Agriculture Reference
In-Depth Information
Table 3.1
Absorbance, refl ectance and transmittance of the solar radiation spectrum by clouds
(Data from Liou
1976
, altered, transmittance added)
Type of cloud
Thickness (m)
Absorbance (%)
Refl ectance (%)
Transmittance (%)
Cumulonimbus
6,000
10-20
80-90
0-10
Nimbostratus
4,000
10-20
80-90
0-10
Altostratus
600
8-15
57-77
8-35
Cumulus
450
4-9
68-85
6-28
Stratus
100
1-6
45-72
22-54
The gaseous molecules that block the radiation are oxygen, water and carbon
dioxide. The transmittance of ultraviolet light is partly prevented by oxygen mole-
cules, which is a blessing for human health.
The situation is quite different when
clouds
are present. Clouds are generated by
water vapor near the condensation point. They consist of aerosol-sized particles of
liquid water that absorb or scatter electromagnetic radiation of waves with less than
about 0.1 cm length. Consequently only radar-, micro-and radiowaves are capable
of penetrating clouds without being scattered, refl ected or absorbed. This is a very
important point for remote sensing, since in many areas of the world it is necessary
to reckon with cloud covers.
So for sensing of visible- and infrared radiation by satellites, clouds can com-
pletely alter the possibilities. And on the average, clouds occupy regularly more
than 50 % of the planet earth's atmosphere (Liou
1976
). There are of course large
regional differences in the incidence of clouds. Their attenuation of the transmit-
tance depends on the wavelengths. Within the visible- and infrared range, the longer
the waves, the more attenuation occurs. Short visible waves still have the best
chance to penetrate the clouds and thus provide for some diffuse illumination of the
earth's surface during an overcast day.
Even a thin stratus cloud reduces the transmittance on the average to almost one
third (Table
3.1
). With thick clouds, the transmittance drops to 10 % or less. The
problem for recording data from satellites by visible and infrared refl ectance is that
indeed the terrestrial area might be regularly passed overhead, yet in regions with
humid climate the actual sensing possibilities are not predictable.
3.4
Sensing from Satellites , Aerial Platforms
and Field Machines
Electromagnetic sensing of soil- or crop properties can be achieved with passive- or
active sensors. Passive sensors rely on natural electromagnetic waves that are pro-
vided either by solar energy or by radiation that is emitted from the earth. Hence pas-
sive sensing of visible light is confi ned to daytime. Active sensors have their own
artifi cial radiation sources. This means that they can operate at night as well, even if
visible radiation is needed for the sensing process. In case radiation outside the visible
