Editing Diffuse sky radiation

{{Short description|Solar radiation reaching the Earth's surface}}
{{Redirect|Red sky||Red Sky (disambiguation)}}
{{Use mdy dates|date=November 2024}}
{{Use American English|date=September 2020}}
[[File:Rayleigh sunlight scattering.png|thumb|upright=1.9|In [[Earth's atmosphere]], the dominant scattering efficiency of [[sky blue|blue light]] is compared to [[Visible spectrum#Spectral colors|red or green]] light. Scattering and absorption are major causes of the [[attenuation]] of sunlight radiation by the atmosphere. During broad [[daylight]], the sky is blue due to [[Rayleigh scattering]], while around sunrise or sunset, and especially during [[twilight]], [[Chappuis absorption|absorption]] of irradiation by [[Ozone#Spectroscopic properties|ozone]] helps maintain blue color in the evening sky. At sunrise or sunset, tangentially incident solar rays illuminate clouds with orange to red hues.]]
[[File:Spectrum of blue sky.svg|thumb|upright=1.9|The visible spectrum, approximately 380 to 740 nanometers (nm),<ref>{{cite book | title = Biology: Concepts and Applications | author = Starr, Cecie | publisher = Thomson Brooks/Cole | year = 2006| isbn = 978-0-534-46226-0 | url = https://archive.org/details/biologyconceptsa06edstar| url-access = registration | page = [https://archive.org/details/biologyconceptsa06edstar/page/94 94] }}</ref> shows the atmospheric water absorption band and the solar [[Fraunhofer lines]]. The blue sky spectrum contains light at all visible wavelengths with a broad maximum around 450–485 nm, the wavelengths of the color blue.]]

'''Diffuse sky radiation''' is [[solar radiation]] reaching the [[Earth]]'s surface after having been [[scattering|scattered]] from the direct solar beam by [[molecule]]s or [[particulates]] in the [[Earth's atmosphere|atmosphere]]. It is also called '''sky radiation''',  the determinative process for changing the colors of the [[sky]]. Approximately 23% of direct incident radiation of total [[sunlight]] is removed from the direct solar beam by scattering into the atmosphere; of this amount (of incident radiation)  about two-thirds ultimately reaches the earth as [[photon diffusion|photon diffused]] skylight radiation.{{citation needed|date=March 2016}}

The dominant radiative scattering processes in the atmosphere are [[Rayleigh scattering]] and [[Mie scattering]]; they are [[elastic scattering|elastic]], meaning that a photon of light can be deviated from its path without being absorbed and without changing wavelength.

Under an overcast sky, there is no direct sunlight, and all light results from diffused skylight radiation.

Proceeding from analyses of the aftermath of the eruption of the Philippines volcano Mount Pinatubo (in June 1991) and other studies:<ref name="web.archive.org"/><ref>{{cite journal |last1=Young |first1=Donald |last2=Smith |first2=William |title=Effect of Cloudcover on Photosynthesis and Transpiration in the Subalpine Understory Species Arnica Latifolia |journal=Ecology |pages=681–687 |doi=10.2307/1937189 |date=1983|volume=64 |issue=4 |jstor=1937189 |bibcode=1983Ecol...64..681Y }}</ref>  Diffused skylight, owing to its intrinsic structure and behavior, can illuminate under-canopy leaves, permitting more efficient total whole-plant photosynthesis than would otherwise be the case; this in stark contrast to the effect of totally clear skies with direct sunlight that casts shadows onto understory leaves and thereby limits plant photosynthesis to the top canopy layer, [[#The diffused skylight effect|(see below)]].

==Color==
[[File:Trees-sky.jpg|thumb|upright=1.5|A clear daytime sky, looking toward the [[zenith]]]]

