Editing Direct insolation (section)

==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>