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:
“Anton Petrovich turned into the passage, followed the arrow to men, mankind, human beings, marched past the toilet, past the kitchen, gave a start when a cat darted under his feet, quickened his step, reached the end of the passage, pushed open a door, and a shower of sunlight splashed his face.”
—Vladimir Nabokov (1899–1977)
“Prostration is our natural position. A wormlike movement from a spot of sunlight to a spot of shade, and back, is the type of movement that is natural to men.”
—Wyndham Lewis (1882–1957)
“Summer of man its sunlight and its flower,
Spring-time of man all April in a face.”
—John Masefield (1878–1967)