[[atmosphere of Earth|Earth's atmosphere]] scatters short-[[wavelength]] light more efficiently than that of longer wavelengths. Because its wavelengths are shorter,  blue light is more strongly scattered than the longer-wavelength lights, red or green. Hence, the result that when looking at the sky away from the direct incident [[sunlight]], the human eye perceives the sky to be blue.<ref>"[http://search.eb.com/eb/article-9062822 Rayleigh scattering]." ''Encyclopædia Britannica''. 2007. Encyclopædia Britannica Online. retrieved November 16, 2007.</ref> The color perceived is similar to that presented by a monochromatic blue (at wavelength {{nobr|474–476 [[Nanometer|nm]]}}) mixed with white light, that is, an [[Saturation (color theory)|unsaturated]] blue light.<ref>{{cite journal |author=Glenn S. Smith |url=http://www.patarnott.com/atms749/pdf/blueSkyHumanResponse.pdf |title=Human color vision and the unsaturated blue color of the daytime sky |journal=American Journal of Physics |volume=73 |issue=7 |pages=590–597 |date=July 2005 |doi=10.1119/1.1858479|bibcode = 2005AmJPh..73..590S }}</ref> The explanation of blue color by  [[John William Strutt, 3rd Baron Rayleigh|Lord Rayleigh]] in 1871 is a famous example of applying [[dimensional analysis]] to solving problems in physics.<ref>{{cite web|url=https://application.wiley-vch.de/books/sample/3527403205_c01.pdf|title=Craig F. Bohren, "Atmospheric Optics", Wiley-VCH Verlag GmbH, page 56.|website=wiley-vch.de|access-date=April 4, 2018}}</ref>

Scattering and absorption are major causes of the [[attenuation]] of sunlight radiation by the atmosphere. Scattering varies as a function of the ratio of [[particle size|particle diameters]] (of [[particulates]] in the atmosphere) to the wavelength of the incident radiation. When this ratio is less than about one-tenth, [[Rayleigh scattering]] occurs. (In this case, the scattering coefficient varies inversely with the fourth power of the wavelength. At larger ratios scattering varies in a more complex fashion, as described for spherical particles by the [[Mie scattering|Mie theory]].) The laws of [[geometric optics]] begin to apply at higher ratios.

Daily at any global venue experiencing [[sunrise]] or [[sunset]], most of the solar beam of visible sunlight arrives nearly [[tangent lines to circles|tangentially]] to Earth's surface. Here, the [[optical path|path]] of sunlight [[air mass (astronomy)|through the atmosphere]] is [[optical path length|elongated]] such that much of the blue or green light is scattered away from the line of perceivable visible light. This phenomenon leaves the Sun's rays, and the clouds they illuminate, abundantly orange-to-red in colors, which one sees when looking at a sunset or sunrise.

For the example of the Sun at [[zenith]], in broad daylight, the sky is blue due to Rayleigh scattering, which also involves the [[diatomic]] gases [[nitrogen|{{chem|N|2}}]] and [[oxygen|{{chem|O|2}}]]. Near sunset and especially during [[twilight]], [[Chappuis absorption|absorption]] by [[ozone]] ({{chem|O|3}}) significantly contributes to maintaining [[blue hour|blue color]] in the evening sky.

==Under an overcast sky==
There is essentially no direct sunlight under an [[overcast]] sky, so all light is then diffuse sky radiation. The flux of light is not very wavelength-dependent because the cloud droplets are larger than the light's wavelength and [[Mie scattering|scatter all colors]] approximately equally. The light passes through the [[transparency and translucency|translucent]] clouds in a manner similar to [[frosted glass]]. The intensity ranges (roughly) from {{frac|1|6}} of direct sunlight for relatively thin clouds down to {{frac|1|1000}} of direct sunlight under the extreme of thickest storm clouds.{{Citation needed|date=February 2009}}

==As a part of total radiation==
One of the equations for total solar radiation is:<ref>{{cite book|last1=Mukherjee|first1=D.|last2=Chakrabarti|first2=S.|title=Fundamentals of Renewable Energy Systems|url=https://books.google.com/books?id=fdxdezgeXAcC&pg=PA22|year=2004|publisher=New Age International|isbn=978-81-224-1540-7|page=22}}</ref>

: <math>H_t= H_b R_b + H_d R_d + (H_b+H_d) R_r </math>

where ''H<sub>b</sub>'' is the beam radiation irradiance, ''R<sub>b</sub>'' is the tilt factor for beam radiation, ''H<sub>d</sub>'' is the diffuse radiation irradiance, ''R<sub>d</sub>'' is the tilt factor for diffuse radiation and ''R<sub>r</sub>'' is the tilt factor for reflected radiation.

