Editing Cloud albedo (section)

== 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" />