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| {{Expert-subject|date = July 2015}}'''Solar irradiance''' is the power per unit area produced by the Sun in the form of electromagnetic radiation. Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering. Total solar irradiance (TSI), is a measure of the solar radiative power per unit area normal to the rays, incident on the Earth's upper atmosphere. The ] is a conventional measure of mean TSI at a distance of one ] (AU). Irradiance is a function of distance from the Sun, the solar cycle, and cross-cycle changes.<ref name="Boxwell">Michael Boxwell, ''Solar Electricity Handbook: A Simple, Practical Guide to Solar Energy'' (2012), p. 41–42.</ref> Irradiance on Earth is most intense at points directly facing (normal to) the Sun. | {{Expert-subject|date = July 2015}}'''Solar irradiance''' (also Insolation from ] ''insolare'', to expose to the sun)<ref>{{cite web|url = http://www.merriam-webster.com/dictionary/insolation|title = Insolation - Definition of insolation by Merriam-Webster|work = merriam-webster.com}}</ref><ref>{{cite web|url = http://www.etymonline.com/index.php?allowed_in_frame=0&search=insolation&searchmode=none|title = Online Etymology Dictionary|work = etymonline.com}}</ref> is the power per unit area produced by the Sun in the form of electromagnetic radiation. Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering. Total solar irradiance (TSI), is a measure of the solar radiative power per unit area normal to the rays, incident on the Earth's upper atmosphere. The ] is a conventional measure of mean TSI at a distance of one ] (AU). Irradiance is a function of distance from the Sun, the solar cycle, and cross-cycle changes.<ref name="Boxwell">Michael Boxwell, ''Solar Electricity Handbook: A Simple, Practical Guide to Solar Energy'' (2012), p. 41–42.</ref> Irradiance on Earth is most intense at points directly facing (normal to) the Sun.] (TOA) and at the planet's surface]] | ||
| == Units == | |||
| The unit recommended by the ] is the megajoule per square metre (MJ/m<sup href="Earth Radiation Budget Satellite">2</sup>) or joule per square millimetre (J/mm<sup href="Solar Heliospheric Observatory">2</sup>).<ref>{{cite web|url = http://www.wmo.int/pages/prog/www/IMOP/publications/CIMO-Guide/CIMO%20Guide%207th%20Edition,%202008/Part%20I/Chapter%201.pdf|title = World Meteorological Organization - WMO|author = WMO Webteam|work = wmo.int}}</ref> | |||
| An alternate unit of measure is the ] (1 ] per square centimeter or 41,840 J/m<sup>2</sup>) or irradiance per unit time. | |||
| == Measurement == | |||
| All TSI satellite instruments employ a common approach, ]. This technique applies measured electrical heating to maintain an absorptive blackened cavity in thermal equilibrium while incident sunlight passes through a precision ] of calibrated area. The aperture is modulated via a ]. In orbit, ] calibrations drift for reasons including solar degradation of the cavity, electronic degradation of the heater, surface degradation of the precision aperture and varying surface emissions and temperatures that alter thermal backgrounds. These calibrations require compensation to preserve consistent measurements.<ref name=Kopp>{{cite journal |title=A new, lower value of total solar irradiance: Evidence and climate significance |first1=Greg |last1=Kopp |first2=Judith L. |last2=Lean |date=14 January 2011 |journal= Geophysical Research Letters |doi= 10.1029/2010GL045777 |url=http://onlinelibrary.wiley.com/doi/10.1029/2010GL045777/full |accessdate= July 2015}}</ref> | |||
| The ] business uses ] per square metre (Wh/m<sup>2</sup>). Divided by the recording time, this measure becomes insolation, another unit of irradiance. | |||
| The space-based TSI record comprises measurements from more than ten radiometers spanning three solar cycles. For various reasons, the sources do not always agree. The Solar Radiation and Climate Experiment/Total Irradiance Measurement (]/TIM) TSI values are lower than prior measurements by the Earth Radiometer Budget Experiment (ERBE) on the ] (ERBS), VIRGO on the ] (SoHO) and the ACRIM instruments on the ] (SMM), ] (UARS), and ]. Ground calibrations relied on component rather than system level measurements, since irradiance standards prior to their launches lacked absolute accuracies.<ref name=Kopp/> | |||
| Insolation can be measured in space, at the edge of the atmosphere or at a terrestrial object. | |||
| Uncertainties of individual irradiance observations exceed solar irradiance variations (∼0.1%). Thus, instrument stability and measurement continuity are relied upon to compute real variations. Instrument stability involves exposing redundant radiometer cavities to different accumulations of solar radiation to quantify exposure-dependent degradation effects that are then compensated for in final data. Sequential radiometer observation overlaps permits corrections for absolute offsets and validation of instrumental drifts.<ref name=Kopp/> | |||
