Figure 1: Electromagnetic absorption of the earth's atmosphere, ① heavy rain, ② fog, clouds, ③ moderate rain, ④ molecular dispersion
Figure 1: Electromagnetic absorption of the earth's atmosphere
The electromagnetic wave may be partially or totally absorbed by an absorbing medium due to atomic and molecular interactions (In this process energy is transferred to the substance and this may cause significant changes to occur within the absorbing medium; the radiant energy is converted into heat mostly.)
The absorption of electromagnetic radiation is insignificant at low frequencies less than 3 Gigahertzes (or a wavelength longer than ten centimeters, or 4 inches) by clear weather conditions. Not till then the frequency reach the resonant frequency of some molecules contained into the earth's atmosphere, the intensity of a beam of electromagnetic radiation is attenuated in passing through this medium considerably.
This absorption depends on the frequency and the path length therefore.
Not condensed water vapour, or the so-called relative humidity depends on the temperature too. The air temperature and the air humidity affect adversely the absorption too.
Atmospheric absorption losses consist of an atmospheric basic absorption as well as a strongly weather-dependent auxiliary absorption by fog and rain. The electromagnetic waves are weakened when penetrating air and water vapor layers. In this process mainly water vapor and diatomic oxygen are involved. A part of the electromagnetic energy is converted into heat, another part becomes scattered due to the molecular dipole function.
The diagram shows that the absorption increase as the rates of humidity rises. Furthermore, absorption increases with higher transmitter frequency.
Also, one can see that all frequencies are not equally suitable for all radar applications (e.g., a high frequency is not suitable for long-range radars). The very strong absorption at about 75 gigahertz's caused by the oxygen molecules practice the radar-based Pre-Safe Brake Assist Plus developed by Mercedes. The maximum range of the radar is limited on the desired measure by this and mutual disturbing influences therefore are to avoided.
In summary atmospheric absorption losses are always present and cannot be avoided.
Figure 2: Rain attenuation
The graphic in Figure 2 shows the effect that the rain attenuation has a different value at different wavelengths. The top (blue) curve shows the unattenuated weather; a storm 20 km in diameter with rainfall rising to 100 mm/hr in the center. The next (mauve) curve shows the returns as seen by an S- Band radar. The next two curves (yellow and light blue) show the outputs from a C- Band and an X- Band radar. All the results have been normalised.
It is obvious that X- Band suffers from attenuation and cannot see far into a severe storm, while S- Band has little attenuation. C- Band offers a good compromise. For these reasons, X band weather radars are only used for short ranges, S band radars are used in the tropics because they can see beyond a severe storm, and C- Band is favoured in temperate latitudes, having good sensitivity and range.
Reference: Recommendation ITU-R P.676-10 “Attenuation by atmospheric gases”