The India Meteorological Department’s (IMD) Doppler Radar in Mumbai, which surveys weather patterns and forecasts, stopped working after heavy rainfalls.

How does a Doppler radar work?

• In radars, a beam of energy– called radio waves– is emitted from an antenna.
• When this beam strikes an object in the atmosphere, the energy scatters in all directions, with some reflecting directly back to the radar.
• The larger the object deflecting the beam, the greater is the amount of energy that the radar receives in return.
• Observing the time required for the beam to be transmitted and returned to the radar allows weather forecasting departments to “see” raindrops in the atmosphere, and measure their distance from the radar.

What makes a Doppler radar special?

• It can provide information on both the position of targets as well as their movement.
• It does this by tracking the ‘phase’ of transmitted radio wave pulses; phase meaning the shape, position, and form of those pulses.
• As computers measure the shift in phase between the original pulse and the received echo, the movement of raindrops can be calculated.
• Thus it is possible to tell whether the precipitation is moving toward or away from the radar.

Types of Doppler radar

• In India, Doppler radars of varying frequencies — S-band, C-band and X-band — are commonly used.
• They help track the movement of weather systems and cloud bands and gauge rainfall over its coverage area of about 500 km.
• The radars guide meteorologists, particularly in times of extreme weather events like cyclones and associated heavy rainfall.
• An X-band radar is used to detect thunderstorms and lightning whereas C-band guides in cyclone tracking.

Why are they called ‘Doppler’ radars?

• The phase shift in these radars works on the same lines as the “Doppler effect” observed in sound waves.
• It tells that the sound pitch of an object approaching the observer is higher due to the compression of sound waves (a change in their phase).
• As this object moves away from the observer, the sound waves stretch, resulting in lower frequency.
• This effect explains why an approaching train’s whistle sounds louder than the whistle when the train moves away.
• The discovery of the phenomenon is attributed to Christian Doppler, a 19th-century Austrian physicist.

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