seasonmap field guide
How to read weather radar
This page covers how to read the three main radar products — reflectivity, velocity, and correlation coefficient — and the common false echoes that mimic weather.
What radar measures
A NEXRAD radar sweeps a beam and listens for what bounces back. Two consequences follow. First, it measures what's in the air, not what reaches the ground — rain evaporating before landing (virga) still paints an echo. Second, the beam climbs with distance: 100+ miles out, it samples a storm at 10,000+ feet or higher, so distant snow showers can go undetected while distant thunderstorm tops appear intense. Most apparent radar inconsistencies trace back to one of these two facts.
Reflectivity: the dBZ scale
| dBZ | Typical meaning |
|---|---|
| 10–25 | Drizzle, light rain, or snow (snow reads ~10 dBZ lower than rain at the same intensity) |
| 30–40 | Solid rain — the classic green-to-yellow transition |
| 45–55 | Heavy rain, small hail possible — orange/red core territory |
| 60+ | Large hail likely aloft |
A common artifact: in cool-season rain, a ring of enhanced echo often appears around the radar site — the "bright band," where melting snowflakes acquire a water coating and reflect more strongly, like large raindrops. It marks the melting layer, not a ring of heavier precipitation.
Velocity: the rotation product
Doppler velocity colors motion along the beam: conventionally green toward the radar, red away. Most of the display shows ordinary wind. The signal to look for is a couplet — bright green pixels adjacent to bright red, indicating air moving toward and away from the radar at the same location. That pattern indicates rotation. When the couplet is tight ("gate-to-gate") and strong, it indicates a mesocyclone, which is why tornado warnings can be issued before a tornado is visually confirmed.
Two limitations: velocity only detects the along-beam component, so rotation viewed end-on can be underrepresented, and beyond roughly 80 miles the beam is often sampling above the storm level that matters for surface rotation.
Correlation coefficient: the debris signal
Dual-pol radar transmits horizontal and vertical pulses and compares the returns. Raindrops and snowflakes are uniformly shaped, so the two channels agree closely — correlation near 1.0. Tornado debris (lumber, shingles, vegetation tumbling in random orientations) produces disordered returns and a sharp drop in correlation. A compact area of low correlation coinciding with a velocity couplet inside high reflectivity is a debris ball — radar evidence that a tornado is on the ground causing damage, distinct from a radar-indicated rotation signature without confirmed debris.
Correlation coefficient also marks the melting layer (the bright-band ring drops below roughly 0.97) and flags non-precipitation returns such as smoke, insects, and birds.
Common false echoes
- Ground clutter — a persistent speckle pattern near the radar site; filtered by processing, but not completely.
- Anomalous propagation — on calm, clear nights, temperature inversions bend the beam toward the ground, producing sprawling echoes that do not move like weather.
- Biological returns — expanding ring patterns at sunrise are birds leaving roosts; broad nighttime returns in summer are insects and migrating birds. Correlation coefficient distinguishes these from precipitation.
- Wind farms — stationary flickering patches with spurious velocity values.
Ryzhkov, A. V., & Zrnić, D. S. (2019). Radar Polarimetry for Weather Observations. Springer.
NOAA/NWS WSR-88D documentation (Radar Operations Center).