Environmental Engineering Reference
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(@2 W/m 2 ) during the same period (Wielicki et al.
2005). Whatever the actual direction and the rate of
change, changing albedo affects average global tempera-
tures as well as global ocean heat storage.
All SW radiation that is not reflected gets absorbed and
transformed into longwave (LW) flux. The process starts
in the stratosphere and continues throughout the tropo-
sphere, but most of it takes place on the Earth's surface.
Stratospheric O 3 contributes by far the largest share to
the drastic reduction of UV waves ( < 300 nm). Visible
light passes through the atmosphere largely unaffected,
but water and CO 2 and, to a lesser extent, CH 4 ,N 2 O,
and O 3 absorb much of the infrared flux. The total ab-
sorption profile shows two peaks in the near infrared and
three extended blocks, 1.5-2, 2.5-3.5, and 4-8 mm (fig.
2.3). Changing concentrations of absorptive gases alter
both the quantity and the quality of surface insolation.
The fate of stratospheric O 3 is of the greatest concern:
its destruction by such anthropogenic pollutants as
chlorofluorocarbons and N 2 O would have progressively
injurious effects on biota, including higher frequencies
of erythemal damage, skin cancers, and genetic malfor-
mations (see section 11.4). Sky color is due to molecular
scattering that diffuses the incoming radiation with effi-
ciency inversely proportional to the fourth power of the
wavelength: the blue light at 400 nm is scattered more
than nine times as much as the red one at 700 nm. Nat-
ural and anthropogenic aerosols (above all, black carbon
and sulfates) are a variable but often important cause of
SW absorption in the troposphere.
The sources of radiation extinction (O 3 -dominated
absorption, Rayleigh scattering, and turbidity) can be
combined in a single attenuation coefficient (extinction
optical thickness) and inserted, together with the depth
of the air mass, as the negative exponent in the Beer-
Bouguer-Lambert formula for calculating the fraction of
the solar constant reaching the surface. Values for these
extinction factors are available for individual wavelengths
for the standard atmosphere (Thekaekara 1977). Combi-
nations of different air masses (solar zenith angles) and
extinction optical thicknesses result in very different sur-
face irradiances. For one air mass and very clear air, the
rate is nearly 960 W/m 2 or 70% of the solar constant,
whereas with seven air masses and heavy pollution, the
rate is merely 133 W/m 2 , less than 10% of the solar con-
stant. Fractions of the total energy carried by UV, visible,
and IR waves also differ: 5%, 47%, and 48% in the first in-
stance, 0.1%, 30%, and 70% in the other.
The UV disparity is by far the greatest, the reason for
the skin-burning effect of noontime summer radiation in
high mountains and a key factor in the prevalence of
rachitis in heavily polluted nineteenth-century northern
cities under low winter sun. Typical attenuation ranges
can be also combined to give the daily maxima of clear
sky sunlight penetration. Absorption can eliminate 11%-
23% of incoming radiation; scattering can return 1.1%-
11% and send 5%-15% of the beam downward as diffuse
sunlight, leaving 56%-83% in the direct beam. Total mid-
latitude peak insolation would then be 970-1203 W/
m 2 . Typical shares of radiation received under different
cloud types are (all for solar altitude of 65 ) 85% for cirri,
52% for altocumuli, 35% for stratocumuli, and 15% for
nimbostrati. The percentage of possible sunshine ranges
seasonally from less than 10% in the world's cloudiest
regions (North Pacific and Atlantic) to around 90% in
subtropical deserts (peaks in Chile's Atacama, southern
Egypt, and northern Sudan).
Because of the selective absorption by atmospheric
gases the SW radiation that reaches the Earth's surface
has a notably different wavelength profile than does the
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