Daylight

World map showing the areas of Earth receiving daylight around 13:00 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. For an explanation of why it is not exactly half, see here). However, the outdoor illuminance can vary 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, and volcanic ash.

Intensity in different conditions

Illuminance Example
120,000 lux Brightest sunlight
111,000 lux Bright sunlight
109,880 lux AM 1.5 global solar spectrum sunlight (= 1,000.4 W/m2)[circular reference]
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)
<200 lux Extreme of thickest storm clouds, midday
40 lux Fully overcast, sunset/sunrise
<1 lux Extreme of thickest storm clouds, sunset/rise

For comparison, nighttime illuminance levels are:

Illuminance Example
<1 lux Moonlight, clear night sky
0.25 lux A full Moon, clear night sky
0.01 lux A quarter Moon, clear night sky
0.002 lux Starlight, clear moonless night sky, including airglow
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.

See also


This page was last updated at 2024-04-16 12:01 UTC. Update now. View original page.

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