Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun, particularly infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through the Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon.
When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by the clouds or reflects off other objects, it is experienced as diffused light.
The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter.
Sunlight may be recorded using a sunshine recorder, pyranometer or pyrheliometer.
Sunlight takes about 8.3 minutes to reach the Earth. On average, it takes energy between 10,000 and 170,000 years to leave the sun's interior and then be emitted from the surface as light.
Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. Bright sunlight provides illuminance of approximately 100,000 lux or lumens per square meter at the Earth's surface. Sunlight's composition at ground level, per square meter, with the sun at the zenith, is about 527 watts of infrared radiation, 445 watts of visible light, and 32 watts of ultraviolet radiation. At the top of the atmosphere sunlight is about 30% more intense with more than three times the fraction of ultraviolet (UV), with most of the extra UV consisting of biologically-damaging shortwave ultraviolet.
Sunlight is a key factor in photosynthesis, a process vital for many living beings on Earth.
Read more about Sunlight: Composition and Power, Calculation, Solar Constant, Total (TSI) and Spectral Solar Irradiance (SSI) Upon Earth, Intensity in The Solar System, Surface Illumination, Climate Effects, Past Variations in Solar Irradiance, Life On Earth, Cultural Aspects, Effects On Human Health
Famous quotes containing the word sunlight:
“Calm as that second summer which precedes
The first fall of the snow,
In the broad sunlight of heroic deeds,
The City bides the foe.”
—Henry Timrod (1828–1867)
“The sunlight on the garden
Hardens and grows cold,
We cannot cage the minute
Within its nets of gold,”
—Louis MacNeice (1907–1963)
“Just across the Green from the post office is the county jail, seldom occupied except by some backwoodsman who has been intemperate; the courthouse is under the same roof. The dog warden usually basks in the sunlight near the harness store or the post office, his golden badge polished bright.”
—Administration for the State of Con, U.S. public relief program (1935-1943)