| Insolation can also be expressed in Suns, where one Sun equals 1000 W/m<sup>2 at the point of arrival, with kWh/m<sup>2</sup>/day expressed as hours/day.<ref> retrieved 29 October 2012</ref> | |||
| Despite the fact that ACRIM I, ACRIM II, ACRIM III, VIRGO, and TIM all track degradation with redundant cavities, notable and unexplained differences remain in irradiance and the modeled influences of ] and ]. Features not easily attributable to solar activity include an annual cycle that is nearly in phase with the Sun-Earth distance in ACRIM III data, and 90-day spikes in the VIRGO data coincident with SoHO spacecraft maneuvers that are most apparent during the 2008 solar minimum. Disagreement among overlapping observations indicates unresolveded drifts that suggest the TSI record is not sufficiently stable to discern solar changes on decadal time scales. Only the ACRIM composite shows irradiance increasing by ∼1 W m−2 between 1986 and 1996; this change is also absent in the model.<ref name=Kopp/> | |||
| == Absorption and reflection == | |||
| Reaching an object, part of the irradiance is absorbed and the remainder reflected. Usually the absorbed radiation is converted to thermal energy, increasing the object's temperature. Manmade or natural systems, however, can convert part of the absorbed radiation into another form such as electricity or chemical bonds, as in the case of ] cells or ]. The proportion of reflected radiation is the object's ] or ]. | |||
| == Projection effect == | |||
| ]Insolation onto a surface is largest when the surface directly faces (is normal to) the sun. As the angle between the surface and the Sun moves from normal, the insolation is reduced in proportion to the angle's ]; see ]. | |||
| In the figure, the angle shown is between the ground and the sunbeam rather than between the vertical direction and the sunbeam; hence the sine rather than the cosine is appropriate. A sunbeam one mile (1.6 km) wide arrives from directly overhead, and another at a 30° angle to the horizontal. The ] of a 30° angle is 1/2, whereas the sine of a 90° angle is 1. Therefore, the angled sunbeam spreads the light over twice the area. Consequently, half as much light falls on each square mile. | |||
| Recommendations to resolve the instrument discrepancies include validating optical measurement accuracy by comparing ground-based instruments to laboratory references, such as those at ] (NIST); NIST validation of aperture area calibrations using spares from each instrument; and applying ] corrections from the view-limiting aperture.<ref name=Kopp/> | |||
| This 'projection effect' is the main reason why Earth's ] are much colder than ]. On an annual average the poles receive less insolation than does the equator, because the poles are always angled more away from the sun than the tropics. At a lower angle the light must travel through more atmosphere. This attenuates it (by absorption and scattering) further reducing insolation. | |||
| For ACRIM, NIST determined that diffraction from the view-limiting aperture contributes a 0.13% signal not accounted for in the three ACRIM instruments. This correction lowers the reported ACRIM values, bringing ACRIM closer to TIM. In ACRIM and all other instruments, the aperture is deep inside the instrument, with a larger view-limiting aperture at the front. Depending on edge imperfections this can directly scatter light into the cavity. This design admits two to three times the amount of light intended to be measured; if not completely absorbed or scattered, this additional light produces erroneously high signals. In contrast, TIM's design places the precision aperture at the front so that only desired light enters.<ref name=Kopp/> | |||
| == Categories == | |||
| ] | |||
| ] is measured at a given location with a surface element perpendicular to the Sun. It excludes diffuse insolation (radiation that is scattered or reflected by atmospheric components). Direct insolation is equal to the ] minus the atmospheric losses due to ] and ]. While the solar constant varies, losses depend on time of day (length of light's path through the atmosphere depending on the ]), ], ] content and other ]. Insolation affects plant metabolism and animal behavior.<ref>C.Michael Hogan. 2010. . Washington DC</ref> | |||
| Diffuse insolation is the contribution of light scattered by the atmosphere to total insolation. | |||
| === TSI Radiometer Facility === | |||
| TIM's high absolute accuracy creates new opportunities for measuring climate variables. TSI Radiometer Facility (TRF) is a cryogenic ] that operates in a vacuum with controlled light sources. L-1 Standards and Technology (LASP) designed and built the system, completed in 2008. It was calibrated for optical power against the NIST Primary Optical Watt Radiometer, a cryogenic radiometer that maintains the NIST radiant power scale to an uncertainty of 0.02% (1''σ''). As of 2011 TRF was the only facility that approached the desired <0.01% uncertainty for pre-launch validation of solar radiometers measuring irradiance (rather than merely optical power) at solar power levels and under vacuum conditions.<ref name="Kopp" /> | |||