''R<sub>b</sub>'' is given by:

: <math>R_b=\frac{\sin(\delta) \sin(\phi-\beta)+ \cos(\delta)\cos(h) \cos(\phi-\beta)}{\sin(\delta) \sin(\phi)+ \cos(\delta)\cos(h) \cos(\phi)}</math>

where ''δ'' is the [[solar declination]], ''Φ'' is the latitude, ''β'' is an angle from the horizontal and ''h'' is the solar [[hour angle]].

''R<sub>d</sub>'' is given by:

: <math>R_d=\frac{1+\cos(\beta)}{2}</math>

and ''R<sub>r</sub>'' by:

: <math>R_r=\frac{\rho(1-\cos(\beta))}{2}</math>

where ''ρ'' is the [[reflectivity]] of the surface.

==Agriculture and the eruption of Mt. Pinatubo==
[[File: Pinatubo dust layer.jpg|thumb|upright=1.25|A Space Shuttle (Mission [[STS-43]]) photograph of the Earth over [[South America]] taken on August 8, 1991, which captures the double layer of Pinatubo aerosol clouds (dark streaks) above lower cloud tops.]]

The eruption of the [[Philippines]] [[volcano]] - [[Mount Pinatubo]] in June 1991 ejected roughly {{convert|10|km3|cumi|abbr=on}} of magma and "17 million [[metric ton]]s"(17 [[Kilogram#SI multiples|teragrams]]) of [[sulfur dioxide]] SO<sub>2</sub> into the air, introducing ten times as much total SO<sub>2</sub> as the [[Kuwaiti oil fires|1991 Kuwaiti fires]],<ref>{{cite book |author= John C McCain |author2= Muhammad Sadiq |author3= M Sadiq |title=The Gulf War Aftermath: An Environmental Tragedy |publisher=Springer |date=1993 |page=60 |isbn=978-0-792-32278-8 }}</ref> mostly during the explosive [[Plinian eruption|Plinian/Ultra-Plinian]] event of June 15, 1991, creating a [[Volcanic winter|global stratospheric SO<sub>2</sub> haze layer]] which persisted for years. This resulted in the global average temperature dropping by about {{convert|0.5|C-change|F-change|1}}.<ref name="Science News">{{cite web|url=http://www.thefreelibrary.com/Mt.+Pinatubo's+cloud+shades+global+climate.-a012467057|title=Mt. Pinatubo's cloud shades global climate|publisher=Science News|access-date=2010-03-07}}</ref> Since [[volcanic ash]] falls out of the atmosphere rapidly,<ref>{{cite web|url=http://hvo.wr.usgs.gov/volcanowatch/archive/2007/07_03_08.html|title=Hawaiian Volcano Observatory|first=Volcano Hazards|last=Program|website=hvo.wr.usgs.gov|access-date=April 4, 2018}}</ref> the negative agricultural, effects of the eruption were largely immediate and localized to a relatively small area in close proximity to the eruption, caused by the resulting thick ash cover.<ref>{{cite web|url=http://pubs.usgs.gov/pinatubo/mercado/|title=Mercado|website=pubs.usgs.gov|access-date=April 4, 2018}}</ref><ref>{{cite web|url=https://sites.google.com/site/earthsciencesystems/students-faculty/larissa-karan/mt-pinatubo-lk-lithosphere|title=Mt. pinatubo (LK): Biosphere - ESS|website=sites.google.com|access-date=April 4, 2018}}</ref> Globally however, despite a several-month 5% drop in overall [[solar irradiation]], and a reduction in direct sunlight by 30%,<ref>{{cite web|url=http://climate.envsci.rutgers.edu/pdf/TreeRingCorrection3.pdf|title=Cooling Following Large Volcanic Eruptions Corrected for the Effect of Diffuse Radiation on Tree Rings. Alan Robock, 2005. See Figure 1 for a graphic of the recorded change in solar iiradiation|website=rutgers.edu|access-date=April 4, 2018}}</ref> there was no negative impact on global agriculture.<ref name="web.archive.org">{{cite web|url=http://earthobservatory.nasa.gov/Newsroom/view.php?id=22098|archive-url=https://web.archive.org/web/20100316225839/http://earthobservatory.nasa.gov/Newsroom/view.php?id=22098|url-status=dead|archive-date=March 16, 2010|title=Large Volcanic Eruptions Help Plants Absorb More Carbon Dioxide From the Atmosphere : News|date=March 16, 2010|access-date=April 4, 2018}}</ref><ref name="ReferenceA">[https://web.archive.org/web/20090801030547/http://www.gsfc.nasa.gov/topstory/20011210co2absorb.html LARGE VOLCANIC ERUPTIONS HELP PLANTS ABSORB MORE CARBON DIOXIDE FROM THE ATMOSPHERE]</ref> Surprisingly, a 3-4 year<ref>{{cite journal|title=The effects and consequences of very large explosive volcanic eruptions|first=S.