Editing Cloud albedo

{{short description|Fraction of incoming sunlight reflected by clouds}}
[[File:NASA graphic representing the distribution of solar radiation.jpg|thumb|NASA graphic representing the distribution of solar radiation]]
'''Cloud albedo''' is a measure of the [[albedo]] or [[Reflectance|reflectivity]] of a [[cloud]]. Clouds regulate the amount of solar radiation absorbed by a planet and its [[Solar irradiance|solar surface irradiance]]. Generally, increased cloud cover correlates to a higher [[albedo]] and a lower absorption of [[solar energy]]. Cloud albedo strongly influences the [[Earth's energy budget]], accounting for approximately half of Earth's albedo.<ref name=":1" /><ref name=":12">{{Cite journal|last1=Mueller|first1=Richard|last2=Trentmann|first2=Jörg|last3=Träger-Chatterjee|first3=Christine|last4=Posselt|first4=Rebekka|last5=Stöckli|first5=Reto|date=2011|title=The Role of the Effective Cloud Albedo for Climate Monitoring and Analysis|journal=Remote Sensing|volume=3|issue=11|pages=2305–2320|doi=10.3390/rs3112305|bibcode=2011RemS....3.2305M |issn=2072-4292|doi-access=free }}</ref> Cloud albedo is influenced by the conditions of cloud formation and variations in cloud albedo depend on the total mass of water, the size and shape of the droplets or particles and their distribution in space.<ref name=":02">{{Cite book|last=Hartmann|first=Dennis|title=Global Physical Climatology|date=2016|publisher=Elsevier|isbn=978-0-12-328531-7|location=Australia|pages=76–78}}</ref> Thick clouds reflect a large amount of incoming solar radiation, translating to a high albedo. Thin clouds tend to transmit more solar radiation and, therefore, have a low albedo. Changes in cloud albedo caused by variations in cloud properties have a significant effect on [[Climate|global climate]], having the ability to spiral into feedback loops.<ref name=":02" />

== Cloud condensation nuclei and cloud albedo ==
On a microscopic scale, clouds are formed through the [[condensation]] of water on [[cloud condensation nuclei]]. These nuclei are [[Aerosol|aerosols]] such as dust or sea salt but also include certain forms of [[pollution]].<ref name=":1">{{Cite book |last=Hay |first=William W. |title=Experimenting on a small planet: a history of scientific discoveries, a future of climate change and global warming |date=2016 |publisher=Springer |isbn=978-3-319-27404-1 |edition=Second |location=Switzerland |pages=355–371}}</ref> Nuclei come from a variety of natural or [[Human impact on the environment|anthropogenic]] sources. For example dust can arise from windblown desserts or from human agricultural or construction activities, similarly even pollutants like [[VOCs]] or sulfates may be emitted by plant life or volcanic activity respectively.<ref name=":1" /> The size, concentration, structure, and chemical composition of these particles influence cloud albedo.<ref name=":22">{{Cite journal |last1=Kuniyal |first1=Jagdish Chandra |last2=Guleria |first2=Raj Paul |date=2019 |title=The current state of aerosol-radiation interactions: A mini review |url=http://dx.doi.org/10.1016/j.jaerosci.2018.12.010 |journal=Journal of Aerosol Science |volume=130 |pages=45–54 |bibcode=2019JAerS.130...45K |doi=10.1016/j.jaerosci.2018.12.010 |issn=0021-8502 |s2cid=104356406|url-access=subscription }}</ref><ref name=":03">Lohmann, U.; Feichter, J. (2005). [https://acp.copernicus.org/articles/5/715/2005/ "Global indirect aerosol effects: a review"]. ''Atmospheric Chemistry and Physics''. '''5''': 715–737.</ref> For example, [[black carbon]] aerosol particles absorb more solar radiation and [[sulfate aerosol|sulfate aerosols]] reflects more solar radiation. Smaller particles form smaller cloud droplets, which tend to decrease precipitation efficiency of a cloud and increasing cloud albedo.<ref name=":22" /> Additionally, more cloud condensation nuclei increases the size of a cloud and the amount of reflected solar radiation.<ref name=":03" />

== Causes of cloud albedo variation ==
Cloud albedo on a planet varies from less than 10% to more than 90% and depends on liquid [[water]]/ice content, thickness of the cloud, droplet sizes, solar zenith angle, etc.<ref name=":02" />

=== Water content ===
[[File:Cirrus cloud in Russia. img 028.jpg|thumb|Image of cirrus clouds taken in Russia uploaded to Wikimedia Commons by user [[c:User:Knopik-som|Knopik-som]]]]
Higher liquid water and ice content in a cloud increases its albedo, which is the dominant factor in determining the cloud's reflectivity.<ref name=":2">{{Cite journal |last1=Han |first1=Qingyuan |last2=Rossow |first2=William B. |last3=Chou |first3=Joyce |last4=Welch |first4=Ronald M. |date=1998 |title=Global Survey of the Relationships of Cloud Albedo and Liquid Water Path with Droplet Size Using ISCCP |journal=Journal of Climate |volume=11 |issue=7 |pages=1516–1528 |bibcode=1998JCli...11.1516H |doi=10.1175/1520-0442(1998)011<1516:GSOTRO>2.0.CO;2 |issn=0894-8755 |doi-access=free}}</ref><ref name=":0">{{Cite book |last=Hartmann |first=Dennis |title=Global Physical Climatology |date=2016 |publisher=Elsevier |isbn=978-0-12-328531-7 |location=Australia |pages=76–78}}</ref> The change in albedo is greater for clouds with less water content to start with and larger clouds begin to receive diminishing returns with increased content. Water content taking the form of ice is common in high altitude clouds such as [[Cirrus cloud|cirrus]].<ref name=":0" /> 