| == Earth == | |||
| TRF encloses both the reference radiometer and the instrument under test in a common vacuum system that contains a stationary, spatially uniform illuminating beam. A precision aperture with area calibrated to 0.0031% (1''σ'') determines the beam's measured portion. The test instrument's precision aperture is positioned in the same location, without optically altering the beam, for direct comparison to the reference. Variable beam power provides linearity diagnostics, and variable beam diameter diagnoses scattering from different instrument components.<ref name="Kopp" /> | |||
| Average annual solar radiation arriving at the top of the Earth's atmosphere is roughly 1366 W/m<sup>2.<ref></ref><ref name="www.pmodwrc.ch.91">{{cite web | |||
| |title = Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present|url = http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant|accessdate = February 2, 2009|at = Figure 4 & figure 5}}</ref> The radiation is distributed across the ], although most is ]. The Sun's rays are ] as they pass through the ], leaving maximum normal surface irradiance at approximately 1000 W /m<sup>2 at ] on a clear day.{{Clarify|reason = At what distance from the sun?|pre-text = |date = July 2015}}], a component of a temporary remote meteorological station, measures insolation on ], ].]]The actual figure varies with the Sun's angle and atmospheric circumstances. Ignoring clouds, the daily average irradiance for the Earth is approximately 250 W/m<sup>2//hr = 6 kWh/m<sup>2. | |||
| The output of, for example, a ] panel, partly depends on the angle of the sun relative to the panel. One Sun is a unit of ], not a standard value for actual insolation. Sometimes this unit is referred to as a Sol, not to be confused with a ''sol'', meaning ].<ref>{{cite web |url = http://www.giss.nasa.gov/tools/mars24/help/notes.html|title = Technical Notes on Mars Solar Time|author = Michael Allison|author2 = Robert Schmunk|last-author-amp = yes|date = 5 August 2008|publisher = ]|accessdate = 16 January 2012}}</ref>{{clear}} | |||
| The Glory/TIM and PICARD/PREMOS flight instrument absolute scales are now traceable to the TRF in both optical power and irradiance. The resulting high accuracy reduces the consequences of any future gap in the solar irradiance record.<ref name="Kopp" /> | |||
| === Solar potential maps === | |||
| {| class="wikitable" | |||
| <gallery> | |||
| |+ Difference Relative to TRF<ref name="Kopp" /> | |||
| File:SolarGIS-Solar-map-North-America-en.png|North America | |||
| !Instrument!!Irradiance: View-Limiting Aperture Overfilled!!Irradiance: Precision Aperture Overfilled!!Difference Attributable To Scatter Error!!Measured Optical Power Error!!Residual Irradiance Agreement||Uncertainty | |||
| File:SolarGIS-Solar-map-Latin-America-en.png|South America | |||
| |- | |||
| File:SolarGIS-Solar-map-Europe-en.png|Europe | |||
| |SORCE/TIM ground||NA||−0.037%||NA||−0.037%||0.000%||0.032% | |||
| File:SolarGIS-Solar-map-Africa-and-Middle-East-en.png|Africa and Middle East | |||
| |- | |||
| File:SolarGIS-Solar-map-South-And-South-East-Asia-en.png|South and South-East Asia | |||
| |Glory/TIM flight||NA||−0.012%||NA||−0.029%||0.017%||0.020% | |||
| File:SolarGIS-Solar-map-Australia-en.png|Australia | |||
| |- | |||
| </gallery> | |||
| |PREMOS-1 ground||−0.005%||−0.104%||0.098%||−0.049%||−0.104%||∼0.038% | |||
| === Top of the atmosphere === | |||
| |- | |||
| ]]The distribution of solar radiation at the top of the atmosphere is determined by Earth's sphericity and orbital parameters. This applies to any unidirectional beam incident to a rotating sphere. Insolation is essential for ] and understanding ] and ]. Its application to ] is known as ]. | |||
| |PREMOS-3 flight||0.642%||0.605%||0.037%||0.631%||−0.026%||∼0.027% | |||
| |- | |||
| |VIRGO-2 ground||0.897%||0.743%||0.154%||0.730%||0.013%||∼0.025% | |||
| |} | |||
| Distribution is based on a fundamental identity from ], the ]: | |||
| == Models == | |||
| <dd><math>\cos(c) = \cos(a) \cos(b) + \sin(a) \sin(b) \cos(C) \, </math></dd> | |||
| where ''a'', ''b'' and ''c'' are arc lengths, in radians, of the sides of a spherical triangle. <i>C</i> is the angle in the vertex opposite the side which has arc length <i>c</i>. Applied to the calculation of ] Θ, the following applies to the ]: | |||
| :<math>C=h \, </math> | |||
| :<math>c=\Theta \, </math> | |||
| === Double dynamo === | |||
