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A weather radar is a remote sensing instrument which allows observation of precipitation by means of electromagnetic waves. The radar system transmits electromagnetic pulses which propagate through the atmosphere in a narrow radar beam. The transmitted wave is scattered and absorbed by precipitation particles. A small portion of the incident energy is returned back to the antenna. The returned echo depends upon the number, size, shape and state (i.e., ice, liquid or mixed) of the precipitation particles.
Working of a radar
Most operational weather radars in Europe operate at C-band in the microwave range of frequencies. The wavelength and the frequency are near 5 cm and 5 GHz, respectively. Weather radars at S-band (frequency around 3 Ghz) and X-band (frequency around 10 Ghz) are also in use. C-band weather radars only detect precipitation particles. They are not suited for cloud observations.
Modern radars scan the atmosphere at multiple elevation angles which allows three-dimensional observations of the precipitation field. The maximum detection range lies generally around 250 km.
Most current radars have the Doppler capability. The radial velocity of the particles is measured using the well known Doppler effect. The last generation of weather radars are dual-polarization radars. These radars are capable of transmitting and receiving waves in both horizontal and vertical polarizations while conventional radar are single-polarization. Additional information on the shape and type of precipitation particles (i.e., rain, snow, hail) can be extracted and used to improve the quality of precipitation observations.