Doppler Radar for weather forecast as well as the radar used by defence forces has same origin. German physicist Heinrich Rudolph Hertz in 19th century displayed that radio waves could be reflected by metallic objects. Another German inventor and physicist Christian Hulsmeyer developed the first patented device that used radio waves for detecting the presence of distant objects. With time this developed into the radar as we know today. Reflection and scattering of radio waves by obstacles in the path is the basis for RADAR (radio detecting and ranging).
A transmitter within the radar emits a beam of short-wavelength energy in short pulses. A receiver listens for any waves that may be reflected or scattered back to the antenna. From the elapsed time between the transmitted pulse and its return, the distance of the ‘obstacle’ can be easily calculated by the direction the antenna is pointed.
Till World War II, radar was a secret war weapon, which was later used to detect the weather. A Doppler Radar is used to probe the atmosphere for liquid and solid water particles. The amount of backscatter from these particles depends strongly on their size and the wavelength of the radio waves.
Meteorological radar like the Doppler Radar sees only rain bearing clouds or those clouds that produce precipitation. So it is an essential tool to know where it is raining or dense clouds are being formed. It also gives a very good estimate of the intensity of the rains or precipitation because a greater returning signal means the presence of more and larger raindrops. This information helps in monitoring growth and path of cyclones. Weather Radar is designed to either sweep in a horizontal circle or up and down. It can have an effective horizontal range up to about 500KM before the curvature of the Earth hides too much of the lowest regions of the atmosphere.
Doppler Radar can provide information about both position and movement of the targets. It tracks the ‘phase’ of transmitted radio wave pulses. Here phase means shape, position and form of these pulses. Computers measure the shift in phase between the original transmitted pulse and that of the echo reflected back. Through this movement of raindrops can be calculated to know whether the precipitation is moving towards or away from the radar.
In India, Doppler Radars of the IMD use diferent frequencies like S-band, C-band and X-band. They have coverage area of around 500KM. Doppler Radars guide meteorologists during extreme weather events like cyclones associated heavy rainfall. X-band radar is used to detect thunder storms. C-band helps in cyclone tracking. Radar observations are usually updated every 10 minutes, for timely predictions of weather changes.