Editing Altostratus cloud (section)

== Description ==
[[File:2017-06-22_16_57_51_Sun_shining_dimly_through_an_altostratus_cloud_layer_over_Ladybank_Lane_in_the_Chantilly_Highlands_section_of_Oak_Hill,_Fairfax_County,_Virginia.jpg|upright=1.3|thumb|right|alt=The sun shines diumly through a largely-featureless gray altostratus cloud.|Sun shines dimly though the ''translucidus'' variant of altostratus clouds]]
Altostratus clouds are generally gray or blue-tinged with a largely-uniform blanket-like appearance. They do not have distinct features, and usually do not produce [[precipitation]]. The name "altostratus" comes from the conjugation of the [[Latin]] words "altum", meaning "high", and "stratus", meaning "flat" or "spread out".<ref name="ICA-AS">{{harvnb|Cohn|Bruhn|Anderson|Atkinson|2017|loc=Section 2.3.5.1}}</ref><ref name="MetOffice">{{cite web|url=https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/clouds/mid-level-clouds/altostratus|title=Altostratus clouds|publisher=Meteorological Office of the UK|access-date=25 March 2022}}</ref> Altostratus clouds can produce [[virga]], causing the cloud base to appear hazy.<ref name="WMO-Desc" /> While they do not produce significant precipitation, altostratus clouds can cause light sprinkles or even small rain showers.<ref name="Cloud-Classification">{{cite web |last=Funk |first=Ted |title=Cloud Classifications and Characteristics |url=https://www.weather.gov/media/lmk/soo/cloudchart.pdf|access-date=25 March 2022 |work=The Science Corner |publisher=[[National Oceanic and Atmospheric Administration|NOAA]] |page=1}}</ref> Consistent rainfall and lowering of the cloud base causes altostratus to become [[nimbostratus]].<ref name="Ahrens-144">{{harvnb|Ahrens|2006|p=194}}</ref>

Unlike most other types of clouds, altostratus clouds are not subdivided into [[cloud species]] due to their largely-featureless appearance.<ref name="ICA-Species">{{harvnb|Cohn|Bruhn|Anderson|Atkinson|2017|loc=Section 2.3.5.2}}</ref> However, they still appear in five varieties: ''Altostratus duplicatus'', ''opacus'', ''radiatus'', ''translucidus'', and ''undulatus''.<ref name="ICA-Varieties">{{harvnb|Cohn|Bruhn|Anderson|Atkinson|2017|loc=Section 2.3.5.3}}</ref> ''Altostratus duplicatus'' is a rare form of altostratus clouds composed of two or more layers of cloud.<ref>{{harvnb|Cohn|Bruhn|Anderson|Atkinson|2017|loc=Section 2.3.5.3.3}}</ref> ''Translucidus'' is a [[translucent]] form of altostratus clouds, meaning that the sun or moon can be seen through the cloud,<ref>{{harvnb|Cohn|Bruhn|Anderson|Atkinson|2017|loc=Section 2.3.5.3.1}}</ref> whereas the ''opacus'' variety is opaque.<ref>{{harvnb|Cohn|Bruhn|Anderson|Atkinson|2017|loc=Section 2.3.5.3.2}}</ref> ''Radiatus'' is another rare variety. It has parallel bands of cloud that stretch toward the horizon.<ref>{{harvnb|Cohn|Bruhn|Anderson|Atkinson|2017|loc=Section 2.3.5.3.5}}</ref> The ''undulatus'' variety has an wavy appearance&mdash;the underside of the cloud appears to rise and fall.<ref>{{harvnb|Cohn|Bruhn|Anderson|Atkinson|2017|loc=Section 2.3.5.3.4}}</ref>

Altostratus and [[altocumulus cloud]]s, both of which are mid-level clouds,<ref name="Cloud-Classification" /> are commonly measured together in cloud cover studies. Together, they cover around 25% of the Earth's surface on average<ref name="Sassen-688" /> based on [[CALIPSO]] satellite data.<ref name="Sassen-679">{{harvnb|Sassen|Wang|2012|p=679}}</ref> This constitutes roughly one third of the Earth's total cloud cover.<ref name="Sassen-688" /> By itself, separated from altocumulus, altostratus covers ~16% of the Earth's surface.<ref name="Sassen-688">{{harvnb|Sassen|Wang|2012|p=688}}</ref> Altostratus cloud cover varies seasonally in temperate regions, with significantly less coverage in the summer months as compared to the other seasons. Additionally, altostratus cloud cover varies by [[latitude]], with tropical regions having vastly fewer altostratus clouds when compared to temperate or polar regions.<ref name="Sassen-686">{{harvnb|Sassen|Wang|2012|p=686}}</ref> Altostratus and altocumulus cover roughly 22% of the ocean's surface based on surface measurements, with minimal variation based on season.<ref>{{harvnb|Warren|Hahn|London|Chervin|Jenne|1988|loc=Table 9b}}</ref>

Altostratus clouds are warmest at the bottom and coldest at the top,<ref name="Yang-6010">{{harvnb|Yang|Zou|2013|p=6010}}</ref> with a fairly consistent<ref name="Yang-6013" /> [[lapse rate]] of 5 to 7&nbsp;°C per kilometer (14 to 20&nbsp;°F per mile) inside the cloud. The lapse rate is the rate at which the temperature decreases with altitude.<ref name="Yang-6011">{{harvnb|Yang|Zou|2013|p=6011}}</ref> Higher lapse rates (i.e. the faster temperature drops with increasing altitude) were associated with colder clouds.<ref name="Yang-6013">{{harvnb|Yang|Zou|2013|p=6013}}</ref> The average temperature of altostratus clouds, based on data collected from roughly 45° to 80° latitude, varied from around {{convert|-16|to|-45|C|F|sigfig=2}}. Warmer temperatures occurred during summer and colder temperatures during winter.<ref name="Yang-6010" />

Inside altostratus clouds, the [[relative humidity]] is generally greatest towards the top of the cloud decreasing slowly and roughly linearly towards the bottom. The lowest part of the cloud has the lowest relative humidity.<ref name="Yang-6010" /> Below the bottom of the cloud, the relative humidity drops rapidly.<ref name="Danne-181">{{harvnb|Danne|Quante|Milferstädt|Lemke|1999|p=181}}</ref>

=== Microphysical properties ===
Altostratus can be composed of water droplets, [[supercooled]] water droplets, and ice crystals,<ref name="Cloud-Classification" /> but ice crystals make up the vast majority.<ref>{{harvnb|Sassen|Wang|2012|pp=679–680}}</ref> In some altostratus clouds made of ice crystals, very thin horizontal sheets of water droplets can appear seemingly at random, but they quickly disappear.<ref name="platt-344">{{harvnb|Platt|1977|p=344}}</ref> The sizes of the ice crystals in the cloud tended to increase as altitude decreased. However, close to the bottom of the cloud, the particles decreased in size again. During the sampling of one cloud, the scientists noted a [[Halo (optical phenomenon)|halo]] while flying near the top of the cloud, which indicated that the ice crystals were hexagonal near the top. However, farther down, the ice crystals became more conglomerated.<ref name="field-1929">{{harvnb|Field|1999|p=1929}}</ref><ref name="field-1933">{{harvnb|Field|1999|p=1933}}</ref> Mixed-phase (containing both ice and water) altostratus clouds contain a "melt layer", below which the ice crystals tend to melt into water droplets. These water droplets are spheres and thus fall much faster than ice crystals, collecting at the bottom of the cloud.<ref name="Danne-182">{{harvnb|Danne|Quante|Milferstädt|Lemke|1999|p=182}}</ref>