| In 2015, a new model of the solar cycle was published that produced more accurate predictions of solar irregularities. The model draws on dynamo effects in two layers of the Sun, one close to the surface and one deep within its ]. Model predictions suggest that solar activity will fall by 60 per cent during the 2030s to conditions last seen during the ']' that began in 1645. Prior models included only the deeper dynamo.<ref name=":1">{{Cite web|title = Solar activity predicted to fall 60% in 2030s, to 'mini ice age' levels: Sun driven by double dynamo|url = http://www.sciencedaily.com/releases/2015/07/150709092955.htm|website = www.sciencedaily.com|accessdate = 2015-07-11|publisher = Science Daily|date = July 9, 2015}}</ref> | |||
| :<math>a=\tfrac{1}{2}\pi-\phi \, </math> | |||
| The model features paired magnetic wave components. Both components have a frequency of approximately 11 years, although their frequencies are slightly different and temporally offset. Over the cycle, the waves fluctuate between the Sun's northern and southern hemispheres.<ref name=":1" /> | |||
| : <math>b=\tfrac{1}{2}\pi-\delta \, </math> | |||
| The model used ]' of the ] observations from the Wilcox Solar Observatory. They examined magnetic field activity from ]-], covering 1976-2008. They also compared their predictions to average ] numbers. The model was 97% accurate in predicting solar activity fluctuations.<ref name=":1" /> | |||
| : <math>\cos(\Theta) = \sin(\phi) \sin(\delta) + \cos(\phi) \cos(\delta) \cos(h) \, </math> | |||
| The separation of Earth from the sun can be denoted R<sub>E</sub> and the mean distance can be denoted R<sub>0</sub>, approximately 1 AU. The ] is denoted S<sub>0</sub>. The solar flux density (insolation) onto a plane tangent to the sphere of the Earth, but above the bulk of the atmosphere (elevation 100 km or greater) is: | |||
| : <math>Q = S_o \frac{R_o^2}{R_E^2}\cos(\Theta)\text{ when }\cos(\Theta)>0</math> | |||
| and | |||
| : <math>Q=0\text{ when }\cos(\Theta)\le 0 \, </math> | |||
| The average of ''Q'' over a day is the average of ''Q'' over one rotation, or the hour angle progressing from ''h'' = π to ''h'' = −π: | |||
| : <math>\overline{Q}^{\text{day}} = -\frac{1}{2\pi}{\int_{\pi}^{-\pi}Q\,dh}</math> | |||
| Let ''h''<sub>0</sub> be the hour angle when Q becomes positive. This could occur at sunrise when <math>\Theta=\tfrac{1}{2}\pi</math>, or for ''h''<sub>0</sub> as a solution of | |||
| : <math>\sin(\phi) \sin(\delta) + \cos(\phi) \cos(\delta) \cos(h_o) = 0 \,</math> | |||
| or | |||
| : <math>\cos(h_o)=-\tan(\phi)\tan(\delta)</math> | |||
| If tan(φ)tan(δ) > 1, then the sun does not set and the sun is already risen at ''h'' = π, so h<sub>o</sub> = π. | |||
| If tan(φ)tan(δ) < −1, the sun does not rise and <math>\overline{Q}^{\mathrm{day}}=0</math>. | |||
| <math>\frac{R_o^2}{R_E^2}</math> is nearly constant over the course of a day, and can be taken outside the integral | |||
| : <math>\int_\pi^{-\pi}Q\,dh = \int_{h_o}^{-h_o}Q\,dh = S_o\frac{R_o^2}{R_E^2}\int_{h_o}^{-h_o}\cos(\Theta)\, dh </math> | |||
| : <math> \int_\pi^{-\pi}Q\,dh = S_o\frac{R_o^2}{R_E^2}\left_{h=h_o}^{h=-h_o}</math> | |||
| : <math> \int_\pi^{-\pi}Q\,dh = -2 S_o\frac{R_o^2}{R_E^2}\left</math> | |||
| : <math> \overline{Q}^{\text{day}} = \frac{S_o}{\pi}\frac{R_o^2}{R_E^2}\left</math> | |||
| Let θ be the conventional polar angle describing a planetary ]. Let ''θ'' = 0 at the vernal ]. The ] δ as a function of orbital position is | |||
| : <math>\sin \delta = \sin \varepsilon~\sin(\theta - \varpi )\, </math> | |||
| where ε is the ]. The conventional ] ϖ is defined relative to the vernal equinox, so for the elliptical orbit: | |||
| : <math>R_E=\frac{R_o}{1+e\cos(\theta-\varpi)}</math> | |||
| or | |||
| : <math>\frac{R_o}{R_E}={1+e\cos(\theta-\varpi)}</math> | |||
| With knowledge of ϖ, ε and ''e'' from astrodynamical calculations <ref> {{dead link|date = July 2015}} | |||
| </ref> and S<sub>o</sub> from a consensus of observations or theory, <math>\overline{Q}^{\mathrm{day}}</math>can be calculated for any latitude φ and θ. Because of the elliptical orbit, and as a consequence of ], ''θ'' does not progress uniformly with time. Nevertheless, ''θ'' = 0° is exactly the time of the vernal equinox, ''θ'' = 90° is exactly the time of the summer solstice, ''θ'' = 180° is exactly the time of the autumnal equinox and ''θ'' = 270° is exactly the time of the winter solstice. | |||
| === Milankovitch cycles === | |||
| Obtaining a time series for a <math>\overline{Q}^{\mathrm{day}}</math> for a particular time of year, and particular latitude, is a useful application in the theory of Milankovitch cycles. For example, at the summer solstice, the declination δ is equal to the obliquity ε. The distance from the sun is | |||
| : <math>\frac{R_o}{R_E} = 1+e\cos(\theta-\varpi) = 1+e\cos(\tfrac{\pi}{2}-\varpi) = 1 + e \sin(\varpi)</math> | |||
