Diffuse sky radiation

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In Earth's atmosphere, the dominant scattering efficiency of blue light is compared to 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, absorption of irradiation by ozone helps maintain blue color in the evening sky. At sunrise or sunset, tangentially incident solar rays illuminate clouds with orange to red hues. Rayleigh sunlight scattering.png
In Earth's atmosphere, the dominant scattering efficiency of blue light is compared to 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, absorption of irradiation by ozone helps maintain blue color in the evening sky. At sunrise or sunset, tangentially incident solar rays illuminate clouds with orange to red hues.
The visible spectrum, approximately 380 to 740 nanometers (nm), 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. Spectrum of blue sky.svg
The visible spectrum, approximately 380 to 740 nanometers (nm), 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 scattered from the direct solar beam by molecules or particulates in the 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 diffused skylight radiation.[ citation needed ]

Contents

The dominant radiative scattering processes in the atmosphere are Rayleigh scattering and Mie scattering; they are 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: [2] [3] 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, (see below).

Color

A clear daytime sky, looking toward the zenith Trees-sky.jpg
A clear daytime sky, looking toward the zenith

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. [4] The color perceived is similar to that presented by a monochromatic blue (at wavelength 474–476 nm ) mixed with white light, that is, an unsaturated blue light. [5] The explanation of blue color by Rayleigh in 1871 is a famous example of applying dimensional analysis to solving problems in physics; [6] .

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 N
2 and O
2. Near sunset and especially during twilight, absorption by ozone (O
3) significantly contributes to maintaining 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 scatter all colors approximately equally. The light passes through the translucent clouds in a manner similar to frosted glass. The intensity ranges (roughly) from 1⁄6 of direct sunlight for relatively thin clouds down to 1⁄1000 of direct sunlight under the extreme of thickest storm clouds.[ citation needed ]

As a part of total radiation

One of the equations for total solar radiation is: [7]

where Hb is the beam radiation irradiance, Rb is the tilt factor for beam radiation, Hd is the diffuse radiation irradiance, Rd is the tilt factor for diffuse radiation and Rr is the tilt factor for reflected radiation.

Rb is given by:

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

Rd is given by:

and Rr by:

where ρ is the reflectivity of the surface.

Agriculture and the eruption of Mt. Pinatubo

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. Pinatubo dust layer.jpg
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 10 km3 (2.4 cu mi) of magma and "17,000,000 metric tons"(17 teragrams) of sulfur dioxide SO2 into the air, introducing ten times as much total SO2 as the 1991 Kuwaiti fires, [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 0.5 °C (0.9 °F). [9] Since volcanic ash falls out of the atmosphere rapidly, [10] 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. [11] [12] Globally however, despite a several-month 5% drop in overall solar irradiation, and a reduction in direct sunlight by 30%, [13] there was no negative impact on global agriculture. [2] [14] Surprisingly, a 3-4 year [15] increase in global Agricultural productivity and forestry growth was observed, excepting boreal forest regions. [16]

Under more-or-less direct sunlight, dark shadows that limit photosynthesis are cast onto understorey leaves. Within the thicket, very little direct sunlight can enter. ZeaMays.jpg
Under more-or-less direct sunlight, dark shadows 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". [17] 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. [2] [14] 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. [18] [19] Instead the advantageous anomaly was relatively firmly [20] linked to an unprecedented increase in the growth/net primary production, [21] of global plant life, resulting in the increase of the carbon sink effect of global photosynthesis. [2] [14] 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. [2] [18] [22] [14]

The diffused skylight effect

Well lit understorey areas due to overcast clouds creating diffuse/soft sunlight conditions, that permits photosynthesis on leaves under the canopy. Canopy.jpg
Well lit understorey areas due to overcast 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, [2] [14] and also increasing evaporative cooling, from vegetated surfaces. [23] 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. [2] [14] 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. [16] [24] [25]

See also

Related Research Articles

<span class="mw-page-title-main">Albedo</span> Ratio of how much light is reflected back from a body

Albedo is the fraction of sunlight that is diffusely reflected by a body. It is measured on a scale from 0 to 1. Surface albedo is defined as the ratio of radiosity Je to the irradiance Ee received by a surface. The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface. These factors vary with atmospheric composition, geographic location, and time.

