Doppler shift from hydrometeors
Raindrops, snowflakes and hailstones moving toward the radar compress the pulse frequency; those moving away stretch it. The WSR-88D measures this shift to 1 m/s precision across every range bin in a volume scan. Green usually means rain moving toward you; red means away. Where they touch, rotation — the tornado signature.
Reflectivity (dBZ)
The radar also measures how much energy bounces back — reflectivity, in decibels relative to Z (dBZ). Light rain: 20 dBZ. Heavy rain: 45 dBZ. Hail core: 60+ dBZ. Your weather app maps these to blue-green-yellow-red-magenta. A trained meteorologist can read storm severity from the colour scale in seconds.
Dual-polarisation upgrade
Since 2011, NEXRAD radars transmit both horizontal and vertical polarisation. Ellipsoidal raindrops reflect differently than spherical hailstones or flat snowflakes. Dual-pol improves rainfall estimates, distinguishes precipitation type, and detects debris balls — the lofted wreckage that confirms a tornado is on the ground.
From research to your phone
The first operational Doppler weather radar was installed in 1968. NEXRAD's 160-site network was completed in 1997. Today, data flows to apps in under 5 minutes. The supercell that spawned a tornado in Oklahoma is visible on your phone before the sirens sound.