| For this summer solstice calculation, the role of the elliptical orbit is entirely contained within the important product <math>e \sin(\varpi)</math>, the ] index, whose variation dominates the variations in insolation at 65° N when eccentricity is large. For the next 100,000 years, with variations in eccentricity being relatively small, variations in obliquity dominate.{{wide image|InsolationSummerSolstice65N.png|600px|Past and future of daily average insolation at top of the atmosphere on the day of the summer solstice, at 65 N latitude. The green curve is with eccentricity ''e'' hypothetically set to 0. The red curve uses the actual (predicted) value of ''e''. Blue dot is current conditions, at 2 ky A.D. | |||
| }} | |||
| == Applications == | == Applications == | ||
| Solar irradiance is useful for capacity planning for ] installations.<ref name="Boxwell"/> | |||
| === Buildings === | |||
| ] is a concern for space travel. For example, the American space agency, ], launched its ] (SORCE) satellite with ]s. | |||
| In construction, insolation is an important consideration when designing a building for a particular site.<ref>{{cite journal | |||
| |last = Nall|first = D. H.|title = Looking across the water: Climate-adaptive buildings in the United States & Europe|journal = The Construction Specifier|volume = 57|issue = 2004-11|pages = 50–56|url = http://www.wspgroup.com/upload/Upload/Dan%20Nall%20Article%20PDF.pdf|format = PDF}}</ref> | |||
| ] | |||
| The projection effect can be used to design buildings that are cool in summer and warm in winter, by providing vertical windows on the equator-facing side of the building (the south face in the ], or the north face in the ]): this maximizes insolation in the winter months when the Sun is low in the sky and minimizes it in the summer when the Sun is high. (The ] through the sky spans 47 degrees through the year). | |||
| === Solar power === | |||
| Insolation figures are used as an input to worksheets to size ].<ref>{{cite web | |||
| |title = Determining your solar power requirements and planning the number of components|url = http://www.solar4power.com/solar-power-sizing.html}}</ref> Because (except for asphalt solar collectors)<ref>{{cite web|url = http://www.icax.co.uk/asphalt_solar_collector.html|title = Asphalt Solar Collector Renewable Heat for IHT - Solar Collectors - Solar Recharge for GSHP - Pavement Solar Collectors - Road Solar Thermal Collector|work = icax.co.uk}}</ref> panels are almost always mounted at an angle<ref>{{cite web|url = http://www.macslab.com/optsolar.html|title = Optimum solar panel angle|work = macslab.com}}</ref> towards the sun, insolation must be adjusted to prevent estimates that are inaccurately low for winter and inaccurately high for summer.<ref>{{cite web|url = http://www.redrok.com/concept.htm#complaint|title = Heliostat Concepts|work = redrok.com}}</ref> In many countries the figures can be obtained from an insolation map or from insolation tables that reflect data over the prior 30–50 years. Photovoltaic panels are rated under standard conditions to determine the Wp rating (]),<ref> {{dead link|date = July 2015}} | |||
| </ref> which can then be used with insolation to determine the expected output, adjusted by factors such as tilt, tracking and shading (which can be included to create the installed Wp rating).<ref></ref> Insolation values range from 800 to 950 kWh/(kWp·y) in ] to up to 2,900 in ]. | |||
| === Climate research === | === Climate research === | ||
| Line 66: | Line 119: | ||
| In 2014 a new ACRIM composite was developed using the updated ACRI M3 record. It added corrections for scattering and diffraction revealed during recent testing and two algorithm updates. The testing was performed at TRF. The algorithm updates were more accurately account for instrument thermal behavior and parsing of shutter cycle data. These corrected a component of the quasi-annual signal and increased the ], respectively. The net effect of these corrections decreased the average ACRIM3 TSI value from as above without affecting the trending in the ACRIM Composite TSI.<ref name=":0">{{Cite journal|title = ACRIM total solar irradiance satellite composite validation versus TSI proxy models|url = http://arxiv.org/abs/1403.7194|journal = Astrophysics and Space Science|date = April 2014|issn = 0004-640X|pages = 421–442|volume = 350|issue = 2|doi = 10.1007/s10509-013-1775-9|first = Nicola|last = Scafetta|first2 = Richard C.|last2 = Willson}}</ref> | In 2014 a new ACRIM composite was developed using the updated ACRI M3 record. It added corrections for scattering and diffraction revealed during recent testing and two algorithm updates. The testing was performed at TRF. The algorithm updates were more accurately account for instrument thermal behavior and parsing of shutter cycle data. These corrected a component of the quasi-annual signal and increased the ], respectively. The net effect of these corrections decreased the average ACRIM3 TSI value from as above without affecting the trending in the ACRIM Composite TSI.<ref name=":0">{{Cite journal|title = ACRIM total solar irradiance satellite composite validation versus TSI proxy models|url = http://arxiv.org/abs/1403.7194|journal = Astrophysics and Space Science|date = April 2014|issn = 0004-640X|pages = 421–442|volume = 350|issue = 2|doi = 10.1007/s10509-013-1775-9|first = Nicola|last = Scafetta|first2 = Richard C.|last2 = Willson}}</ref> | ||