<span class="mw-page-title-main">Afterglow</span> Whitish or rosy light during twilight or after sunset

An afterglow in meteorology consists of several atmospheric optical phenomena, with a general definition as a broad arch of whitish or pinkish sunlight in the twilight sky, consisting of the bright segment and the purple light. Purple light mainly occurs when the Sun is 2–6° below the horizon, from civil to nautical twilight, while the bright segment lasts until the end of the nautical twilight. Afterglow is often in cases of volcanic eruptions discussed, while its purple light is discussed as a different particular volcanic purple light. Specifically in volcanic occurrences it is light scattered by fine particulates, like dust, suspended in the atmosphere. In the case of alpenglow, which is similar to the Belt of Venus, afterglow is used in general for the golden-red glowing light from the sunset and sunrise reflected in the sky, and in particularly for its last stage, when the purple light is reflected. The opposite of an afterglow is a foreglow, which occurs before sunrise.

<span class="mw-page-title-main">Rayleigh scattering</span> Light scattering by small particles

Rayleigh scattering, named after the 19th-century British physicist Lord Rayleigh, is the predominantly elastic scattering of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scattering medium, the amount of scattering is inversely proportional to the fourth power of the wavelength, e.g., a blue color is scattered much more than a red color as light propagates through air.

<span class="mw-page-title-main">Sunlight</span> Light emitted by the Sun

Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere as daylight when the Sun is above the horizon. When direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat (Atmospheric). When blocked by clouds or reflected off other objects, sunlight is diffused. Sources estimate a global average of between 164 watts to 340 watts per square meter over a 24-hour day; this figure is estimated by NASA to be about a quarter of Earth's average total solar irradiance.

<span class="mw-page-title-main">Scattering</span> Range of physical processes

Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of reflected radiation from the angle predicted by the law of reflection. Reflections of radiation that undergo scattering are often called diffuse reflections and unscattered reflections are called specular (mirror-like) reflections. Originally, the term was confined to light scattering. As more "ray"-like phenomena were discovered, the idea of scattering was extended to them, so that William Herschel could refer to the scattering of "heat rays" in 1800. John Tyndall, a pioneer in light scattering research, noted the connection between light scattering and acoustic scattering in the 1870s. Near the end of the 19th century, the scattering of cathode rays and X-rays was observed and discussed. With the discovery of subatomic particles and the development of quantum theory in the 20th century, the sense of the term became broader as it was recognized that the same mathematical frameworks used in light scattering could be applied to many other phenomena.

<span class="mw-page-title-main">Sunrise</span> Time of day when the sun appears above the horizon

Sunrise is the moment when the upper rim of the Sun appears on the horizon in the morning. The term can also refer to the entire process of the solar disk crossing the horizon.

<span class="mw-page-title-main">Sunset</span> Daily falling of the Sun below the horizon

Sunset is the disappearance of the Sun below the horizon of the Earth due to its rotation. As viewed from everywhere on Earth, it is a phenomenon that happens approximately once every 24 hours, except in areas close to the poles. The equinox Sun sets due west at the moment of both the spring and autumn equinoxes. As viewed from the Northern Hemisphere, the Sun sets to the northwest in the spring and summer, and to the southwest in the autumn and winter; these seasons are reversed for the Southern Hemisphere.

<span class="mw-page-title-main">Sky</span> View upward from the surface of the Earth

The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space.

<span class="mw-page-title-main">Atmosphere of Earth</span> Gas layer surrounding Earth

The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth creates pressure, absorbs most meteoroids and ultraviolet solar radiation, warms the surface through heat retention, and reduces temperature extremes between day and night, maintaining conditions allowing life and liquid water to exist on the Earth's surface.

Direct insolation is the insolation measured at a given location on Earth with a surface element perpendicular to the Sun's rays, excluding diffuse insolation. Direct insolation is equal to the solar irradiance above the atmosphere minus the atmospheric losses due to absorption and scattering. While the solar irradiance above the atmosphere varies with the Earth–Sun distance and solar cycles, the losses depend on the time of day, cloud cover, humidity, and other impurities.

