diffuse sky radiation

File:Trees-sky.jpg]]

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."[http://search.eb.com/eb/article-9062822 Rayleigh scattering]." Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. retrieved November 16, 2007. The color perceived is similar to that presented by a monochromatic blue (at wavelength {{nobr|474–476 nm}}) mixed with white light, that is, an unsaturated blue light.{{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 }} The explanation of blue color by Lord Rayleigh in 1871 is a famous example of applying dimensional analysis to solving problems in physics.{{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}}

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 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 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 tangentially to Earth's surface. Here, the path of sunlight through the atmosphere is 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 and oxygen. Near sunset and especially during twilight, absorption by ozone ({{chem|O|3}}) significantly contributes to maintaining blue color in the evening 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 scatter all colors approximately equally. The light passes through the 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}}

One of the equations for total solar radiation is:{{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}}

: $H\_t=\; H\_b\; R\_b\; +\; H\_d\; R\_d\; +\; (H\_b+H\_d)\; R\_r$

where H_b is the beam radiation irradiance, R_b is the tilt factor for beam radiation, H_d is the diffuse radiation irradiance, R_d is the tilt factor for diffuse radiation and R_r is the tilt factor for reflected radiation.

R_b is given by:

: $R\_b=\backslash frac\{\backslash sin(\backslash delta)\; \backslash sin(\backslash phi-\backslash beta)+\; \backslash cos(\backslash delta)\backslash cos(h)\; \backslash cos(\backslash phi-\backslash beta)\}\{\backslash sin(\backslash delta)\; \backslash sin(\backslash phi)+\; \backslash cos(\backslash delta)\backslash cos(h)\; \backslash cos(\backslash phi)\}$

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_d is given by:

: $R\_d=\backslash frac\{1+\backslash cos(\backslash beta)\}\{2\}$

and R_r by:

: $R\_r=\backslash frac\{\backslash rho(1-\backslash cos(\backslash beta))\}\{2\}$

where ρ is the reflectivity of the surface.

File: Pinatubo dust layer.jpg) 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 tons"(17 teragrams) of sulfur dioxide SO₂ into the air, introducing ten times as much total SO₂ as the 1991 Kuwaiti fires,{{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 }} mostly during the explosive Plinian/Ultra-Plinian event of June 15, 1991, creating a global stratospheric SO₂ haze layer which persisted for years. This resulted in the global average temperature dropping by about {{convert|0.5|C-change|F-change|1}}.{{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}} Since volcanic ash falls out of the atmosphere rapidly,{{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}} 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.{{cite web|url=http://pubs.usgs.gov/pinatubo/mercado/|title=Mercado|website=pubs.usgs.gov|access-date=April 4, 2018}}{{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}} Globally however, despite a several-month 5% drop in overall solar irradiation, and a reduction in direct sunlight by 30%,{{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}} there was no negative impact on global agriculture.{{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}}[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] Surprisingly, a 3-4 year{{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}} increase in global Agricultural productivity and forestry growth was observed, excepting boreal forest regions.[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.]File:ZeaMays.jpgs that limit photosynthesis are 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 (CO₂) was filling the atmosphere was observed, which is charted in what is known as the "Keeling Curve".{{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}} 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 respiration, indicating a deleterious impact on global agriculture from the volcanic haze layer. 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.{{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}}{{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}} Instead the advantageous anomaly was relatively firmly{{cite web|url=http://www.co2science.org/subject/v/summaries/volcanobio.php|title=CO2 Science|website=www.co2science.org|access-date=April 4, 2018}} linked to an unprecedented increase in the growth/net primary production,http://earthobservatory.nasa.gov/Features/GlobalGarden/ Global Garden gets greener. NASA 2003 of global plant life, resulting in the increase of the carbon sink effect of global photosynthesis. 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 sunlight.{{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}}

File:Canopy.jpg clouds creating diffuse/soft sunlight conditions, that permits photosynthesis on leaves under the canopy.]]

This diffused skylight, owing to its intrinsic nature, can illuminate under-canopy leaves permitting more efficient total whole-plant photosynthesis than would otherwise be the case, and also increasing evaporative cooling, from vegetated surfaces.{{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}} 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. 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.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}}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}}

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{{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}}

{{Reflist}}

• {{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 }}

• [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