| Differences between ACRIM and PMOD TSI composites are evident, but the most significant is the solar minimum-to-minimum trends during ]-]. ACRIM established an increase of +0.037%/decade from 1980 to 2000 and a decrease thereafter. PMOD instead presents a steady decrease since 1978. Significant differences can also be seen during the peak of solar cycles 21 and 22. These arise from the fact that ACRIM uses the original TSI results published by the satellite experiment teams while PMOD significantly modifies some results to conform them to specific TSI proxy models. . The implications of increasing TSI during the global warming of the last two decades of the 20th century are that solar forcing of climate change may be a significantly larger factor than represented in the ] ].<ref name=":0"/> | Differences between ACRIM and PMOD TSI composites are evident, but the most significant is the solar minimum-to-minimum trends during ]-]. ACRIM established an increase of +0.037%/decade from 1980 to 2000 and a decrease thereafter. PMOD instead presents a steady decrease since 1978. Significant differences can also be seen during the peak of solar cycles 21 and 22. These arise from the fact that ACRIM uses the original TSI results published by the satellite experiment teams while PMOD significantly modifies some results to conform them to specific TSI proxy models. . The implications of increasing TSI during the global warming of the last two decades of the 20th century are that solar forcing of climate change may be a significantly larger factor than represented in the ] ].<ref name=":0" /> | ||
| === Space travel === | |||
| Insolation is the primary variable affecting ] in ] design and ]. | |||
| ] is a concern for space travel. For example, the American space agency, ], launched its ] (SORCE) satellite with ]s.<ref name="Boxwell" /> | |||
| === Civil engineering === | |||
| In ] and ], numerical models of ] runoff use observations of insolation. This permits estimation of the rate at which water is released from a melting snowpack. Field measurement is accomplished using a ]. | |||
| {| class="wikitable" border="1" | |||
| ! colspan="6" style="background:lightgreen;" |Conversion factor (multiply top row by factor to obtain side column) | |||
| |- | |||
| ! | |||
| ! W/m<sup>2</sup> | |||
| ! kW·h/(m<sup>2</sup>·day) | |||
| ! sun hours/day | |||
| ! kWh/(m<sup>2</sup>·y) | |||
| ! kWh/(kWp·y) | |||
| |- | |||
| ! W/m<sup>2</sup> | |||
| | 1 | |||
| | 41.66666 | |||
| | 41.66666 | |||
| | 0.1140796 | |||
| | 0.1521061 | |||
| |- | |||
| ! kW·h/(m<sup>2</sup>·day) | |||
| | 0.024 | |||
| | 1 | |||
| | 1 | |||
| | 0.0027379 | |||
| | 0.0036505 | |||
| |- | |||
| ! sun hours/day | |||
| | 0.024 | |||
| | 1 | |||
| | 1 | |||
| | 0.0027379 | |||
| | 0.0036505 | |||
| |- | |||
| ! kWh/(m<sup>2</sup>·y) | |||
| | 8.765813 | |||
| | 365.2422 | |||
| | 365.2422 | |||
| | 1 | |||
| | 1.333333 | |||
| |- | |||
| ! kWh/(kWp·y) | |||
| | 6.574360 | |||
| | 273.9316 | |||
| | 273.9316 | |||
| | 0.75 | |||
| | 1 | |||
| |} | |||
| == Measurement == | |||
| All TSI satellite instruments employ a common approach, ]. This technique applies measured electrical heating to maintain an absorptive blackened cavity in thermal equilibrium while incident sunlight passes through a precision ] of calibrated area. The aperture is modulated via a ]. In orbit, ] calibrations drift for reasons including solar degradation of the cavity, electronic degradation of the heater, surface degradation of the precision aperture and varying surface emissions and temperatures that alter thermal backgrounds. These calibrations require compensation to preserve consistent measurements.<ref name=Kopp>{{cite journal |title=A new, lower value of total solar irradiance: Evidence and climate significance |first1=Greg |last1=Kopp |first2=Judith L. |last2=Lean |date=14 January 2011 |journal= Geophysical Research Letters |doi= 10.1029/2010GL045777 |url=http://onlinelibrary.wiley.com/doi/10.1029/2010GL045777/full |accessdate= July 2015}}</ref> | |||
| The space-based TSI record comprises measurements from more than ten radiometers spanning three solar cycles. For various reasons, the sources do not always agree. The Solar Radiation and Climate Experiment/Total Irradiance Measurement (]/TIM) TSI values are lower than prior measurements by the Earth Radiometer Budget Experiment (ERBE) on the ] (ERBS), VIRGO on the ] (SoHO) and the ACRIM instruments on the ] (SMM), ] (UARS), and ]. Ground calibrations relied on component rather than system level measurements, since irradiance standards prior to their launches lacked absolute accuracies.<ref name=Kopp/> | |||