<span class="mw-page-title-main">Global dimming</span> Reduction in the amount of sunlight reaching Earths surface

Global dimming is a decline in the amount of sunlight reaching the Earth's surface, a measure also known as global direct solar irradiance. It was observed soon after the first systematic measurements of solar irradiance began in the 1950s, and this weakening of visible sunlight proceeded at the rate of 4–5% per decade until 1980s. Yet, solar activity did not vary more than the usual during this period. Instead, global dimming was due to an increase in atmospheric particulate matter, predominantly sulfate aerosols, as the result of rapidly growing air pollution due to post-war industrialization. Since the 1980s, a decrease in air pollution has led to a partial reversal of the dimming trend, sometimes referred to as global brightening. The reversal of dimming is not complete and varies worldwide. Brightening in developed countries during the 1980s and 1990s was offset by increased dimming in developing countries and by the expansion of the global shipping industry. During 2010s, air pollution mitigation in developing countries has also improved rapidly.

<span class="mw-page-title-main">Sunbeam</span> Rays of sunlight that appear to radiate from the point in the sky where the sun is located

A sunbeam, in meteorological optics, is a beam of sunlight that appears to radiate from the position of the Sun. Shining through openings in clouds or between other objects such as mountains and buildings, these beams of particle-scattered sunlight are essentially parallel shafts separated by darker shadowed volumes. Their apparent convergence in the sky is a visual illusion from linear perspective. The same illusion causes the apparent convergence of parallel lines on a long straight road or hallway at a distant vanishing point. The scattering particles that make sunlight visible may be air molecules or particulates.

<span class="mw-page-title-main">Skyglow</span> Diffuse luminance of the night sky

Skyglow is the diffuse luminance of the night sky, apart from discrete light sources such as the Moon and visible individual stars. It is a commonly noticed aspect of light pollution. While usually referring to luminance arising from artificial lighting, skyglow may also involve any scattered light seen at night, including natural ones like starlight, zodiacal light, and airglow.

<span class="mw-page-title-main">Tyndall effect</span> Scattering of light by tiny particles in a colloidal suspension

The Tyndall effect is light scattering by particles in a colloid such as a very fine suspension. Also known as Tyndall scattering, it is similar to Rayleigh scattering, in that the intensity of the scattered light is inversely proportional to the fourth power of the wavelength, so blue light is scattered much more strongly than red light. An example in everyday life is the blue colour sometimes seen in the smoke emitted by motorcycles, in particular two-stroke machines where the burnt engine oil provides these particles. The same effect can also be observed with tobacco smoke whose fine particles also preferentially scatter blue light.

<span class="mw-page-title-main">Solar irradiance</span> Measurement of electromagnetic radiation

Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m2) in SI units.

The air mass coefficient defines the direct optical path length through the Earth's atmosphere, expressed as a ratio relative to the path length vertically upwards, i.e. at the zenith. The air mass coefficient can be used to help characterize the solar spectrum after solar radiation has traveled through the atmosphere.

<span class="mw-page-title-main">Stratospheric aerosol injection</span> Putting particles in the stratosphere to reflect sunlight to limit global heating

Stratospheric aerosol injection is a proposed method of solar geoengineering to reduce global warming. This would introduce aerosols into the stratosphere to create a cooling effect via global dimming and increased albedo, which occurs naturally from volcanic winter. It appears that stratospheric aerosol injection, at a moderate intensity, could counter most changes to temperature and precipitation, take effect rapidly, have low direct implementation costs, and be reversible in its direct climatic effects. The Intergovernmental Panel on Climate Change concludes that it "is the most-researched [solar geoengineering] methodagreement that it could limit warming to below 1.5 °C (2.7 °F)." However, like other solar geoengineering approaches, stratospheric aerosol injection would do so imperfectly and other effects are possible, particularly if used in a suboptimal manner.

<span class="mw-page-title-main">Sunset (color)</span> Shade of orange

The color sunset is a pale tint of orange. It is a representation of the average color of clouds when the sunlight from a sunset is reflected from them.

<span class="mw-page-title-main">Rayleigh sky model</span>

The Rayleigh sky model describes the observed polarization pattern of the daytime sky. Within the atmosphere, Rayleigh scattering of light by air molecules, water, dust, and aerosols causes the sky's light to have a defined polarization pattern. The same elastic scattering processes cause the sky to be blue. The polarization is characterized at each wavelength by its degree of polarization, and orientation.

<span class="mw-page-title-main">Earth's shadow</span> Shadow that Earth itself casts through its atmosphere and into outer space

Earth's shadow is the shadow that Earth itself casts through its atmosphere and into outer space, toward the antisolar point. During the twilight period, the shadow's visible fringe – sometimes called the dark segment or twilight wedge – appears as a dark and diffuse band just above the horizon, most distinct when the sky is clear.

