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, and volcanic ash.
Intensity in different conditions
Illuminance Example 120,000 luxBrightest sunlight 111,000 luxBright sunlight 109,880 luxAM 1.5 global solar spectrum sunlight (= 1,000.4 W/m2) 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:
Illuminance Example <1 luxMoonlight, clear night sky 0.25 luxA full Moon, clear night sky 0.01 luxA quarter Moon, clear night sky 0.002 luxStarlight and airglow, clear moonless night sky 0.0002 luxStarlight, clear moonless night sky, excluding airglow 0.00014 luxVenus at brightest, clear night sky 0.0001 luxStarlight, overcast moonless night sky
See also
References
External links
Length of Day and Twilight – Deriving the formulas to calculate the length of day