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Diffuse sky radiation

 
Diffuse Sky Radiation

 
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Diffuse sky radiation
 
 
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Diffuse sky radiation is solar radiation reaching the Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
's surface after having been scattered
Scattering

Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles,are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass....
 from the direct solar beam by molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
s or suspensoids in the atmosphere
Earth's atmosphere

The Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. Dry air contains roughly 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% Carbon dioxide in the Earth's atmosphere, and trace amounts of other gases....
. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky. Of the total light
Light

Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
 removed from the direct solar beam by scattering in the atmosphere (approximately 25% of the incident radiation when the sun is high in the sky, depending on the amount of dust and haze in the atmosphere), about two-thirds ultimately reaches the earth as diffuse sky
Sky

The sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons....
 radiation.

The important processes in the atmosphere (Rayleigh scattering
Rayleigh scattering

Rayleigh scattering is the elastic scattering of light or other electromagnetism radiation by particles much smaller than the wavelength of the light....
 and Mie scattering
Mie theory

Mie theory, also called Lorenz-Mie theory or Lorenz-Mie-Debye theory, is an analytical solution of Maxwell's equations for the scattering of electromagnetic radiation by spherical particles ....
) are elastic
Elastic scattering

In scattering theory and in particular in particle physics, elastic scattering is one of the specific forms of scattering. In this process, the energy of the incident Elementary particle is conserved, only their direction of propagation is modified ....
.






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Diffuse sky radiation is solar radiation reaching the Earth
Earth

Earth is the third planet from the Sun. Earth is the largest of the terrestrial planets in the Solar System in diameter, mass and density. It is also referred to as the World and Wiktionary:Terra.Note that by International Astronomical Union convention, the term "Terra" is used for naming extensive land masses, rather...
's surface after having been scattered
Scattering

Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles,are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass....
 from the direct solar beam by molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
s or suspensoids in the atmosphere
Earth's atmosphere

The Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. Dry air contains roughly 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% Carbon dioxide in the Earth's atmosphere, and trace amounts of other gases....
. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the colour of the sky. Of the total light
Light

Light, or visible light, is electromagnetic radiation of a wavelength that is Visible spectrum to the human eye , or up to 380?750 nm. In the broader field of physics, light is sometimes used to refer to electromagnetic radiation of all wavelengths, whether visible or not....
 removed from the direct solar beam by scattering in the atmosphere (approximately 25% of the incident radiation when the sun is high in the sky, depending on the amount of dust and haze in the atmosphere), about two-thirds ultimately reaches the earth as diffuse sky
Sky

The sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons....
 radiation.

The important processes in the atmosphere (Rayleigh scattering
Rayleigh scattering

Rayleigh scattering is the elastic scattering of light or other electromagnetism radiation by particles much smaller than the wavelength of the light....
 and Mie scattering
Mie theory

Mie theory, also called Lorenz-Mie theory or Lorenz-Mie-Debye theory, is an analytical solution of Maxwell's equations for the scattering of electromagnetic radiation by spherical particles ....
) are elastic
Elastic scattering

In scattering theory and in particular in particle physics, elastic scattering is one of the specific forms of scattering. In this process, the energy of the incident Elementary particle is conserved, only their direction of propagation is modified ....
. No energy transformation results, only a change in the spatial distribution of the radiation.

Colour

Trees Sky
The sunlit sky appears blue because air
AIR

Air is the part of Earth's atmosphere that humans breath and as such Air .Air may also refer to:...
 scatters short-wavelength
Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
 light more than longer wavelengths. Since blue light is at the short wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than long wavelength red light. The result is that the human eye perceives blue when looking toward parts of the sky other than the sun. Near sunrise
Sunrise

Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight....
 and sunset
Sunset

File:Sunset 2007-1.jpgSunset is the daily disappearance of the sun below the horizon as a result of the Earth's rotation. The atmospheric conditions created by the setting of the sun are also commonly referred to as "a sunset"....
, most of the light we see comes in nearly tangent to the Earth's surface, so that the light's path through the atmosphere is so long that much of the blue and even green light is scattered out, leaving the sun rays and the clouds it illuminates red. Therefore, when looking at the sunset and sunrise, you will see the color red more than any of the other colors.

Scattering and absorption are major causes of the attenuation
Attenuation

In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, and X-rays are attenuated by lead....
 of radiation by the atmosphere. Scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation. When this ratio is less than about one-tenth, Rayleigh scattering
Rayleigh scattering

Rayleigh scattering is the elastic scattering of light or other electromagnetism radiation by particles much smaller than the wavelength of the light....
 occurs in which the scattering coefficient varies inversely as the fourth power of the wavelength. At larger values of the ratio of particle diameter to wavelength, the scattering varies in a complex fashion described, for spherical particles, by the Mie theory
Mie theory

Mie theory, also called Lorenz-Mie theory or Lorenz-Mie-Debye theory, is an analytical solution of Maxwell's equations for the scattering of electromagnetic radiation by spherical particles ....
; at a ratio of the order of 10, the laws of geometric optics begin to apply.

Neutral points

There are three commonly detectable points of zero polarization
Polarization

Polarization is a property of waves that describes the orientation of their oscillations. For transverse waves such as many electromagnetic waves, it describes the orientation of the oscillations in the plane perpendicular to the wave's direction of travel....
 of diffuse sky radiation (known as neutral points) lying along the vertical circle
Vertical circle

In astronomy, a vertical circle is a great circle on the celestial sphere that is perpendicular to the horizon. Therefore it passes through the zenith and the nadir....
 through the sun
Sun

The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
.
  • The Arago point, named after its discoverer, is customarily located at about 20° above the antisolar point
    Antisolar point

    The antisolar point is the imaginary point on the celestial sphere exactly opposite the Sun. It is where anticrepuscular rays appear to converge, and on a moonless night away from city lights, it is often possible to see the gegenschein there....
    ; but it lies at higher altitudes in turbid air. The latter property makes the Arago distance a useful measure of atmospheric turbidity
    Turbidity

    Turbidity is the cloudiness or haze of a fluid caused by individual Particle that are generally invisible to the naked eye, similar to smoke in air....
    .
  • The Babinet point, discovered by Babinet in 1840, is located about 15° to 20° above the sun, hence it is difficult to observe because of solar glare.
  • The Brewster point, discovered by Brewster in 1840, is located about 15° to 20° below the sun; hence it is difficult to observe because of solar glare.


Under an overcast sky

There is essentially no direct sunlight under an overcast sky, so all light is then diffuse sky radiation. The flux of light is not very wavelength dependent because the cloud droplets are larger than the light's wavelength and scatter all colours approximately equally. The light passes through the translucent clouds in a manner similar to frosted glass. The intensity ranges (roughly) from 1/6 of direct sunlight for relatively thin clouds down to 1/1000 of direct sunlight under the extreme of thickest storm clouds.

See also

  • Aerial perspective
    Aerial perspective

    File:Mount Feathertop, Australia - May 2005.jpgAerial perspective or atmospheric perspective is the effect on the appearance of an object by the atmosphere between it and a viewer ....
  • Daylight
    Daylight

    Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the Daytime . This includes direct sunlight, diffuse sky radiation, and both of these reflected from the Earth and terrestrial objects....
  • Nighttime airglow
    Airglow

    The airglow is the very weak emission of light by the Earth's atmosphere; as a result, the night sky is never completely dark. It was first noticed in 1868 by Anders ?ngstr?m....
  • Tyndall effect
    Tyndall effect

    The Tyndall effect is an effect of light scattering by colloid particles or particles in Suspension . It is named after the 19th century Irish scientist John Tyndall....


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