Editing Direct insolation

{{disputed|date=November 2017}}
'''Direct insolation''' is the [[insolation]] measured at a given location on [[Earth]] with a surface element [[perpendicular]] to the [[Sun's ray]]s, excluding [[diffuse sky radiation|diffuse insolation]] (the solar radiation that is [[scattering|scattered]] or [[reflection (physics)|reflected]] by atmospheric components in the [[sky]]). Direct insolation is equal to the [[solar irradiance]] above the atmosphere minus the atmospheric losses due to [[absorption (electromagnetic radiation)|absorption]] and scattering. While the solar irradiance above the atmosphere varies with the Earth&ndash;Sun distance and [[solar cycle]]s, the losses depend on the time of day (length of light's path through the atmosphere depending on the [[solar elevation angle]]), [[cloud cover]], [[humidity]], and other [[air pollution|impurities]].

==Simplified formula==
A simple formula gives the approximate level of direct insolation when there are no clouds:<ref>{{cite web|title=Calculation of Solar Insolation|url=https://www.pveducation.org/pvcdrom/properties-of-sunlight/calculation-of-solar-insolation|website=PVEducation.org|archiveurl=https://web.archive.org/web/20160129052640/http://www.pveducation.org/pvcdrom/properties-of-sunlight/calculation-of-solar-insolation|archivedate=January 29, 2016|url-status=live}}</ref>

<math display="block">I_D=1.353\text{ kW/m}^2\times 0.7^{AM^{0.678}}</math>

where ''AM'' is the [[airmass]] given by

<math display="block">AM=\frac 1{\cos\theta}</math>

with θ being the zenith angle (90° minus the [[altitude (astronomy)|altitude]]) of the sun.

For the sun at the [[zenith]], this gives 947&nbsp;W/m<sup>2</sup>. However, another source states that [[direct sunlight]] under these conditions, with 1367&nbsp;W/m<sup>2</sup> above the atmosphere, is about 1050&nbsp;W/m<sup>2</sup>, and total insolation about 1120&nbsp;W/m<sup>2</sup>.<ref name="Solar constant at ground level">{{cite web|title=Introduction to Solar Radiation|url= http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx|publisher= Newport Corporation|url-status= live|archiveurl= https://web.archive.org/web/20131029234117/http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx|archivedate=October 29, 2013}}</ref>

==Average direct insolation==
For practical purposes, a time-average of the direct insolation over the course of the year is commonly used. This averaging takes into account the absence of sunlight during the night, increased scatter in the morning and evening hours, average effects of cloud cover and [[smog]], as well as seasonal variations of the mid-day solar elevation.

==Units of measurement==
Direct insolation is measured in watts per square metre (W/m<sup>2</sup>) or kilowatt-hours per square meter per day (kW·h/(m<sup>2</sup>·day)). 

{{block indent|1=1 kW·h/(m<sup>2</sup>·day) = 1,000 W · 1 hour / ( 1 m<sup>2</sup> · 24 hours) = 41.67 W/m<sup>2</sup>}}

In the case of photovoltaics, average direct insolation is commonly measured in terms of peak direct insolation as kWh/(kWp·y) (kilowatt hours per year per kilowatt peak rating).

==Applications==
Since radiation directly from the sun can be focussed with mirrors and lens, it can be applied to concentrated solar thermal (CST) systems.<ref>{{Cite journal|last=Boerema|first=Nicholas|last2=Morrison|first2=Graham|last3=Taylor|first3=Robert|last4=Rosengarten|first4=Gary|date=2013-11-01|title=High temperature solar thermal central-receiver billboard design|journal=Solar Energy|volume=97|pages=356–368|doi=10.1016/j.solener.2013.09.008|bibcode=2013SoEn...97..356B}}</ref><ref>{{Cite journal|last=Boerema|first=Nicholas|last2=Taylor|first2=Robert A.|last3=Morrison|first3=Graham|last4=Rosengarten|first4=Gary|date=2015-09-01|title=Solid–liquid phase change modelling of metallic sodium for application in solar thermal power plants|journal=Solar Energy|volume=119|pages=151–158|doi=10.1016/j.solener.2015.06.024|bibcode=2015SoEn..119..151B}}</ref><ref>{{Cite journal|last=Boerema|first=Nicholas|last2=Morrison|first2=Graham|last3=Taylor|first3=Robert|last4=Rosengarten|first4=Gary|date=2012-09-01|title=Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems|journal=Solar Energy|volume=86|issue=9|pages=2293–2305|doi=10.1016/j.solener.2012.05.001|bibcode=2012SoEn...86.2293B}}</ref> Due to clouds and aerosols, the direct insolation can fluctuate throughout the day, so forecasting the available resource is important in these applications <ref>{{Cite journal|last=Law|first=Edward W.|last2=Kay|first2=Merlinde|last3=Taylor|first3=Robert A.|date=2016-02-01|title=Calculating the financial value of a concentrated solar thermal plant operated using direct normal irradiance forecasts|journal=Solar Energy|volume=125|pages=267–281|doi=10.1016/j.solener.2015.12.031|bibcode=2016SoEn..125..267L}}</ref><ref>{{Cite journal|last=Law|first=Edward W.|last2=Prasad|first2=Abhnil A.|last3=Kay|first3=Merlinde|last4=Taylor|first4=Robert A.|date=2014-10-01|title=Direct normal irradiance forecasting and its application to concentrated solar thermal output forecasting – A review|journal=Solar Energy|volume=108|pages=287–307|doi=10.1016/j.solener.2014.07.008|bibcode=2014SoEn..108..287L}}</ref>

==See also==
* [[Solar irradiance]]- main article discussing total solar irradiance and subcomponents ''direct normal'', diffuse horizontal, global horizontal, global tilted and global normal irradiances.

==References==
{{reflist}}

==External links==
* [https://web.archive.org/web/20050420085336/http://www.nsdl.arm.gov/Library/glossary.shtml#Direct_insolation National Science Digital Library - Direct Insolation]

[[Category:Atmospheric radiation]]
[[Category:Visibility]]