Velocity is the radar product that finds tornadoes
Reflectivity tells you where precipitation is. Velocity tells you which direction the precipitation is moving relative to the radar — and that's how you find rotation. Almost every tornado warning issued by radar (rather than by spotter) is justified by a velocity signature.
If you only ever learn one advanced radar product, learn velocity.
How velocity radar works
The radar sends out a pulse and listens for the return. Beyond just measuring the strength of the return (reflectivity), it measures the frequency shift of the returned pulse — the Doppler effect. Targets moving toward the radar shift the frequency up; targets moving away shift it down. The software color-codes that:
- Green = motion toward the radar (inbound)
- Red = motion away from the radar (outbound)
- Brightness = magnitude (faster = brighter)
The signature you're looking for: the couplet
A velocity couplet is a tight pair of inbound (green) and outbound (red) pixels right next to each other. It means air on one side is moving toward the radar and air right next to it is moving away — in other words, rotation.
Two important details:
- The green pixel should be on the left of the red pixel for cyclonic (counter-clockwise) rotation in the northern hemisphere — which is what tornadoes do.
- The tighter and brighter the couplet, the stronger the rotation.
A loose, diffuse couplet is usually a mid-level meso. A tight, intense, low-elevation couplet is often a tornado.
Tornado Vortex Signature (TVS)
A TVS is an algorithm-flagged extreme velocity couplet — typically 70+ kt of gate-to-gate shear. When you see a TVS marker on radar, the algorithm is saying "there is almost certainly a tornado at this location right now."
TVS is not infallible. False positives happen with very strong mesos that aren't tornadic yet. False negatives happen with tornadoes too small to resolve at long range. But TVS is one of the highest-confidence radar signatures available.
Tornado Debris Signature (TDS)
Dual-polarization radar added a powerful new product: the TDS, sometimes called a "debris ball." A TDS shows up on the correlation coefficient (CC) product as a small drop in CC value (below ~0.8) co-located with the velocity couplet.
When you see CC drop sharply at the exact location of a couplet, the radar is telling you it's seeing not raindrops but lofted debris — boards, dirt, vegetation, sometimes building material. A TDS is a near-certain confirmation that a tornado is on the ground.
TDS doesn't show up for every tornado — small ones over open prairie may not loft enough debris. But when it appears, it removes essentially all doubt.
Range folding and aliasing
Two technical limitations to know about:
- Range folding ("purple haze") appears as solid purple pixels where the radar can't unambiguously assign motion. Mostly cosmetic; ignore those areas.
- Velocity aliasing happens when wind speeds exceed the radar's Nyquist velocity. The radar may show a tight green/red boundary that's actually one continuous fast wind, not a couplet. Modern dual-pol radars handle this better but it still happens.
If a "couplet" only appears at one elevation and not at the next sweep, suspect aliasing.
Storm-relative vs. base velocity
Most modern radar apps offer storm-relative velocity as an option. This subtracts the storm motion vector from the wind field. The result: the broad inbound/outbound pattern of a moving storm disappears and only the rotation embedded in the storm is visible.
For finding mesos and tornadoes, storm-relative velocity is usually easier to read than base velocity — especially with fast-moving cells.
How chasers actually use velocity
- After you find an interesting storm on reflectivity, immediately switch to velocity.
- Look at the lowest tilt (0.5°) first — that's the closest the radar gets to the ground.
- Find any couplets. Tight + bright + co-located with the hook = real.
- Switch to CC (correlation coefficient) and look for a debris signature at the same location.
- Use storm-relative velocity to confirm the couplet isn't a cosmetic artifact.
- Compare consecutive scans (every 4–6 min on SAILS): is the couplet tightening, weakening, moving as expected?
Common mistakes
- Reading only reflectivity. A perfect hook with no couplet often isn't tornadic yet.
- Looking at high elevations. Tornadoes are surface phenomena; use the lowest tilt.
- Trusting velocity at long range. Beyond ~100 km, the beam is well above the ground and can miss low-level couplets.
- Ignoring CC. A debris signature is the single highest-confidence remote confirmation of a tornado.
Sources to know
How Meso connects
Velocity reading is a manual skill — Meso doesn't replace it. But Meso surfaces nearby NWS warning polygons, TVS-flagged storms, and reported tornado spotter ground-truth in real time, so you have context for the couplet you're staring at on your tablet. When a storm you're tracking suddenly gets a TVS or TDS flag, Meso surfaces the change under "Did anything important just change?" — one of the eight core chaser questions — so you can re-evaluate your position and bail route immediately rather than scrolling Twitter for confirmation.