|last=Self|s2cid=28228518|date=August 15, 2006|journal=Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences|volume=364|issue=1845|pages=2073–2097|doi=10.1098/rsta.2006.1814|pmid=16844649|bibcode=2006RSPTA.364.2073S}}</ref> increase in global [[Agricultural productivity]] and forestry growth was observed, excepting [[boreal forest]] regions.<ref name="dx.doi.org">[https://dx.doi.org/10.3402/tellusb.v66.21808 Evaluating aerosol direct radiative effects on global terrestrial ecosystem carbon dynamics from 2003 to 2010. Chen et al., ''Tellus B'' 2014; 66, 21808, Published by the international meteorological institute in Stockholm.]</ref>[[File:ZeaMays.jpg|thumb|upright|Under more-or-less direct sunlight, dark [[shadow]]s that limit photosynthesis are [[shadow|cast]] onto understorey [[leaves]]. Within the [[thicket]], very little direct sunlight can enter.]] The means of discovery was that initially, a mysterious drop in the rate at which [[carbon dioxide]] (CO<sub>2</sub>) was filling the atmosphere was observed, which is charted in what is known as the "[[Keeling Curve]]".<ref>{{cite web|url=http://climate.envsci.rutgers.edu/pdf/TreeRingCorrection3.pdf|title=Cooling Following Large Volcanic Eruptions Corrected for the Effect of Diffuse Radiation on Tree Rings. Alan Robock, 2005. See Figure 2 for a record of this|website=rutgers.edu|access-date=April 4, 2018}}</ref> This led numerous scientists to assume that the reduction was due to the lowering of Earth's temperature, and with that, a, slowdown in plant and soil [[Ecosystem respiration|respiration]], indicating a [[deleterious]] impact on global agriculture from the volcanic haze layer.<ref name="web.archive.org"/><ref name="ReferenceA"/> However upon investigation, the reduction in the rate at which carbon dioxide filled the atmosphere did not match up with the hypothesis that plant respiration rates had declined.<ref name="adsabs.harvard.edu">{{cite journal|bibcode=2001AGUFM.B51A0194G|title=Roles of volcanic eruptions, aerosols and clouds in global carbon cycle|first1=Gu|last1=L.|first2=Baldocchi|last2=D.|date=December 1, 2001|journal=AGU Fall Meeting Abstracts|volume=2001|pages=B51A–0194}}</ref><ref>{{cite web|url=http://research.eeescience.utoledo.edu/lees/papers_pdf/gu%20science%20paper%20mar%202003.pdf|title=Response of a Deciduous Forest to the Mount Pinatubo Eruption: Enhanced Photosynthesis. Gu et al., 28 March 2003 Journal of Science Vol 299|website=utoledo.edu|access-date=April 4, 2018|archive-url=https://web.archive.org/web/20160304101728/http://research.eeescience.utoledo.edu/lees/papers_pdf/gu%20science%20paper%20mar%202003.pdf|archive-date=March 4, 2016|url-status=dead}}</ref> Instead the advantageous anomaly was relatively firmly<ref>{{cite web|url=http://www.co2science.org/subject/v/summaries/volcanobio.php|title=CO2 Science|website=www.co2science.org|access-date=April 4, 2018}}</ref> linked to an unprecedented increase in the growth/[[net primary production]],<ref>http://earthobservatory.nasa.gov/Features/GlobalGarden/ Global Garden gets greener. NASA 2003</ref> of global plant life, resulting in the increase of the [[carbon sink]] effect of global photosynthesis.<ref name="web.archive.org"/><ref name="ReferenceA"/> The mechanism by which the increase in plant growth was possible, was that the 30% reduction of direct sunlight can also be expressed as an increase or "enhancement" in the amount of [[Diffuse reflection|diffuse]] sunlight.<ref name="web.archive.org"/><ref name="adsabs.harvard.edu"/><ref>{{cite web|url=http://climate.envsci.rutgers.edu/pdf/TreeRingCorrection3.pdf|title=Cooling Following LargeVolcanic Eruptions Corrected for the Effect of Diffuse Radiation on Tree Rings. Alan Robock, 2005. Figure 1|website=rutgers.edu|access-date=April 4, 2018}}</ref><ref name="ReferenceA"/>

