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the photons might hit in the atmosphere. But these material barriers can induce
additional attenuation.
However, there is another factor that affects the results on the target area and thus
the sensing records as well. This is the
sensitivity
of the target area in the sensor to
the energy of the radiation. A high sensitivity can compensate for attenuated sig-
nals. The progress that has taken place in remote sensing must be attributed partly
to the fact that highly sensible receivers of the radiation have made up for inevitable
effects of the inverse square law.
3.3
Atmospheric Windows and Clouds
The solar radiation that is directed towards the earth hits molecules and aerosols in
the atmosphere. The result is scattering and absorption of radiation. Hence the radi-
ation that fi nally gets to the surface of the earth is fi ltered by the atmosphere.
However, this fi ltering effect of the atmosphere depends very much on the type of
radiation.
Atmospheric windows
show, which radiation types are transmitted to the
surface of the earth or
vice versa
(Fig.
3.3
).
The respective white regions show, which radiation is transmitted. Black areas
indicate radiation that is either absorbed or refl ected back into space. The transmit-
tance shown is for a sky without clouds.
There are two main regions of rather unimpeded transmittance: the range of the
visible light and the range of the radar-, micro- and radiowaves. The situation in the
infrared region depends on the respective wavelengths. Here ranges with blocked
transmittance alternate with regions with rather free penetration. Thermal infrared
radiation with long waves hardly is transmitted.
ultra-
violet
visible
reflected
infrared
radar-, micro-,
radiowaves
thermal (emitted) infrared
100
H
2
O
O
3
CO
2
H
2
H
2
O
O
2
H
2
O
O
2
H
2
O
CO
2
O
3
0
0.2
0.5
1.0
5
10
20
100
0.1 1.0 100
wavelength in cm
wavelength in micrometers
Fig. 3.3
The atmospheric windows (
white
) show the wavelengths that penetrate the cloudless
atmosphere of the earth. The gaseous molecules that can block the transmission of wavelength
ranges are indicated. For the boundaries between some radiation types, see legend to Fig.
3.1
(From NASA Earth Observatory
2010
, altered and redrawn)
