An instrument that sends out microwave pulses and listens for the echo from raindrops, showing where rain is falling right now, how hard, and which way it is moving.
ForecastingWhat is Weather Radar?
Most weather observations tell you about one point. A rain gauge in Chennai tells you what fell at that gauge; a satellite shows the tops of clouds. Weather radar produces a map of the rain itself, out to a few hundred kilometres, refreshed every few minutes, from the ground looking up through the storm rather than from orbit looking down at its lid.
That is why radar is the instrument behind almost every useful short-notice warning in South Asia — the squall line running towards Kolkata on an April evening, the band of rain that will shut Mumbai’s suburban railway in the next hour, the eye of a cyclone in its final approach to the Odisha coast.
The word is an acronym — RAdio Detection And Ranging — and the technology was developed to find aircraft. Early operators were annoyed to find rain cluttering their screens. The clutter turned out to be the more useful signal.
How a radar sees rain
A radar antenna sends out a short, powerful pulse of microwave energy and then listens. Raindrops, hailstones and snowflakes scatter a tiny fraction of it back. Range comes from how long the echo takes to return — the pulse travels at the speed of light, so a round trip to a storm 150 km away takes about a thousandth of a second — and direction comes from where the antenna was pointing.
Combine thousands of these measurements as the antenna rotates and you have a map. The antenna then tilts up a fraction of a degree and sweeps again, building a three-dimensional volume of the atmosphere every few minutes.
The strength of the echo is reported as reflectivity, in dBZ on a logarithmic scale. Light drizzle returns around 20 dBZ; steady moderate rain sits in the thirties; heavy monsoon rain reaches the mid-forties and above; values past the mid-fifties usually mean hail, because hailstones are far larger than raindrops and the echo grows steeply with particle size. Converting reflectivity to a rain rate relies on an empirical relationship between drop sizes and echo strength, which is why radar-estimated totals and gauge totals rarely match exactly.
Wavelength matters a great deal in the tropics. Longer-wavelength S-band radars penetrate heavy rain with little loss. Shorter C-band and X-band radars are cheaper, but in the rain the monsoon produces, the near end of a storm can absorb so much of the beam that the far side is underestimated or disappears entirely — attenuation. Cyclone-prone countries therefore lean on S-band for their coastal radars.
Modern radars also transmit pulses polarised both horizontally and vertically. Because large raindrops flatten as they fall while hailstones tumble, comparing the two returns separates rain from hail from melting snow, and improves rainfall estimates.
Weather radar in South Asia
The India Meteorological Department operates a national network of Doppler weather radars, expanded substantially over the past two decades, with coverage concentrated on the major cities and — deliberately — on the cyclone-exposed east coast. The Pakistan Meteorological Department and the Bangladesh Meteorological Department run their own networks, Bangladesh’s positioned against the Bay of Bengal, the most lethal storm-surge coastline in the world.
Radar has three jobs here that nothing else does as well:
- Cyclone landfall. Satellite tracks a storm across the ocean, but its position estimate carries real uncertainty. Once a cyclone moves within radar range — roughly the last day before landfall — the eye becomes directly visible and the crossing point can be pinned down far more tightly. On the coasts that took the 1999 Odisha Super Cyclone and Cyclone Amphan in 2020, those final hours decide which districts get evacuated.
- Monsoon urban flooding. Cities flood on timescales of an hour or two. Radar tracking of individual rain cells over Mumbai, Chennai, Kolkata or Dhaka is the only warning that operates on that timescale.
- Pre-monsoon severe weather. The violent thunderstorms and squalls of April and May across eastern India and Bangladesh — the nor’westers or kalbaisakhi — develop and move too quickly for anything but radar to follow.
What radar cannot see
Radar’s limitations are geometric, and in South Asia they bite in specific places.
The beam rises. A radar beam travels in a nearly straight line while the Earth curves away beneath it. At long range it is several kilometres up, sampling the middle of a cloud and missing rain falling below it. Coverage maps look generous; useful low-level coverage is much smaller.
Mountains block it. A radar in a valley sees the ridge in front of it and nothing behind. This is the core problem in the Himalaya and along the Western Ghats — precisely where cloudbursts and landslide-triggering rainfall occur, in Uttarakhand, Himachal Pradesh, the northeast and northern Pakistan. Siting radars on peaks helps with blocking but worsens the beam-height problem for the valleys below.
Ground clutter and false echoes. Buildings, hills and the sea surface return echoes. Strong temperature inversions — routine over the winter plains and the Arabian Sea — bend the beam downward and make the ground itself appear as widespread light rain. Processing removes most of this, but not all.
It only sees precipitation. Radar does not measure temperature, pressure or wind away from precipitation, and it sees nothing over the open ocean beyond its range. It is an observation instrument, not a forecast.
Radar, satellite and models: three different jobs
These three are complementary, not competing. Radar covers zero to about three hours with high spatial detail, over land, within range. Satellite covers everything including the open ocean, sees cloud-top structure rather than the rain below, and is the primary tool for a cyclone still far from the coast. Numerical models cover hours to weeks ahead everywhere, but cannot say exactly where an individual thunderstorm cell will be in forty minutes.
The forecasts on this site come from numerical models, which is what makes multi-day city forecasts possible. For live radar imagery during an unfolding storm, the national meteorological services publish their own products, and that is the right place to look.
Frequently asked questions
How far can a weather radar see? A few hundred kilometres for detecting that something is there, but far less for reliable measurement of rain near the ground, because the beam climbs with distance. Heavy rain close to the radar weakens the beam and shortens the useful range further.
Why does radar show rain when it is dry outside? Usually ground clutter or anomalous propagation — the beam bent downward by a temperature inversion and reflecting off terrain or buildings. It can also be genuine rain evaporating before it reaches the ground, which is common over the dry northwest.
Can radar see a cyclone before it arrives? Only in the last stage of its approach, once it is within range. Before that, satellite does the tracking. Radar’s contribution is precision about where and when landfall happens, which is exactly what evacuation decisions need.
Where can I get the forecast for a city in the radar-covered coastal belt? See the current conditions and multi-day outlook for Chennai, Mumbai, Kolkata, Bhubaneswar, Visakhapatnam and Dhaka.