Radar reflectivity
Models & data Radar reflectivity factor Reflectivity factor
Radar reflectivity describes how strongly particles in the atmosphere scatter a radar signal back. It helps detect precipitation but is not yet a direct rainfall measurement.
Radar listens for its echo
Weather radar transmits microwaves. Raindrops, snowflakes and hailstones scatter a small part back. Reflectivity is derived from that echo. Larger or more numerous particles generally produce stronger signals.
What dBZ means
Displays often use dBZ, a logarithmic scale. An increase of 10 dBZ means a tenfold increase in the reflectivity factor. Forty dBZ is therefore not simply twice twenty dBZ. Rain rate does not increase in the same proportion either.
Why a red patch does not tell the whole story
Conversion to mm/h depends on particle size and type. Hail and melting snow can produce strong echoes. Insects, birds and ground reflections can also return signals that processing tries to remove.
Radar measures above the ground
The beam is higher farther from the radar. Precipitation may evaporate beneath it or drift in the wind, so an echo does not guarantee rain has already reached the ground directly below. Colour scales also differ: some show dBZ, others estimated rainfall rates.