View source for Altocumulus cloud

{{Short description|Genus of mid-level cloud}}
{{Infobox cloud
|name=Altocumulus clouds
|image location=Altocumulus.jpg
|image name=Altocumulus cloud formation
|abbreviation=Ac
|symbol=Clouds CM 3.svg
|genus=alto-, meaning high, and cumulo-, meaning heap
| species = * Castellanus
* Floccus
* Lenticularis
* Stratiformis
* Volutus
| variety= * Duplicatus
* Lacunosus
* Opacus
* Perlucidus
* Radiatus
* Translucidus
* Undulatus
|altitude_m=2,000–7,000
|altitude_ft=7,000–23,000
| level = medium
|appearance=Middle-altitude [[Stratocumulus]] clouds arranged in groups or rolls. Has smaller clouds separate from each other
|precipitation=[[Virga]] only
| thickness      = 
| Ice content    = 
}}

'''Altocumulus''' ({{etymology|la|{{Wikt-lang|la|altus}}|high||{{Wikt-lang|la|cumulus}}|heaped}})<ref>{{cite web |url=https://weatheronline.co.uk/reports/wxfacts/Altocumulus.htm |title=Weather Facts: Altocumulus |publisher=WeatherOnline |access-date=2015-02-03}}</ref> is a middle-altitude [[list of cloud types|cloud genus]] that belongs mainly to the {{em|stratocumuliform}} physical category, characterized by globular masses or rolls in layers or patches{{snd}}the individual elements being larger and darker than those of [[cirrocumulus]] and smaller than those of [[stratocumulus]].<ref name="Altocumulus">{{cite book |publisher=[[World Meteorological Organization]] |title=Manual on the Observation of Clouds and Other Meteors |chapter=Altocumulus, International Cloud Atlas |volume=I |year=1975 |url=https://archive.org/details/manualonobservat00worl/page/31 |pages=31–35 |isbn=92-63-10407-7 |access-date=2014-08-26}}</ref>  However, if the layers become tufted in appearance due to increased airmass instability, then the altocumulus clouds become more purely ''cumuliform'' in structure.  Like other cumuliform and stratocumuliform clouds, altocumulus signifies [[convection]]. A sheet of partially conjoined altocumulus perlucidus is sometimes found preceding a weakening warm [[weather front|front]], where the altostratus is starting to fragment, resulting in patches of altocumulus perlucidus between the areas of altostratus. Altocumulus is also commonly found between the warm and cold fronts in a depression, although this is often hidden by lower clouds.
[[File:Altocumulus at Norah Head.jpg|thumb|left|Some '''''Altocumulus floccus''''' clouds in a gold tinge due to [[Rayleigh scattering]] in [[Norah Head]].]]
Towering altocumulus, known as [[altocumulus castellanus]], frequently signals the development of [[thunderstorms]] later in the day, as it shows instability and convection in the middle levels of the [[troposphere]], the area where towering cumulus clouds can turn into [[cumulonimbus]].<ref name="theairlinepilots.com">{{cite web |url=https://theairlinepilots.com/met/altocumuluscastellanus.htm |title=Altocumulus Castellanus |website=Clouds |publisher=TheAirlinePilots.com |access-date=}}</ref> It is therefore one of three warning clouds often recorded by the aviation industry, the other two being towering cumulus and cumulonimbus.<ref name="theairlinepilots.com"/> Altocumulus generally forms at about {{cvt|2,000|to|6,100|m|ft}} above ground level, a similar level to altostratus formations, and satellite photography reveals that the two types of cloud can create formations that can stretch for thousands of square miles.<ref>{{cite web |url=https://theairlinepilots.com/met/altocumulus.htm |title=Altocumulus |website=Clouds |publisher=TheAirlinePilots.com |access-date=}}</ref> Extensive altocumulus formations, particularly if they take the form of undulatus, are often referred to as altocumulus [[mackerel sky]].