| Uncertainties of individual irradiance observations exceed solar irradiance variations (∼0.1%). Thus, instrument stability and measurement continuity are relied upon to compute real variations. Instrument stability involves exposing redundant radiometer cavities to different accumulations of solar radiation to quantify exposure-dependent degradation effects that are then compensated for in final data. Sequential radiometer observation overlaps permits corrections for absolute offsets and validation of instrumental drifts.<ref name=Kopp/> | |||
| Despite the fact that ACRIM I, ACRIM II, ACRIM III, VIRGO, and TIM all track degradation with redundant cavities, notable and unexplained differences remain in irradiance and the modeled influences of ] and ]. Features not easily attributable to solar activity include an annual cycle that is nearly in phase with the Sun-Earth distance in ACRIM III data, and 90-day spikes in the VIRGO data coincident with SoHO spacecraft maneuvers that are most apparent during the 2008 solar minimum. Disagreement among overlapping observations indicates unresolveded drifts that suggest the TSI record is not sufficiently stable to discern solar changes on decadal time scales. Only the ACRIM composite shows irradiance increasing by ∼1 W m−2 between 1986 and 1996; this change is also absent in the model.<ref name=Kopp/> | |||
| Recommendations to resolve the instrument discrepancies include validating optical measurement accuracy by comparing ground-based instruments to laboratory references, such as those at ] (NIST); NIST validation of aperture area calibrations using spares from each instrument; and applying ] corrections from the view-limiting aperture.<ref name=Kopp/> | |||
| For ACRIM, NIST determined that diffraction from the view-limiting aperture contributes a 0.13% signal not accounted for in the three ACRIM instruments. This correction lowers the reported ACRIM values, bringing ACRIM closer to TIM. In ACRIM and all other instruments, the aperture is deep inside the instrument, with a larger view-limiting aperture at the front. Depending on edge imperfections this can directly scatter light into the cavity. This design admits two to three times the amount of light intended to be measured; if not completely absorbed or scattered, this additional light produces erroneously high signals. In contrast, TIM's design places the precision aperture at the front so that only desired light enters.<ref name=Kopp/> | |||
| === TSI Radiometer Facility === | |||
| TIM's high absolute accuracy creates new opportunities for measuring climate variables. TSI Radiometer Facility (TRF) is a cryogenic ] that operates in a vacuum with controlled light sources. L-1 Standards and Technology (LASP) designed and built the system, completed in 2008. It was calibrated for optical power against the NIST Primary Optical Watt Radiometer, a cryogenic radiometer that maintains the NIST radiant power scale to an uncertainty of 0.02% (1''σ''). As of 2011 TRF was the only facility that approached the desired <0.01% uncertainty for pre-launch validation of solar radiometers measuring irradiance (rather than merely optical power) at solar power levels and under vacuum conditions.<ref name="Kopp" /> | |||
| TRF encloses both the reference radiometer and the instrument under test in a common vacuum system that contains a stationary, spatially uniform illuminating beam. A precision aperture with area calibrated to 0.0031% (1''σ'') determines the beam's measured portion. The test instrument's precision aperture is positioned in the same location, without optically altering the beam, for direct comparison to the reference. Variable beam power provides linearity diagnostics, and variable beam diameter diagnoses scattering from different instrument components.<ref name="Kopp" /> | |||
| The Glory/TIM and PICARD/PREMOS flight instrument absolute scales are now traceable to the TRF in both optical power and irradiance. The resulting high accuracy reduces the consequences of any future gap in the solar irradiance record.<ref name="Kopp" /> | |||
| {| class="wikitable" | |||
| |+ Difference Relative to TRF<ref name="Kopp" /> | |||
| !Instrument!!Irradiance: View-Limiting Aperture Overfilled!!Irradiance: Precision Aperture Overfilled!!Difference Attributable To Scatter Error!!Measured Optical Power Error!!Residual Irradiance Agreement||Uncertainty | |||
| |- | |||
| |SORCE/TIM ground||NA||−0.037%||NA||−0.037%||0.000%||0.032% | |||
| |- | |||
| |Glory/TIM flight||NA||−0.012%||NA||−0.029%||0.017%||0.020% | |||
| |- | |||
| |PREMOS-1 ground||−0.005%||−0.104%||0.098%||−0.049%||−0.104%||∼0.038% | |||
| |- | |||
| |PREMOS-3 flight||0.642%||0.605%||0.037%||0.631%||−0.026%||∼0.027% | |||
| |- | |||
| |VIRGO-2 ground||0.897%||0.743%||0.154%||0.730%||0.013%||∼0.025% | |||
| |} | |||
| ==References== | ==References== | ||
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| ==See also== | ==See also== | ||
| {{Portal|Renewable energy|Energy}} | |||