References

  1. ↑ Starr, Cecie (2006). Biology: Concepts and Applications . Thomson Brooks/Cole. p.  94. ISBN   978-0-534-46226-0.
  2. 1 2 3 4 5 6 7 "Large Volcanic Eruptions Help Plants Absorb More Carbon Dioxide From the Atmosphere : News". March 16, 2010. Archived from the original on March 16, 2010. Retrieved April 4, 2018.
  3. ↑ Young, Donald; Smith, William (1983). "Effect of Cloudcover on Photosynthesis and Transpiration in the Subalpine Understory Species Arnica Latifolia". Ecology. 64 (4): 681–687. Bibcode:1983Ecol...64..681Y. doi:10.2307/1937189. JSTOR   1937189.
  4. ↑ "Rayleigh scattering." Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. retrieved November 16, 2007.
  5. ↑ Glenn S. Smith (July 2005). "Human color vision and the unsaturated blue color of the daytime sky" (PDF). American Journal of Physics. 73 (7): 590–597. Bibcode:2005AmJPh..73..590S. doi:10.1119/1.1858479.
  6. ↑ "Craig F. Bohren, "Atmospheric Optics", Wiley-VCH Verlag GmbH, page 56" (PDF). wiley-vch.de. Retrieved April 4, 2018.
  7. ↑ Mukherjee, D.; Chakrabarti, S. (2004). Fundamentals of Renewable Energy Systems. New Age International. p. 22. ISBN   978-81-224-1540-7.
  8. ↑ John C McCain; Muhammad Sadiq; M Sadiq (1993). The Gulf War Aftermath: An Environmental Tragedy. Springer. p. 60. ISBN   978-0-792-32278-8.
  9. ↑ "Mt. Pinatubo's cloud shades global climate". Science News. Retrieved March 7, 2010.
  10. ↑ Program, Volcano Hazards. "Hawaiian Volcano Observatory". hvo.wr.usgs.gov. Retrieved April 4, 2018.
  11. ↑ "Mercado". pubs.usgs.gov. Retrieved April 4, 2018.
  12. ↑ "Mt. pinatubo (LK): Biosphere - ESS". sites.google.com. Retrieved April 4, 2018.
  13. ↑ "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" (PDF). rutgers.edu. Retrieved April 4, 2018.
  14. 1 2 3 4 5 6 LARGE VOLCANIC ERUPTIONS HELP PLANTS ABSORB MORE CARBON DIOXIDE FROM THE ATMOSPHERE
  15. ↑ Self, S. (August 15, 2006). "The effects and consequences of very large explosive volcanic eruptions". Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. 364 (1845): 2073–2097. Bibcode:2006RSPTA.364.2073S. doi:10.1098/rsta.2006.1814. PMID   16844649. S2CID   28228518.
  16. 1 2 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.
  17. ↑ "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" (PDF). rutgers.edu. Retrieved April 4, 2018.
  18. 1 2 L., Gu; D., Baldocchi (December 1, 2001). "Roles of volcanic eruptions, aerosols and clouds in global carbon cycle". AGU Fall Meeting Abstracts. 2001: B51A–0194. Bibcode:2001AGUFM.B51A0194G.
  19. ↑ "Response of a Deciduous Forest to the Mount Pinatubo Eruption: Enhanced Photosynthesis. Gu et al., 28 March 2003 Journal of Science Vol 299" (PDF). utoledo.edu. Archived from the original (PDF) on March 4, 2016. Retrieved April 4, 2018.
  20. ↑ "CO2 Science". www.co2science.org. Retrieved April 4, 2018.
  21. ↑ http://earthobservatory.nasa.gov/Features/GlobalGarden/ Global Garden gets greener. NASA 2003
  22. ↑ "Cooling Following LargeVolcanic Eruptions Corrected for the Effect of Diffuse Radiation on Tree Rings. Alan Robock, 2005. Figure 1" (PDF). rutgers.edu. Retrieved April 4, 2018.
  23. ↑ Chakraborty, TC; Lee, Xuhui; Lawrence, David M. (2021). "Strong Local Evaporative Cooling Over Land Due to Atmospheric Aerosols". Journal of Advances in Modeling Earth Systems. 13 (5). Bibcode:2021JAMES..1302491C. doi: 10.1029/2021ms002491 . ISSN   1942-2466. S2CID   236541532.
  24. ↑ 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
  25. ↑ 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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