===The diffused skylight effect===
[[File:Canopy.jpg|thumb|left|Well lit understorey areas due to [[overcast]] clouds creating diffuse/[[soft light|soft sunlight]] conditions, that permits photosynthesis on leaves under the canopy.]] 
This diffused skylight, owing to its intrinsic nature, can illuminate under-[[canopy (biology)|canopy]] leaves permitting more efficient total whole-plant [[photosynthesis]] than would otherwise be the case,<ref name="web.archive.org"/><ref name="ReferenceA"/> and also increasing evaporative cooling, from vegetated surfaces.<ref>{{Cite journal|last1=Chakraborty|first1=TC|last2=Lee|first2=Xuhui|last3=Lawrence|first3=David M.|date=2021|title=Strong Local Evaporative Cooling Over Land Due to Atmospheric Aerosols|journal=Journal of Advances in Modeling Earth Systems|volume=13|issue=5|doi=10.1029/2021ms002491|bibcode=2021JAMES..1302491C |s2cid=236541532 |issn=1942-2466|doi-access=free}}</ref> In stark contrast, for totally clear skies and the direct sunlight that results from it, shadows are cast onto [[understorey]] leaves, limiting plant photosynthesis to the top canopy layer.<ref name="web.archive.org"/><ref name="ReferenceA"/> This increase in global agriculture from the volcanic haze layer also naturally results as a product of other aerosols that are not emitted by volcanoes, such, "moderately thick smoke loading" pollution, as the same mechanism, the "aerosol direct radiative effect" is behind both.<ref name="dx.doi.org"/><ref>Impact of atmospheric aerosol light scattering and absorption on terrestrial net primary productivity, Cohan et al. ''GLOBAL BIOGEOCHEMICAL CYCLES'' 2002 VOL. 16, NO. 4, 1090, {{doi|10.1029/2001GB001441}}</ref><ref>Direct observations of the effects of aerosol loading on net ecosystem CO2 exchanges over different landscapes. Niyogi et al. ''Geophysical Research Letters'' Volume 31, Issue 20, October 2004 {{doi|10.1029/2004GL020915}}</ref>
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==See also==
{{Div col|small=yes}}
*[[Atmospheric diffraction]]
*[[Aerial perspective]]
*[[Cyanometer]]
*[[Daylight]]
*Nighttime [[airglow]]
*[[Rayleigh scattering]]
*[[Rayleigh sky model]]
*[[Sunshine duration]]
*[[Sunset#Colors]]
*[[Sunrise#Colors]]
*[[Tyndall effect]]
{{Div col end}}

== References ==
{{Reflist}}

==Further reading==
*{{cite book | first=Peter | last=Pesic | title=Sky in a Bottle | year=2005 | publisher=The MIT Press | isbn=978-0-262-16234-0 | url-access=registration | url=https://archive.org/details/skyinbottle00pesi }}

==External links==
*[http://www.arvindguptatoys.com/arvindgupta/raman.htm Dr. C. V. Raman lecture: Why is the sky blue?]
*[http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Why is the sky blue?]
*[http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html Blue Sky and Rayleigh Scattering]
*[http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf Atmospheric Optics (.pdf), Dr. Craig Bohren] {{Webarchive|url=https://web.archive.org/web/20131206175627/http://homepages.wmich.edu/%7Ekorista/atmospheric_optics.pdf |date=December 6, 2013 }}

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[[Category:Sun]]
[[Category:Light]]
[[Category:Visibility]]
[[Category:Atmospheric optical phenomena]]