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Daylight
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<h2 id="definition">Definition</h2> <p>Daylight is present at a particular location, to some degree, whenever the <a href="/facts/Sun/CadPCVHh">Sun</a> is above the local <a href="/facts/Horizon/LxwxWfC2">horizon</a>. This is true for slightly more than 50% of the <a href="/facts/Earth/HLLmDfj0">Earth</a> at any given time, since the Earth's atmosphere refracts some sunlight even when the Sun is below the horizon. </p><p>Outdoor <a href="/facts/Illuminance/6Dj4pKmu">illuminance</a> varies from 120,000 <a href="/facts/Lux/xa3AAqWd">lux</a> for direct <a href="/facts/Sunlight/WPkJRC7s">sunlight</a> at <a href="/facts/Noon/bEe082CA">noon</a>, which may cause eye <a href="/facts/Pain/70E79oUn">pain</a>, to less than 5 <a href="/facts/Lux/xa3AAqWd">lux</a> for thick storm <a href="/facts/Cloud/KDgo0Wqu">clouds</a> with the Sun at the horizon (even <1 lux for the most extreme case), which may make shadows from distant <a href="/facts/Street_light/Uuw2oH1y">street lights</a> visible. It may be darker under unusual circumstances like a <a href="/facts/Solar_eclipse/2XsCFG1a">solar eclipse</a> or very high levels of atmospheric <a href="/facts/Particulates/RleGr5E6">particulates</a>, which include smoke (see <a href="/facts/New_England%27s_Dark_Day/Y70hmIrE">New England's Dark Day</a>), dust,<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> and volcanic ash.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> </p> <h2 id="intensity-in-different-conditions">Intensity in different conditions</h2> <table><tbody><tr><th>Illuminance</th><th>Example</th></tr><tr><td>120,000 lux</td><td>Brightest <a href="/facts/Sunlight/WPkJRC7s">sunlight</a></td></tr><tr><td>111,000 lux</td><td>Bright sunlight</td></tr><tr><td>109,880 lux</td><td>AM 1.5 global solar spectrum sunlight (= 1,000.4 W/m2)<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a>[<i>circular reference</i>]</td></tr><tr><td>20,000 lux</td><td><a href="/facts/Shade_(shadow)/nuJRxKGj">Shade</a> illuminated by entire clear <a href="/facts/Diffuse_sky_radiation/HqPS9Ugv">blue sky</a>, midday</td></tr><tr><td>1,000–2,000 lux</td><td>Typical <a href="/facts/Overcast/1KmWPIYR">overcast</a> day, midday</td></tr><tr><td>400 lux</td><td><a href="/facts/Sunrise/hMAQ2oh2">Sunrise</a> or <a href="/facts/Sunset/vQ9h1P1J">sunset</a> on a clear day (ambient illumination)</td></tr><tr><td><200 lux</td><td>Extreme of thickest storm clouds, midday</td></tr><tr><td>40 lux</td><td>Fully overcast, sunset/sunrise</td></tr><tr><td><1 lux</td><td>Extreme of thickest storm clouds, sunset/rise</td></tr></tbody></table> <p>For comparison, nighttime illuminance levels are: </p> <table><tbody><tr><th>Illuminance</th><th>Example</th></tr><tr><td><1 lux</td><td><a href="/facts/Moonlight/dNlzKrem">Moonlight</a>,<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> clear <a href="/facts/Night_sky/IM1PakDh">night sky</a></td></tr><tr><td>0.25 lux</td><td>A <a href="/facts/Full_Moon/NRMWFPh2">full Moon</a>, clear night sky<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a><a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a></td></tr><tr><td>0.01 lux</td><td>A <a href="/facts/Lunar_phase/0JalCYrE">quarter Moon</a>, clear night sky</td></tr><tr><td>0.002 lux</td><td><a href="/facts/Starlight/nXTYlHSz">Starlight</a>, clear moonless night sky, including <a href="/facts/Airglow/9yMhcjB4">airglow</a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a></td></tr><tr><td>0.0002 lux</td><td>Starlight, clear moonless night sky, excluding airglow<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a></td></tr><tr><td>0.00014 lux</td><td>Venus at brightest,<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> clear night sky</td></tr><tr><td>0.0001 lux</td><td>Starlight, overcast moonless night sky<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a></td></tr></tbody></table> <p>For