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Daylight

Natural light during the daytime

World map showing the areas of Earth receiving daylight around 1:00 pm UTC in April

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

Intensity in different conditions

Illuminance	Example
120,000 lux	Brightest sunlight
111,000 lux	Bright sunlight
109,880 lux	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}}
20,000 lux	Shade illuminated by entire clear blue sky, midday
1,000–2,000 lux	Typical overcast day, midday
400 lux	Sunrise or sunset on a clear day (ambient illumination)
	Extreme of thickest storm clouds, midday
40 lux	Fully overcast, sunset/sunrise
	Extreme of thickest storm clouds, sunset/rise

For comparison, nighttime illuminance levels are:

Illuminance	Example
	Moonlight, clear night sky
0.25 lux	first=Paul	last=Schlyter	url=http://stjarnhimlen.se/comp/radfaq.html#10	title=Radiometry and photometry in astronomy FAQ	year=2006}}
0.01 lux	A quarter Moon, clear night sky
0.002 lux	Starlight and airglow, clear moonless night sky
0.0002 lux	Starlight, clear moonless night sky, excluding airglow
0.00014 lux	Venus at brightest, clear night sky
0.0001 lux	Starlight, overcast moonless night sky

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

References

Cox, Clifford. "Dust Bowl".
"Volcanic Ash Impacts & Mitigation". [[USGS]].
"Air mass (solar energy)". Wikipedia.
Bunning, Erwin. (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.
Schlyter, Paul. (2006). "Radiometry and photometry in astronomy FAQ".
"Petzl reference system for lighting performance".

Info: Wikipedia Source

This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page.

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