=== Cloud thickness ===
[[File:Big Cumulonimbus.JPG|thumb|Archetypical anvil shaped cumulonimbus cloud photographed by Simon Eugster in April 2005]]
Thicker clouds have a higher albedo than thinner ones.<ref name=":1" /><ref name=":02" /><ref name=":2" /> In fact thick clouds and thin clouds will occasionally respond differently to differences in other factors such as droplet size. Clouds that tend to be thicker and have higher albedos include [[Cumulus cloud|cumulus]], [[Stratocumulus cloud|stratocumulus]], and [[Cumulonimbus cloud|cumulonimbus]] clouds.<ref name=":02" /><ref name=":1" />

==== Liquid water path ====
Water content and cloud thickness together make a cloud's [[liquid water path]]. This value also notably varies with changing cloud droplet size.<ref name=":2" /> Liquid water path is typically measured in units of g/m<sup>2</sup> and in excess of 20 g/m<sup>2</sup> clouds typically will become opaque to long-wavelength light although this may not hold true with cirrus clouds.<ref name=":0" />

=== Droplet size ===
In general smaller droplet size is associated with increased albedo. That said, depending on the cloud location, thin clouds may actually have the opposite hold true.<ref name=":2" /> In the general and more influential cases however, decreased particle size makes clouds possess higher albedos by having a larger surface areas relative to their volumes. This makes the droplets whiter or more reflective.<ref name=":1" /><ref name=":0" />

=== The Twomey Effect (Aerosol Indirect Effect) ===
[[File:Aerosol effect on cloud albedo.jpg|thumb|Increased cloud droplet concentration and albedo due to aerosol effect]]
The [[Twomey effect|Twomey Effect]] is increased cloud albedo due to cloud nuclei from pollution.<ref>{{Cite journal|last=Twomey|first=S.|date=1974|title=Pollution and the Planetary Albedo|url=https://doi.org/10.1016/0004-6981(74)90004-3|journal=Atmospheric Environment|volume=8|issue=12 |pages=1251–1256|doi=10.1016/0004-6981(74)90004-3 |bibcode=1974AtmEn...8.1251T |url-access=subscription}}</ref><ref name=":22" /> Increasing [[Aerosol|aerosol concentration]] and aerosol density leads to higher cloud droplet concentration, smaller cloud droplets, and higher cloud albedo.<ref name=":2" /><ref name=":0" /> In macrophysically identical clouds, a cloud with few larger drops will have a lower albedo than a cloud with more smaller drops. The smaller cloud particles similarly increase cloud albedo by reducing precipitation and prolonging the lifetime of a cloud. This subsequently increases cloud albedo as solar radiation is reflected over a longer period of time. The [[Albrecht effect|Albrecht Effect]] is the related concept of increased cloud lifetime from cloud nuclei.<ref name=":03" />

=== Zenith angle ===
Cloud albedo increases with the total water content or depth of the cloud and the [[solar zenith angle]].<ref name=":0" /> The variation of albedo with zenith angle is most rapid when the sun is near the horizon, and least when the sun is overhead. Absorption of solar radiation by plane-parallel clouds decreases with increasing zenith angle because radiation that is reflected to space at the higher zenith angles penetrates less deeply into the cloud and is therefore less likely to be absorbed.<ref name=":0" />

== Influence on global climate ==
Cloud albedo indirectly affects global climate through solar radiation [[scattering]] and [[Absorption (electromagnetic radiation)|absorption]] in Earth's radiation budget.<ref name=":12" /> Variations in cloud albedo cause atmospheric instability that influences the [[hydrological cycle]], weather patterns, and [[atmospheric circulation]].<ref name=":22" /> These effects are parameterized by [[Cloud forcing|cloud radiative forcing]], a measure of short-wave and long-wave radiation in relation to [[cloud cover]]. The [[Earth Radiation Budget Experiment]] demonstrated that small variations in cloud coverage, structure, altitude, droplet size, and phase have significant effects on the climate. A five percent increase in short-wave reflection from clouds would counteract the greenhouse effect of the past two-hundred years.<ref name=":22" />

=== Cloud albedo-climate feedback loops ===
There are a variety of positive and negative [[Cloud feedback|cloud albedo-climate feedback loops]] in cloud and climate models. An example of a negative cloud-climate feedback loop is that as a planet warms, cloudiness increases, which increases a planet's albedo. An increase in albedo reduces absorbed solar radiation and leads to cooling. A counteracting positive feedback loop considers the rising of the high cloud layer, reduction in the vertical distribution of cloudiness, and decreased albedo.<ref>{{Cite journal|last1=Wetherald|first1=R. T.|last2=Manabe|first2=S.|date=1988|title=Cloud Feedback Processes in a General Circulation Model|journal=Journal of the Atmospheric Sciences|language=EN|volume=45|issue=8|pages=1397–1416|doi=10.1175/1520-0469(1988)045<1397:CFPIAG>2.0.CO;2 |bibcode=1988JAtS...45.1397W |issn=0022-4928|doi-access=free}}</ref>

[[Air pollution]] can result in variation in cloud condensation nuclei, creating a feedback loop that influences atmospheric temperature, relative humility, and cloud formation depending on cloud and regional characteristics. For example, increased [[Sulfate aerosol|sulfate aerosols]] can reduce precipitation efficiency, resulting in a positive feedback loop in which decreased precipitation efficiency increases aerosol atmospheric longevity.<ref name=":03" /> On the other hand, a negative feedback loop can be established in mixed-phase clouds in which [[black carbon]] aerosol can increase ice phase precipitation formation and reduce aerosol concentrations.<ref name=":03" />

==References==
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[[Category:Atmospheric radiation]]
[[Category:Clouds]]
[[Category:Satellite meteorology]]