==Subtypes==
[[File:Midcloudcymbols.gif|thumb|300px|Middle cloud weather map symbols: Includes middle-étage and downward growing vertical.]]
:*Species:  Altocumulus has four species.  The [[Altocumulus stratiformis|stratiformis]] species (Ac str) is composed of sheets or relatively flat patches of stratocumuliform cloud.  The synoptic coding is determined by the predominant variety or occasionally by the genitus mother cloud. Altocumulus lenticularis (Ac len [[lenticular cloud]]) is a lens-shaped middle cloud which can resemble flying saucers and may occasionally be mistaken for "[[unidentified flying object]]s". This is formed by uplift usually associated with mountains. but usually with at least some grey shading. It is coded C<sub>M</sub>4 on the SYNOP weather observation. Grey shading is also seen with [[altocumulus castellanus]] (Ac cas), a turreted middle cloud that can achieve significant vertical development and signals increasing air mass instability. It is nevertheless usually classified as middle rather than vertical and is coded C<sub>M</sub>8. The [[Altocumulus floccus|floccus]] species (Ac flo) is a tufted middle cloud which is also associated with greater instability. It shares the same code C<sub>M</sub>8. Chaotic altocumulus, which is typically poorly defined with multiple species or transitional forms arranged in several layers,<ref name="Species">{{cite book|editor=World Meteorological Organization|editor-link=World Meteorological Organization|title=Species, International Cloud Atlas|volume=I|year=1975|url=https://archive.org/details/manualonobservat00worl/page/17|pages=[https://archive.org/details/manualonobservat00worl/page/17 17–20]|isbn=92-63-10407-7|access-date=26 August 2014}}</ref> is coded C<sub>M</sub>9.
::*Opacity-based varieties:  Altocumulus stratiformis has three opacity-based varieties; ''Translucidus'' (C<sub>M</sub>3), ''perlucidus'' (C<sub>M</sub>3 or 7 depending on predominant opacity), and ''opacus'' (C<sub>M</sub>7). Varieties based on opacity are not commonly associated with the species lenticularis, castellanus, or floccus.
::*Pattern-based varieties: ''Radiatus'' (arranged in parallel bands) is sometimes seen with the stratformis and castellanus species.  Altocumulus stratiformis radiatus of any opacity is coded C<sub>M</sub>5 if it is increasing in amount.  The ''duplicatus'' or ''undulatus'' varieties are occasionally seen with the stratiformis and lenticularis species.<ref name="Varieties">{{cite book |editor=World Meteorological Organization |title=Varieties, International Cloud Atlas |year=1975 |url=http://library.wmo.int/pmb_ged/wmo_407_en-v1.pdf |pages=20–22 |access-date=26 August 2014 |archive-url=https://web.archive.org/web/20160725172359/http://library.wmo.int/pmb_ged/wmo_407_en-v1.pdf |archive-date=25 July 2016 |url-status=dead }}</ref>  Altocumulus stratiformis duplicatus is coded C<sub>M</sub>7 if it is not overridden by another coding of higher importance.  ''Lacunosus'' is very occasionally associated with altocumulus of the species stratiformis, castellanus, or floccus.
:::*Precipitation-based supplementary feature: Altocumulus often produces virga but usually not precipitation that reaches the ground.
:::*Cloud-based supplementary feature: Mamma caused by localized downdrafts in the cloud layer are occasionally seen with altocumulus.<ref name="Features">{{cite book|editor=World Meteorological Organization|editor-link=World Meteorological Organization|title=Features, International Cloud Atlas|volume=I|year=1975|url=https://archive.org/details/manualonobservat00worl/page/22|pages=[https://archive.org/details/manualonobservat00worl/page/22 22–24]|isbn=92-63-10407-7|access-date=26 August 2014}}</ref> A newly recognized type is the asperitas feature that is characterized by chaotic undulations caused by severe wind shear.

:::*Genitus Mother clouds: Altocumulus stratiformis cumulogenitus or cumulonimbogenitus can form when the middle or upper part of a towering free convective cloud begins to spread horizontally due to a loss of convective lift.  It is coded C<sub>M</sub>6.
:::*Mutatus mother clouds: Altocumulus can form due to the complete transformation of cirrocumulus, altostratus, nimbostratus, or stratocumulus.

==Gallery==
<gallery>
Image:Ac perlucidus.JPG|Altocumulus perlucidus
Image:Lenticular clouds and Mount Hotaka from Mount Otensho 1994-06-25.jpg|Altocumulus lenticularis
Image:Partially illuminated Ac with shadows.JPG|Altocumulus at sunset
Image:Hiranandani-Gardens-3.jpg|Altocumulus floccus with line of castellanus
Image:AltocumulusUndulatusClouds.png|Altocumulus undulatus during a cold front
File:Altocumulus-mamma.jpg|Altocumulus mamma
File:Altocumulus radiatus clouds 1.jpg|Altocumulus stratiformis radiatus
</gallery>

==See also==
* [[Atmospheric convection]]
* [[Mackerel sky]]

==References==
{{Reflist}}

==External links==
{{Commons category|Altocumulus clouds}}
* [http://nephology.eu/altocumulus International Cloud Atlas – Altocumulus] {{Webarchive|url=https://web.archive.org/web/20111008222204/http://nephology.eu/altocumulus |date=2011-10-08 }}
* [http://www.clouds-online.com Clouds-Online.com Cloud Atlas with many photos and description of the different cloud genus]
*[https://web.archive.org/web/20050420085336/http://www.nsdl.arm.gov/Library/glossary.shtml#Altocumulus#Altocumulus National Science Digital Library – Altocumulus]
* [http://www.geographyinaction.co.uk/Assets/Photo_albums/Nine/pages/Altocumulus.html Altocumulus] {{Webarchive|url=https://web.archive.org/web/20061017182539/http://www.geographyinaction.co.uk/Assets/Photo_albums/Nine/pages/Altocumulus.html |date=2006-10-17 }}
* [http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/mdl/altcu.rxml WW2010: University of Illinois]

{{Cloud types}}

[[Category:Cumulus]]