Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 34

Fig. 3.1 Possible frequency ranges for remote sensing of the atmosphere. Selected active and
passive remote sensing methods are marked (LIDAR, FTIR, etc.). Ceilometers operate in the same
frequency range as LIDARs. The frame marks the range shown in more detail in Fig. 3.6

the radiation; Rayleigh scattering occurs if the size of the scattering object is much
smaller than the wavelength of the radiation. Mie scattering has only small wave-
length dependence, because the ratio between particle size and wavelength is close
to the Bragg condition. Rayleigh scattering is proportional to

λ − 4 , i.e. the backscat-
ter intensity increases drastically when coming closer to the Bragg condition. In the
case of Raman scattering the energy of the incoming radiation is contributing tem-
porarily to the rotation or vibration energy of a molecule and is then emitted again
at a different frequency.
A second condition for a successful sounding of the atmosphere is that the atmo-
sphere is not or only weakly absorbing or attenuating the used radiation in order
to allow for a sufficient range of the remote-sensing technique. Wavelength bands
within which the atmosphere is not absorbing are called atmospheric windows.
One window is in the range of visible light, another in the infrared range at 10-15
μ m. For radiation with shorter wavelengths than 0.3 μ m (UV radiation), for some

wavelengths between 0.9 and 10 m, the

atmosphere is more or less opaque. For large wavelength above about 1 mm (micro
wave range) the atmosphere becomes transparent again.
The combination of relevant backscatter processes in the atmosphere and the
wavelength bands within which the atmosphere is transparent has led to the devel-
opment of the different active remote-sensing techniques used today (see Table 3.1

μ m, and for wavelengths greater than 15 μ