Radar guides

Tornadoes on radar

Hook echoes, velocity couplets and debris signatures — and how confident each one should make you.

Radar does not see tornadoes. It sees rotation, and sometimes it sees what a tornado has picked up. Those are different observations with different confidence levels, and conflating them is the most common error in reading a warning-scale storm.

There are three signatures worth knowing, and they form a ladder of increasing certainty.

1. The hook echo — the weakest of the three

The famous one, and the least diagnostic. A hook is a curl of reflectivity on the rear-right flank of a supercell, wrapping counter-clockwise. It forms because precipitation is being drawn around the mesocyclone — the hook is the rain being wrapped, not the tornado.

Many hooks never produce a tornado, and many tornadoes occur with no visible hook, particularly in rain-wrapped storms and linear systems. A hook says “this storm has organized rotation dragging precipitation around it.” That is worth your attention and it is not a detection. Treat it as the thing that makes you look at velocity.

Go deeper: Why hooks form, and the 1953 radar film that first caught one

2. The velocity couplet — the real detection

Look for strong inbound immediately adjacent to strong outbound, with a tight boundary between them. That is rotation on an axis perpendicular to the beam.

  • Tightness beats magnitude. A 60-knot difference across two gates is far more concerning than the same difference spread over several miles. The first is a small violent circulation; the second is broad, ordinary storm-scale rotation.
  • Height above ground. The beam rises with range. A couplet 15,000 feet up in a distant storm is a midlevel mesocyclone, not a tornado. The same couplet at 1,500 feet is another situation entirely.
  • Depth. Rotation continuous through several elevation cuts is a deep, organized mesocyclone. Rotation at one level may be noise, aliasing, or a brief spin-up.
  • Persistence. A couplet surviving several volume scans means something. One that appears for a single scan often does not.

The aliasing trap: velocity wraps past a radar’s maximum unambiguous value, so a very fast inbound can be reported as a fast outbound. It mimics an extremely tight couplet. The tell is a sharp noisy boundary with no supporting structure in reflectivity and no coherence across tilts.

The national radar mosaic across the eastern United States overlaid with hundreds of amber surface observation stations and green isobar contours.
Surface observations drawn over the reflectivity field. Radar gives you the storm; the surface field gives you the environment it is moving into.

3. The debris signature — confirmation, after the fact

This is the only radar signature that constitutes evidence a tornado is on the ground right now, and it comes from dual-polarization. A tornado debris signature requires three things co-located:

  • High reflectivity where the circulation is — often a distinct blob, hence “debris ball”
  • Correlation coefficient dropping below roughly 0.80 — the diagnostic half
  • ZDR near zero — debris tumbles randomly rather than falling in a preferred orientation

The CC drop is the physics. Correlation coefficient measures how uniform targets are within a sample volume. Rain is very uniform and sits near 0.98. Lofted debris — lumber, insulation, soil, vegetation — is wildly non-uniform, and CC collapses. No ordinary precipitation does this.

Go deeper: The debris signature in depth: thresholds, false positives, and what height means

Go deeper: Wedge, rope and rain-wrapped tornadoes: why shape is not intensity · Funnel clouds, landspouts and gustnadoes · The El Reno tornado of 2013, on radar

Reading them together

No single field decides. The confident read is a stack.

EvidenceWhat it establishes
Hook in reflectivityThe storm has organized rotation wrapping precipitation
Tight couplet in velocityThere is a small, intense circulation
Couplet low and persistent across tiltsIt is deep and near the ground
CC below 0.80, co-locatedSomething non-meteorological is airborne
ZDR near zero thereThat something is tumbling

Each field answers a question the others cannot. That is the argument for a tool that shows all of them from the same volume rather than one derived product at a time — you are cross-examining a single moment from several angles.

Go deeper: Tornado Alley, Dixie Alley and when the season peaks · The EF scale: how a tornado gets its rating

What this page will not tell you

Whether to take shelter. That is not a radar-reading question. Warnings come from the National Weather Service, which has training, context and ground truth no radar image carries. If you are in a warned area, act on the warning.

Radar is a tool for understanding storms, not a replacement for the warning system. Anyone telling you otherwise is selling you something.

Next: the supercell, the storm type most of these signatures live inside.

The other guides

  • The hook echoThe most famous shape on weather radar: why it forms, where it came from, and why it is a reason to look at velocity rather than a tornado detection.
  • The tornado debris signatureWhat a TDS is in dual-polarization data, the thresholds forecasters actually use, and the ways low correlation coefficient can fool you.
  • The El Reno tornado of 2013, on radarThe widest tornado on record: its erratic path, the multiple-vortex structure mobile radars resolved, why radar-measured winds near 300 mph still produced an EF3 rating, and what the operational radar could and could not see.
  • Wedge, rope and rain-wrapped tornadoesWhat the shape words spotters use actually describe, why a tornado’s width and shape do not tell you its rating, and what radar sees when the funnel cannot be seen at all.
  • Funnel clouds, landspouts and gustnadoesWhat a funnel cloud is and when it counts as a tornado, why cold-air funnels and landspouts rarely show up in velocity data, and why a gustnado is logged as thunderstorm wind.
  • Tornado Alley, Dixie Alley and tornado seasonWhere and when United States tornadoes happen: why Tornado Alley has no official boundaries, how the Southeast differs, the sourced seasonality, the eastward shift in the research, and why a map of reports is not a map of risk.
  • The EF scale: how an EF5 rating is assignedThe Enhanced Fujita scale rates tornadoes from damage, not from measured wind. The 28 damage indicators, the EF0-EF5 gust ranges, and why an EF5 is so rare.
  • How to read weather radarWhat the colors mean, why velocity matters more than reflectivity, and what the radar physically cannot see.
  • The supercell, on radarMesocyclone, BWER, rear-flank downdraft — what a rotating updraft does to the data.
  • Squall lines, bow echoes and derechosOrganized linear convection, why bowing means damaging wind, and what actually qualifies as a derecho.
  • Outlooks and forecast modelsWhat the SPC categories mean, and the most common way people misread a convection-allowing model.

Figures in these guides are unmodified output from WeatherStory Radar, which reads Level II straight from NOAA’s public archive at full resolution. Nothing is thresholded, smoothed or decimated.

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