Reflectivity — how much rain
The radar emits 750 kW pulses at 2.7-3.0 GHz. Raindrops, hail and snow reflect a tiny fraction back. The strength of the return — measured in dBZ — converts to rainfall rate. Light rain shows green on the map; thunderstorms show red and purple. Hail produces especially strong returns because ice reflects more than water.
Velocity — how fast and which way
Doppler shift converts to wind speed within each range cell. Toward the radar shows as green; away shows as red. The boundary between green and red, with strong shifts on both sides, is a rotating mesocyclone. The famous 'hook echo' shape combined with a tight velocity couplet is the signature of a tornado-producing storm.
Dual polarisation — what's actually in the air
Since the 2010s, NEXRAD radars transmit both horizontally and vertically polarised pulses. Comparing the returns tells the radar what shape the targets are. Round drops vs flat raindrops vs hail vs insects vs debris. Tornado debris signatures — chunks of buildings and trees swirling in a vortex — are now visible directly on radar, often before any storm spotter can see anything.
Why 13 minutes matters
In 1980, average tornado warning lead time in the US was 3 minutes. After NEXRAD deployment in the 1990s, it climbed to 13 minutes. Dual-pol added another margin. Every minute of warning saves lives. The same Doppler radar that started as a fighter intercept tool now saves around 100 American lives per year.