| * ] | * ] | ||
| * ] (photosynthesis-irradiance curve) | * ] (photosynthesis-irradiance curve) | ||
| * ] |
* ] | ||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] ] (TOA) and at the planet's surface]] | |||
| == External links == | |||
| {{Wiktionary|insolation}} | |||
| {{external links|date = July 2015}} | |||
| <!-- Average irradiance across the Earth = irradiance perpendicular to the Sun's rays * cross-sectional area of the Earth / surface area of the Earth = 1000 W/m^2 * (pi * R^2) / (4 * pi * R^2) = 250 W/m^2 where R is the radius of the Earth. -->* | |||
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| * choose "Theoretically Perfect Collector" to receive results for the insolation on a tilted surface | |||
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| * SMARTS, software to compute solar insolation of each date/location of earth | |||
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| ] | ] | ||
Revision as of 06:26, 5 August 2015
 | This article needs attention from an expert on the subject. Please add a reason or a talk parameter to this template to explain the issue with the article. When placing this tag, consider associating this request with a WikiProject. (July 2015) |
Solar irradiance (also Insolation from Latin insolare, to expose to the sun) is the power per unit area produced by the Sun in the form of electromagnetic radiation. Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering. Total solar irradiance (TSI), is a measure of the solar radiative power per unit area normal to the rays, incident on the Earth's upper atmosphere. The solar constant is a conventional measure of mean TSI at a distance of one Astronomical Unit (AU). Irradiance is a function of distance from the Sun, the solar cycle, and cross-cycle changes. Irradiance on Earth is most intense at points directly facing (normal to) the Sun.
Units
The unit recommended by the World Meteorological Organization is the megajoule per square metre (MJ/m) or joule per square millimetre (J/mm).
An alternate unit of measure is the Langley (1 thermochemical calorie per square centimeter or 41,840 J/m) or irradiance per unit time.
The solar energy business uses watt-hour per square metre (Wh/m). Divided by the recording time, this measure becomes insolation, another unit of irradiance.
Insolation can be measured in space, at the edge of the atmosphere or at a terrestrial object.
Insolation can also be expressed in Suns, where one Sun equals 1000 W/m
Absorption and reflection
Reaching an object, part of the irradiance is absorbed and the remainder reflected. Usually the absorbed radiation is converted to thermal energy, increasing the object's temperature. Manmade or natural systems, however, can convert part of the absorbed radiation into another form such as electricity or chemical bonds, as in the case of Photovoltaic cells or Plants. The proportion of reflected radiation is the object's Reflectivity or albedo.
Projection effect
Insolation onto a surface is largest when the surface directly faces (is normal to) the sun. As the angle between the surface and the Sun moves from normal, the insolation is reduced in proportion to the angle's Cosine; see Effect_of_sun_angle_on_climate.
In the figure, the angle shown is between the ground and the sunbeam rather than between the vertical direction and the sunbeam; hence the sine rather than the cosine is appropriate. A sunbeam one mile (1.6 km) wide arrives from directly overhead, and another at a 30° angle to the horizontal. The Sine of a 30° angle is 1/2, whereas the sine of a 90° angle is 1. Therefore, the angled sunbeam spreads the light over twice the area. Consequently, half as much light falls on each square mile.
This 'projection effect' is the main reason why Earth's polar regions are much colder than equatorial regions. On an annual average the poles receive less insolation than does the equator, because the poles are always angled more away from the sun than the tropics. At a lower angle the light must travel through more atmosphere. This attenuates it (by absorption and scattering) further reducing insolation.
Categories
Direct insolation is measured at a given location with a surface element perpendicular to the Sun. It excludes diffuse insolation (radiation that is scattered or reflected by atmospheric components). Direct insolation is equal to the Solar_constant minus the atmospheric losses due to absorption and scattering. While the solar constant varies, losses depend on time of day (length of light's path through the atmosphere depending on the Solar_elevation_angle), Cloud_cover, Moisture content and other contents. Insolation affects plant metabolism and animal behavior.
Diffuse insolation is the contribution of light scattered by the atmosphere to total insolation.
Earth
Average annual solar radiation arriving at the top of the Earth's atmosphere is roughly 1366 W/m
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