Diffuse sky radiation
1 Color 2 Neutral points 3 Under an overcast sky 4 As a part of total radiation 5 Eruption of Mt. Pinatubo and agriculture 6 See also 7 References 8 Further reading 9 External links
Clear blue sky
Comparison of the scattering efficiency of blue light in the atmosphere compared to red light.
The sunlit sky is blue because air scatters short-wavelength 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 when looking toward parts of the sky other than the sun, human
eye perceives them to be blue. The color perceived is similar to
that obtained by a monochromatic blue of a wavelength of 474–476 nm
mixed with white light, i.e., an unsaturated blue light. The
explanation of the blue color by Rayleigh in 1871 is one of the most
famous examples of the application of dimensional analysis in solving
a problem in physics.
Near sunrise and sunset, most of the sunlight arrives nearly
tangentially to the Earth's surface; thus, the light's path through
the atmosphere is so long that much of the blue and even green light
is scattered out along the way, leaving the sun rays and the clouds it
illuminates red. Therefore, when looking at the sunset and sunrise, we
see the color red more than the other colors.
The Arago point, named after its discoverer, is customarily located at
about 20° above the antisolar point; but it lies at higher altitudes
in turbid air. The latter property makes the Arago distance a useful
measure of atmospheric turbidity.
The Babinet point, discovered by
Jacques 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
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 1⁄6 of direct sunlight for relatively thin clouds down to 1⁄1000 of direct sunlight under the extreme of thickest storm clouds. As a part of total radiation One of the equations for total solar radiation is:
displaystyle H_ t =H_ b R_ b +H_ d R_ d +(H_ b +H_ d )R_ r
where Hb is the beam radiation irradiance, Rb is the tilt factor for beam radiation, Hd is the diffuse radiation irradiance, Rd is the tilt factor for diffuse radiation and Rr is the tilt factor for reflected radiation. Rb is given by:
sin ( δ ) sin ( ϕ − β ) + cos ( δ ) cos ( h ) cos ( ϕ − β )
sin ( δ ) sin ( ϕ ) + cos ( δ ) cos ( h ) cos ( ϕ )
displaystyle R_ b = frac sin(delta )sin(phi -beta )+cos(delta )cos(h)cos(phi -beta ) sin(delta )sin(phi )+cos(delta )cos(h)cos(phi )
where δ is the solar declination, Φ is the latitude, β is an angle from the horizontal and h is the solar hour angle. Rd is given by:
1 + cos ( β )
displaystyle R_ d = frac 1+cos(beta ) 2
and Rr by:
ρ ( 1 − cos ( β ) )
displaystyle R_ r = frac rho (1-cos(beta )) 2
where ρ is the reflectivity of the surface. Eruption of Mt. Pinatubo and agriculture See also: photosynthesis
A Space Shuttle (Mission STS-43) photograph of the
The eruption of the
Under more-or-less direct sunlight, dark shadows that limit photosynthesis are cast onto understorey leaves. Within the thicket, very little light can enter.
The means by which this was discovered, is that initially at the time, a mysterious drop in the rate at which carbon dioxide (CO2) was filling the atmosphere was observed, which is charted in what is known as the "Keeling Curve". This led numerous scientists to assume that this reduction was due to the lowering of the Earth's temperature, and with that, a slow down in plant and soil respiration, indicating a deleterious impact to global agriculture from the volcanic haze layer. However upon actual investigation, the reduction in the rate at which carbon dioxide filled the atmosphere did not match up with the hypothesis that plant respiration rates had declined. Instead the advantageous anomaly was relatively firmly linked to an unprecedented increase in the growth/net primary production, of global plant life, resulting in the increase of the carbon sink effect of global photosynthesis. The mechanism by which the increase in plant growth was possible, was that the 30% reduction of direct sunlight can also be expressed as an increase or "enhancement" in the amount of diffuse sunlight.
Well lit understorey areas due to overcast clouds creating diffuse/soft sunlight conditions, that permits photosynthesis on leaves under the canopy.
With, owing to its intrinsic nature, can illuminate under-canopy leaves permitting more efficient total whole-plant photosynthesis than would otherwise be the case. In stark contrast to the effect of totally clear skies and the direct sunlight that results from it, which casts shadows onto understorey leaves, strictly limiting plant photosynthesis to the top canopy layer. This increase in global agriculture from the volcanic haze layer also naturally results as a product of other aerosols that are not emitted by volcanoes, such as man-made "moderately thick smoke loading" pollution, as the same mechanism, the "aerosol direct radiative effect" is behind both. See also
Atmospheric diffraction Aerial perspective Cyanometer Sunshine duration Daylight Nighttime airglow Tyndall effect Rayleigh scattering Rayleigh sky model
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Pesic, Peter (2005).
Dr. C. V. Raman lecture: Why is the sky blue?
Why is the sky blue?