Daylight is the combination of all direct and indirectsunlight during thedaytime. This includes directsunlight,diffuse sky radiation, and (often) both of these reflected byEarth and terrestrial objects, likelandforms and buildings. Sunlightscattered or reflected byastronomical objects is generally not considered daylight. Therefore, daylight excludesmoonlight, despite it being reflected indirect sunlight.
Daylight is present at a particular location, to some degree, whenever theSun is above the localhorizon. This is true for slightly more than 50% of theEarth at any given time, since the Earth's atmosphere refracts some sunlight even when the Sun is below the horizon.
Outdoorilluminance varies from 120,000lux for directsunlight atnoon, which may cause eyepain, to less than 5lux for thick stormclouds with the Sun at the horizon (even <1 lux for the most extreme case), which may make shadows from distantstreet lights visible. It may be darker under unusual circumstances like asolar eclipse or very high levels of atmosphericparticulates, which include smoke (seeNew England's Dark Day), dust,[1] and volcanic ash.[2]
| Illuminance | Example |
|---|---|
| 120,000 lux | Brightestsunlight |
| 111,000 lux | Bright sunlight |
| 109,880 lux | AM 1.5 global solar spectrum sunlight (= 1,000.4 W/m2)[3][circular reference] |
| 20,000 lux | Shade illuminated by entire clearblue sky, midday |
| 1,000–2,000 lux | Typicalovercast day, midday |
| 400 lux | Sunrise orsunset 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,[4] clearnight sky |
| 0.25 lux | Afull Moon, clear night sky[5][6] |
| 0.01 lux | Aquarter Moon, clear night sky |
| 0.002 lux | Starlight, clear moonless night sky, includingairglow[5] |
| 0.0002 lux | Starlight, clear moonless night sky, excluding airglow[5] |
| 0.00014 lux | Venus at brightest,[5] clear night sky |
| 0.0001 lux | Starlight, overcast moonless night sky[5] |
For a table of approximate daylight intensity in theSolar System, seesunlight.
