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Daylight

{{Short description|Natural light during the daytime}}

{{Other uses|Daylight (disambiguation)}}

File:Daylight.png

Daylight is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. Sunlight scattered or reflected by astronomical objects is generally not considered daylight. Therefore, daylight excludes moonlight, despite it being reflected indirect sunlight.

Definition

Daylight is present at a particular location, to some degree, whenever the Sun is above the local horizon. This is true for slightly more than 50% of the Earth at any given time, since the Earth's atmosphere refracts some sunlight even when the Sun is below the horizon.

Outdoor illuminance varies from 120,000 lux for direct sunlight at noon, which may cause eye pain, to less than 5 lux for thick storm clouds with the Sun at the horizon (even <1 lux for the most extreme case), which may make shadows from distant street lights visible. It may be darker under unusual circumstances like a solar eclipse or very high levels of atmospheric particulates, which include smoke (see New England's Dark Day), dust,{{cite web |url=http://www.perryton.com/black.htm |title=Dust Bowl |last=Cox|first=Clifford |website=Perryton.com |access-date=2017-08-01}} and volcanic ash.{{cite web |url=http://volcanoes.usgs.gov/ash/ |title=Volcanic Ash Impacts & Mitigation |publisher=USGS |access-date=2017-08-01}}

Intensity in different conditions

class="wikitable"

! Illuminance !! Example

120,000 luxBrightest sunlight
111,000 luxBright sunlight
109,880 luxAM 1.5 global solar spectrum sunlight (= 1,000.4 W/m2){{cite web |url=https://en.wikipedia.org/w/index.php?title=Air_mass_(solar_energy)&oldid=938950059 |title=Air mass (solar energy) |publisher=Wikipedia |access-date=14 April 2020}}{{circular reference|date=October 2022}}
20,000 luxShade illuminated by entire clear blue sky, midday
1,000–2,000 luxTypical overcast day, midday
400 luxSunrise or sunset on a clear day (ambient illumination)
<200 luxExtreme of thickest storm clouds, midday
40 luxFully overcast, sunset/sunrise
<1 luxExtreme of thickest storm clouds, sunset/rise

For comparison, nighttime illuminance levels are:

class="wikitable"

! Illuminance !! Example

<1 luxMoonlight,{{cite journal |last=Bunning |first=Erwin |author2=Moser, Ilse |date=April 1969 |title=Interference of moonlight with the photoperiodic measurement of time by plants, and their adaptive reaction |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=62 |issue=4|pages=1018–22 |bibcode=1969PNAS...62.1018B |doi=10.1073/pnas.62.4.1018 |pmc=223607 |pmid=16591742 |doi-access=free }}{{cbignore|bot=medic}} clear night sky
0.25 luxA full Moon, clear night sky{{cite web |first=Paul |last=Schlyter |url=http://stjarnhimlen.se/comp/radfaq.html#10 |title=Radiometry and photometry in astronomy FAQ |year=2006}}{{cite web |url=http://en.petzl.com/petzl/frontoffice/Lampes/static/referentiel/present_referentiel_en.jsp |archive-url=https://web.archive.org/web/20080620123040/http://en.petzl.com/petzl/frontoffice/Lampes/static/referentiel/present_referentiel_en.jsp |archive-date=2008-06-20 |title=Petzl reference system for lighting performance |access-date=2007-04-24}}
0.01 luxA quarter Moon, clear night sky
0.002 luxStarlight, clear moonless night sky, including airglow
0.0002 luxStarlight, clear moonless night sky, excluding airglow
0.00014 luxVenus at brightest, clear night sky
0.0001 luxStarlight, overcast moonless night sky

For a table of approximate daylight intensity in the Solar System, see sunlight.

See also

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References

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External links

  • [http://www.gandraxa.com/length_of_day.xml Length of Day and Twilight – Deriving the formulas to calculate the length of day]

{{Parts of a day}}

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Category:Atmospheric optical phenomena

Category:Light

Category:Parts of a day

Category:Visibility