a table of approximate daylight intensity in the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>, see <a href="/facts/Sunlight/WPkJRC7s">sunlight</a>. </p> <h2 id="see-also">See also</h2> <ul><li>Artificial sky</li> <li><a href="/facts/Color_temperature/g77MjsPM">Color temperature</a></li> <li><a href="/facts/Daylight_harvesting/GJBcUfd9">Daylight harvesting</a></li> <li><a href="/facts/Daylight_saving_time/bEWvB1Rt">Daylight saving time</a></li> <li><a href="/facts/Daylighting_(architecture)/fW39xsfW">Daylighting</a></li> <li><a href="/facts/Daytime/nR7raDjV">Daytime</a> – Period of a day in which a location experiences natural illumination</li> <li><a href="/facts/Moonlight/dNlzKrem">Moonlight</a></li> <li><a href="/facts/Right_to_light/nuc7DjXb">Right to light</a></li> <li><a href="/facts/Twilight/hNUKeCT8">Twilight</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="http://www.gandraxa.com/length_of_day.xml">Length of Day and Twilight – Deriving the formulas to calculate the length of day</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Cox, Clifford. "Dust Bowl". Perryton.com. Retrieved 2017-08-01. <a href="http://www.perryton.com/black.htm" target="_blank">http://www.perryton.com/black.htm</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"Volcanic Ash Impacts & Mitigation". USGS. Retrieved 2017-08-01. <a href="https://volcanoes.usgs.gov/ash/" target="_blank">https://volcanoes.usgs.gov/ash/</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Air mass (solar energy)". Wikipedia. Retrieved 14 April 2020. <a href="https://en.wikipedia.org/w/index.php?title=Air_mass_(solar_energy)&oldid=938950059" target="_blank">https://en.wikipedia.org/w/index.php?title=Air_mass_(solar_energy)&oldid=938950059</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Bunning, Erwin; Moser, Ilse (April 1969). "Interference of moonlight with the photoperiodic measurement of time by plants, and their adaptive reaction". Proceedings of the National Academy of Sciences of the United States of America. 62 (4): 1018–22. Bibcode:1969PNAS...62.1018B. doi:10.1073/pnas.62.4.1018. PMC 223607. PMID 16591742. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC223607" target="_blank">https://www.ncbi.nlm.nih.gov/pmc/articles/PMC223607</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Schlyter, Paul (2006). "Radiometry and photometry in astronomy FAQ". <a href="http://stjarnhimlen.se/comp/radfaq.html#10" target="_blank">http://stjarnhimlen.se/comp/radfaq.html#10</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>"Petzl reference system for lighting performance". Archived from the original on 2008-06-20. Retrieved 2007-04-24. <a href="https://web.archive.org/web/20080620123040/http://en.petzl.com/petzl/frontoffice/Lampes/static/referentiel/present_referentiel_en.jsp" target="_blank">https://web.archive.org/web/20080620123040/http://en.petzl.com/petzl/frontoffice/Lampes/static/referentiel/present_referentiel_en.jsp</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Schlyter, Paul (2006). "Radiometry and photometry in astronomy FAQ". <a href="http://stjarnhimlen.se/comp/radfaq.html#10" target="_blank">http://stjarnhimlen.se/comp/radfaq.html#10</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Schlyter, Paul (2006). "Radiometry and photometry in astronomy FAQ". <a href="http://stjarnhimlen.se/comp/radfaq.html#10" target="_blank">http://stjarnhimlen.se/comp/radfaq.html#10</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Schlyter, Paul (2006). "Radiometry and photometry in astronomy FAQ". <a href="http://stjarnhimlen.se/comp/radfaq.html#10" target="_blank">http://stjarnhimlen.se/comp/radfaq.html#10</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Schlyter, Paul (2006). "Radiometry and photometry in astronomy FAQ". <a href="http://stjarnhimlen.se/comp/radfaq.html#10" target="_blank">http://stjarnhimlen.se/comp/radfaq.html#10</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> </ol>