Skywatchweather.in
Skywatchweather.in
Radar colour measures how strongly the atmosphere reflects a pulse, not how much rain is coming. The difference explains most of the misreading seen in public commentary.
Monalisa DuttaRadar imagery has become public property. During any active monsoon spell, colour-coded radar loops circulate widely within minutes, generally accompanied by a confident caption.
Most of those captions are mistaken in the same manner. They treat colour as a measure of how much rain is approaching. It is not. Colour on a radar image indicates how strongly the atmosphere is reflecting a radar pulse back to the receiver, and the relationship between that quantity and what falls upon the ground is looser than is generally assumed.
A Doppler weather radar transmits a pulse of microwave energy and then listens. Raindrops, hailstones, snowflakes and — inconveniently — birds, insects and buildings scatter a portion of that energy back towards the radar. The instrument measures how much returned and how long it took to do so.
The strength of the returned signal is termed reflectivity and is expressed in dBZ, decibels relative to Z. The scale is logarithmic, which is the first point at which intuition fails: a doubling of the number does not represent a doubling of anything readily imagined.
Colour tables vary between agencies, but the widely used convention runs approximately as follows.
| dBZ | Typical colour | Interpretation |
|---|---|---|
| 5 – 20 | Pale blue, light green | Very light rain, drizzle, or non-meteorological returns |
| 20 – 35 | Green | Light to moderate rain |
| 35 – 45 | Yellow | Moderate to heavy rain |
| 45 – 50 | Orange | Heavy rain |
| 50 – 60 | Red | Very heavy rain, hail possible |
| Above 60 | Magenta, purple, white | Large hail almost certain |
As a working rule, values above approximately 50 dBZ are difficult to achieve with raindrops alone. Water droplets reach a maximum stable size and then break apart. Returning that quantity of energy generally requires something larger and, critically, something wet and icy. Hail coated in a film of liquid water is an outstanding radar reflector.
Magenta on a radar image is therefore not an indication of extremely heavy rain. It is a strong indication of hail.
Two situations regularly mislead.
Warm rain processes. Along the Western Ghats and the west coast during active monsoon, a great deal of very heavy rainfall is produced by comparatively shallow cloud containing enormous numbers of small droplets. Many small drops return modest reflectivity. The radar displays unremarkable green and yellow while rain gauges record extraordinary accumulation. Some of the heaviest rainfall days recorded along the Konkan coast appear thoroughly unimpressive on a reflectivity image.
Training. An individual moderate cell is an inconvenience. A line of moderate cells passing repeatedly over the same catchment for six hours produces a flood. Rainfall accumulation is the product of intensity and duration, and a single radar frame conveys only intensity. This is why urban flooding in Mumbai, Chennai and Bengaluru frequently develops beneath a radar picture which never appeared alarming in any individual image.
Most publicly circulated radar images display reflectivity alone. The term Doppler, however, refers to something else entirely, and arguably the more valuable measurement.
By measuring the frequency shift of the returned pulse, the radar determines whether targets are moving towards or away from it. This is displayed as a separate velocity image, conventionally using green tones for motion towards the radar and red tones for motion away from it.
What forecasters examine in velocity imagery is not the colours themselves but their arrangement:
Ground clutter. Buildings, terrain and towers near the radar return energy permanently, appearing as a persistent speckled patch centred upon the radar site which never moves and never develops. Modern processing removes most of it, but on unfiltered imagery it remains a common source of false interpretation. A return which has not moved in an hour is not weather.
The bright band. In stratiform rain there is frequently a distinct ring or band of enhanced reflectivity at the altitude where snow melts into rain. Partially melted snowflakes are large and wet, making them ideal reflectors, and they return considerably more energy than either the snow above or the rain below. It resembles a band of heavier precipitation but is in fact a melting layer, and it routinely causes rainfall to be overestimated.
Attenuation. Radar energy is absorbed as it passes through intense rain, so a very heavy cell may conceal what lies behind it. A region beyond a strong cell may appear clear simply because the beam never reached it. An apparently empty sector downrange of a red core should not be assumed to be genuinely empty.
India's Doppler weather radar network has expanded substantially over the past decade, and coverage of most major population centres is now genuinely good. That imagery represents a remarkable public asset. It simply rewards being read as a physical measurement rather than as a map of how severe the weather is about to become.
Live radar imagery is available alongside satellite and lightning data on our Live Weather Dashboard.
Meteorology consultant and former Squadron Leader in the Indian Air Force, with over eleven years of experience in operational aviation meteorology, weather forecasting and flight safety support. She now advises technology organisations on AI-driven weather forecasting and decision-support systems. Consultancy services →