How to read velocity radar and storm-relative velocity

Velocity radar shows which way the wind is blowing inside a storm — toward the radar or away from it. Strong winds in opposite directions side by side are the signature of rotation.

Toward and away: the only thing velocity measures

Doppler radar measures how fast targets move along the beam — straight toward the radar or straight away from it. By convention, green is toward the radar (inbound) and red is away (outbound). Wind blowing across the beam shows as near zero, even if it is strong.

That means you must always know where the radar is. A storm east of the radar with red on it is moving east; the same red on a storm west of the radar means it is moving west.

Finding rotation: the couplet

When a storm rotates, one side of the circulation moves toward the radar and the other moves away. On velocity, that appears as a tight pair of bright green and bright red pixels next to each other — a velocity couplet. The tighter and stronger the couplet, the stronger the rotation.

• Rotational velocity (Vrot) is half the difference between the strongest inbound and outbound speeds. Forecasters treat higher Vrot — especially at close range and low in the storm — as a stronger tornado threat.
• Gate-to-gate shear — when the strongest inbound and outbound are in adjacent pixels — suggests a very tight circulation.
• Look at more than one tilt. Rotation that extends from near the ground up through the storm is more significant than rotation only aloft.

Why storm-relative velocity (SRV) is better for rotation

Base velocity includes the storm's own motion. A storm racing northeast at 50 mph adds that speed to everything inside it, which can hide the rotation. Storm-relative velocity subtracts the storm's motion so you see only the winds relative to the storm — couplets stand out far more clearly. TALON computes SRV from the radar's super-resolution data using the storm motion it measures (or one you set).

Common pitfalls

• Velocity aliasing — very strong winds can "wrap around" the scale and show up as the opposite color. Real couplets sit inside a storm's rotation; aliasing tends to appear as sharp, isolated color flips.
• Range folding — areas the radar can't resolve are shown as purple or blank.
• Convergence and divergence — green and red that face each other along the beam (rather than side by side across it) mean air coming together or spreading apart, not rotation.

Velocity in TALON

TALON Pro shows super-resolution velocity, SRV and an azimuthal-shear "rotation" view for every NEXRAD radar, and its rotation detector scans each new volume for couplets — showing their strength, depth, trend and track. Pair it with correlation coefficient to look for debris.

Quick answers