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Cloud physics
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== Cloud classification == {{main|List of cloud types}} Clouds in the [[troposphere]], the atmospheric layer closest to Earth, are classified according to the height at which they are found, and their shape or appearance.<ref>{{cite web |author=Sirvatka, P. |title=Cloud Physics: Types of Clouds |website=College of DuPage Weather Lab |url=http://weather.cod.edu/sirvatka/cloudtypes.html}}</ref> There are five ''forms'' based on physical structure and process of formation.<ref name="LANDSAT identification">{{cite web | url =https://ntrs.nasa.gov/search.jsp?R=19760014556| title=The identification of cloud types in LANDSAT MSS images |author1=E.C. Barrett |author2=C.K. Grant | year=1976 | access-date=22 August 2012 | publisher = [[NASA]]}}</ref> ''Cirriform'' clouds are high, thin and wispy, and are seen most extensively along the leading edges of organized weather disturbances. ''Stratiform'' clouds are non-convective and appear as extensive sheet-like layers, ranging from thin to very thick with considerable vertical development. They are mostly the product of large-scale lifting of stable air. Unstable free-convective ''cumuliform'' clouds are formed mostly into localized heaps. ''Stratocumuliform'' clouds of limited convection show a mix of cumuliform and stratiform characteristics which appear in the form of rolls or ripples. Highly convective ''cumulonimbiform'' clouds have complex structures often including cirriform tops and stratocumuliform accessory clouds.{{fact|date=January 2018}} These forms are cross-classified by altitude range or ''level'' into ten ''genus'' types which can be subdivided into species and lesser types. High-level clouds form at altitudes of 5 to 12 kilometers. All cirriform clouds are classified as high-level and therefore constitute a single cloud genus [[cirrus cloud|cirrus]]. Stratiform and stratocumuliform clouds in the high level of the troposphere have the prefix ''cirro-'' added to their names yielding the genera [[cirrostratus cloud|cirrostratus]] and [[cirrocumulus cloud|cirrocumulus]]. Similar clouds found in the middle level (altitude range 2 to 7 kilometers) carry the prefix ''alto-'' resulting in the genus names [[altostratus cloud|altostratus]] and [[altocumulus cloud|altocumulus]].<ref name="Definitions">{{cite web|editor=[[World Meteorological Organization]]|title=Definitions, International Cloud Atlas|year=2017|url=https://www.wmocloudatlas.org/clouds-definitions.html|access-date=30 March 2017|archive-url=https://web.archive.org/web/20170327221058/https://www.wmocloudatlas.org/clouds-definitions.html|archive-date=27 March 2017|url-status=dead}}</ref> Low level clouds have no height-related prefixes, so stratiform and stratocumuliform clouds based around 2 kilometres or lower are known simply as [[stratus cloud|stratus]] and [[stratocumulus cloud|stratocumulus]]. Small [[cumulus cloud|cumulus]] clouds with little vertical development (species humilis) are also commonly classified as low level.<ref name="Definitions"/> Cumuliform and cumulonimbiform heaps and deep stratiform layers often occupy at least two tropospheric levels, and the largest or deepest of these can occupy all three levels. They may be classified as low or mid-level, but are also commonly classified or characterized as vertical or multi-level. [[Nimbostratus cloud|Nimbostratus]] clouds are stratiform layers with sufficient vertical extent to produce significant precipitation. Towering cumulus (species congestus), and [[cumulonimbus cloud|cumulonimbus]] may form anywhere from near the surface to intermediate heights of around 3 kilometres. Of the vertically developed clouds, the cumulonimbus type is the tallest and can virtually span the entire troposphere from a few hundred metres above the ground up to the tropopause.<ref name="Definitions"/> It is the cloud responsible for thunderstorms. Some clouds can form at very high to extreme levels above the troposphere, mostly above the polar regions of Earth. [[Polar stratospheric cloud]]s are seen but rarely in winter at altitudes of 18 to 30 kilometers, while in summer, [[noctilucent]] clouds occasionally form at high latitudes at an altitude range of 76 to 85 kilometers.<ref name=Hsu>{{cite news |last=Hsu |first=Jeremy |title=Strange clouds spotted at the edge of Earth's atmosphere |date=2008-09-03 |work=[[USA Today]] |url=https://www.usatoday.com/tech/science/space/2008-09-02-strange-clouds-space_N.htm}}</ref> These polar clouds show some of the same forms as seen lower in the troposphere. '''Homospheric types determined by cross-classification of forms and levels'''. {| class="wikitable" !Forms and levels !! Stratiform <br> non-convective !! Cirriform <br> mostly non-convective !! Stratocumuliform <br> limited-convective !! Cumuliform <br> free-convective !! Cumulonimbiform <br> strong-convective |- !Extreme level | [[Polar mesospheric cloud|PMC]]: [[Noctilucent cloud|Noctilucent]] veils || Noctilucent billow or whirls || Noctilucent bands || | |- !Very high level | [[Nitric acid]] & [[water]] [[Polar stratospheric cloud|PSC]] ||[[Cirrus cloud|Cirriform]] [[nacreous]] [[Polar stratospheric cloud|PSC]] ||[[Lenticular cloud|Lenticular]] [[nacreous]] [[Polar stratospheric cloud|PSC]] || || |- !High-level | [[Cirrostratus cloud|Cirrostratus]] ||[[Cirrus cloud|Cirrus]] || [[Cirrocumulus cloud|Cirrocumulus]] || || |- !Mid-level | [[Altostratus cloud|Altostratus]] || || [[Altocumulus cloud|Altocumulus]] || || |- !Low-level | [[Stratus cloud|Stratus]] || || [[Stratocumulus cloud|Stratocumulus]] || [[Cumulus cloud|Cumulus humilis]] or [[fractus]] || |- !Multi-level or moderate vertical | [[Nimbostratus cloud|Nimbostratus]] || || || [[Cumulus mediocris]] || |- !Towering vertical | || || || [[Cumulus congestus]] || [[Cumulonimbus cloud|Cumulonimbus]] |} '' Homospheric types include the ten tropospheric genera and several additional major types above the troposphere. The cumulus genus includes four species that indicate vertical size and structure.
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