Diffuse sky radiation
Encyclopedia
Diffuse sky radiation is solar radiation reaching the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

'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. In conventional use, this also includes deviation of...

 from the direct solar beam by molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s or suspensoids in the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

. 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 that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

 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 outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a pale blue surface because the air scatters the sunlight. The sky is sometimes...

 radiation.

The important processes in the atmosphere (Rayleigh scattering
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

 and Mie scattering
Mie theory
The Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere...

) 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 kinetic energy of the incident particles is conserved, only their direction of propagation is modified .-Electron elastic scattering:When an alpha particle...

 processes, by which light can be deviated from its path without being absorbed and with no change in wavelength.

Color

The sunlit sky is blue because air scatters short-wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 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. The color perceived is similar to that obtained by a monochromatic blue of wavelength 474–476 nm mixed with white light, i.e., an unsaturated blue light.

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
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...

, 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, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...

 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, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

 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
The Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere...

; 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 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. There is a vertical circle for any given azimuth, where azimuth is the angle measured east from the north on the celestial horizon....

 through the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

.
  • 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. The apex of a rainbow will also form roughly 42...

    ; 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 haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....

    .
  • 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 colors approximately equally. The light passes through the translucent clouds in a manner similar to frosted glass. The intensity ranges (roughly) from of direct sunlight for relatively thin clouds down to of direct sunlight under the extreme of thickest storm clouds.

See also

  • Aerial perspective
    Aerial perspective
    Aerial perspective or atmospheric perspective refers to the effect the atmosphere has on the appearance of an object as it is viewed from a distance. As the distance between an object and a viewer increases, the contrast between the object and its background decreases, and the contrast of any...

  • Cyanometer
    Cyanometer
    A cyanometer ) is an instrument for measuring 'blueness', specifically the colour intensity of blue sky. It is attributed to Horace-Bénédict de Saussure and Alexander von Humboldt...

  • 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. Sunlight scattered or reflected from objects in outer space is...

  • Nighttime airglow
    Airglow
    Airglow is the very weak emission of light by a planetary atmosphere. In the case of Earth's atmosphere, this optical phenomenon causes the night sky to never be completely dark .-Development:The airglow phenomenon was first identified in 1868 by Swedish scientist...

  • Tyndall effect
    Tyndall effect
    The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a colloid or particles in a fine suspension. It is named after the 19th century physicist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth...

  • Rayleigh scattering
    Rayleigh scattering
    Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...

  • Rayleigh Sky Model
    Rayleigh sky model
    The Rayleigh sky model describes the observed polarization pattern of the daytime sky. Within the atmosphere Rayleigh scattering of light from air molecules, water, dust, and aerosols causes the sky's light to have a defined polarization pattern. The same elastic scattering processes cause the sky...


External links

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