coloration that can resemble brightly illuminated cirrus. Noctilucent clouds may occasionally take on more of a red or orange hue.<ref name="Polar-stratospheric" /> They are not common or widespread enough to have a significant effect on climate.<ref name="simulation studies" /> However, an increasing frequency of occurrence of noctilucent clouds since the 19th century may be the result of climate change.<ref name="noctilucent-cloud">{{Cite web |year=2017 |editor-last=Project Possum |title=About Noctiluent Clouds |url=http://projectpossum.org/research/noctilucent-cloud/about-noctilucent-clouds/ |access-date=6 April 2018}}</ref>

Ongoing research indicates that convective lift in the mesosphere is strong enough during the polar [[summer]] to cause adiabatic cooling of small amount of water vapor to the point of saturation.  This tends to produce the coldest temperatures in the entire atmosphere just below the mesopause.<ref name="simulation studies">{{Cite journal |last1=Turco |first1=R. P. |last2=Toon |first2=O. B. |last3=Whitten |first3=R. C. |last4=Keesee |first4=R. G. |last5=Hollenbach |first5=D. |year=1982 |title=Noctilucent clouds: Simulation studies of their genesis, properties and global influences |journal=Planetary and Space Science |volume=30 |issue=11 |pages=1147–1181 |bibcode=1982P&SS...30.1147T |doi=10.1016/0032-0633(82)90126-X}}</ref> There is evidence that smoke particles from burnt-up meteors provide much of the condensation nuclei required for the formation of noctilucent cloud.<ref name="sounding rocket">{{Cite web |last=Fox, Karen C. |year=2013 |title=NASA Sounding Rocket Observes the Seeds of Noctilucent Clouds |url=http://www.nasa.gov/mission_pages/sunearth/news/CHAMPS-NLCs.html#.UkqY0BAxIgq |access-date=1 October 2013 |archive-date=24 September 2013 |archive-url=https://web.archive.org/web/20130924233857/http://www.nasa.gov/mission_pages/sunearth/news/CHAMPS-NLCs.html#.UkqY0BAxIgq |url-status=dead }}</ref>

Noctilucent clouds have four major types based on physical structure and appearance. Type I veils are very tenuous and lack well-defined structure, somewhat like cirrostratus fibratus or poorly defined cirrus.<ref name="veils">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Type I Veils, International Cloud Atlas |url=https://cloudatlas.wmo.int/noctilucent-clouds-type-I-veils.html |access-date=18 July 2019}}</ref> Type II bands are long streaks that often occur in groups arranged roughly parallel to each other. They are usually more widely spaced than the bands or elements seen with cirrocumulus clouds.<ref name="bands">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Type II Bands, International Cloud Atlas |url=https://cloudatlas.wmo.int/noctilucent-clouds-type-II-bands.html |access-date=18 July 2019}}</ref> Type III billows are arrangements of closely spaced, roughly parallel short streaks that mostly resemble cirrus.<ref name="billows">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Type III Billows, International Cloud Atlas |url=https://cloudatlas.wmo.int/noctilucent-clouds-type-III-billows.html |access-date=18 July 2019}}</ref> Type IV whirls are partial or, more rarely, complete rings of cloud with dark centers.<ref name="whirls">{{Cite web |year=2017 |editor-last=World Meteorological Organization |title=Type IV Whirls, International Cloud Atlas |url=https://cloudatlas.wmo.int/noctilucent-clouds-type-IV-whirls.html |access-date=18 July 2019}}</ref>

Distribution in the mesosphere is similar to the stratosphere except at much higher altitudes. Because of the need for maximum cooling of the water vapor to produce noctilucent clouds, their distribution tends to be restricted to polar regions of Earth.  Sightings are rare more than 45 degrees south of the North Pole or north of the South Pole.<ref name="Polar-stratospheric" />

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