Geoscience Reference
In-Depth Information
25
UV
Visible
Infrared
Sunlight at top of the atmosphere
20
5250K blackbody spectrum
15
10
H
2
O
Radiation at sea level
5
H
2
O
O
2
H
2
O
CO
2
H
2
O
O
3
H
2
O
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
Wavelength (µm)
Figure 4.4 Solar spectral irradiance observed by a satellite outside the earth's
atmosphere (light gray) and observed at the surface (dark gray).
significantly from the solar spectrum observed outside the atmosphere. The
shape is similar to that of an ideal blackbody, but it is clear that when solar
radiation passes through the earth's atmosphere, energy is removed from the
solar spectrum across all wavelengths, and the loss is particularly strong and
even complete at certain wavelengths.
The energy lost from the solar spectrum across a broad range of wave-
lengths is due to reflection and scattering, which change the direction of the
beam of energy but not its wavelength. Two scattering processes are important
in the earth's atmosphere:
(1)
Rayleigh scattering
by atmospheric molecules and small particles (with
diameters < 0.1 mm) is the dominant form of scattering. This process is
most effective at shorter visible wavelengths, that is, the blue region of the
visible spectrum, since the amount of energy scattered is proportional to
l
−4
. Rayleigh scattering removes blue visible light from the solar beam and
scatters it across the sky, which makes the sky appear blue. Most of the
energy that Rayleigh scatters from a molecule is redirected either forward
or backward.
(2)
Mie
(pronounced “me”)
scattering
occurs when solar radiation interacts
with larger particles in the atmosphere, with diameters on the order
of (or larger than) the wavelength of the radiation. Examples are dust,
pollen, smoke, and water droplets. Mie scattering does not have a strong
wavelength dependence so the scattered light is white. Because of Mie
scattering, clouds, mist, and fog appear white, and there is a white glare
around the sun when numerous water droplets are suspended in the air.
Mie scattering is predominantly forward scattering and becomes more so as
the particle size increases.
