How to read radar velocity

Radar velocity is the speed of targets moving directly toward or away from the radar, shown by convention in green for inbound motion (negative values) and red for outbound motion (positive values). It is one component of the wind, never the whole wind, and nearly every skill in reading it comes from keeping that in mind.

The how to read weather radar guide introduces velocity alongside reflectivity and dual-polarization. This page stays on velocity: the geometry, the artifacts, and the patterns that separate rotation from wind simply coming together or spreading apart.

What radial velocity measures

The NWS Warning Decision Training Division (WDTD) defines radial velocity as the component of target motion parallel to the radar radial, and states three principles. Radial velocity is always less than or equal to the true velocity. It equals the true velocity only when the target moves straight toward or away from the radar. And it reads zero when the target is stationary or moving perpendicular to the beam.

The relationship is a cosine, so it is not linear. WDTD’s example: wind blowing 45° off the beam still shows about 70% of its speed, not 50%. At 90° the display reads zero while the wind has not changed at all.

Colors and signs

On NWS displays, inbound velocities are negative and drawn in cool colors, outbound velocities positive and drawn in warm colors. WDTD traces the sign convention to the first Doppler radars, which pointed straight up, so falling air moved toward the antenna. The NSSL Guide for Interpreting Doppler Velocity Patterns (Brown and Wood, 2007) uses the same convention: positive is flow away from the radar. Color tables beyond that basic green-and-red split vary by display; see weather radar colors.

The zero isodop

An isodop is a line of constant radial velocity. The zero isodop is the line where radial velocity is zero, and in widespread precipitation or clear air it is the most informative line on the display.

• Wind direction. Draw a line from the radar to any point on the zero isodop. The wind there blows perpendicular to that line, from the inbound side toward the outbound side. WDTD stresses drawing the arrow perpendicular to the radial, not to the isodop itself.
• Height. Because the beam climbs with distance, range stands in for height. A straight zero isodop means the direction is the same at every level sampled.
• Veering and backing. A zero isodop curved like the letter S means winds turn clockwise with height (veering), which WDTD says generally indicates warm air advection. A backward S means counterclockwise turning (backing), generally associated with cold air advection.
• Diffluence and confluence. A bowed zero isodop with inbound velocities inside the bow means air spreading apart over the radar; with outbound velocities inside, air coming together.

These readings assume the flow is horizontally uniform at each level. Near fronts and storms it is not, and the patterns break down.

Range folding: the purple haze

A pulsed radar assumes each echo belongs to the pulse it just sent. Brown and Wood give the maximum unambiguous range as r_max = c/(2 PRF), where PRF is the pulse repetition frequency. Return from a target beyond r_maxarrives after the next pulse leaves and is plotted at the wrong, shorter range. The WSR-88D’s design lets it recover much of that velocity into the correct range, Brown and Wood note, but not all of it. NWS Louisville’s sampling notes call the remaining range-folded areas “purple haze” and warn that they can obscure features in velocity and spectrum width. Their suggested workarounds: look at other elevation angles, or at another radar.

Aliasing and the Nyquist velocity

The same pulse rate caps the speeds a radar can measure. Brown and Wood give that limit, the Nyquist velocity, as V_max = ±PRF λ/4. The product of V_max and r_max is fixed for a given wavelength, so a longer unambiguous range means a lower velocity limit. This trade is known as the Doppler dilemma.

Speeds beyond the limit wrap around. In the NSSL guide’s example, with a Nyquist velocity of ±30 m/s, a true +36 m/s reads as −24 m/s. Without correction, the guide notes, this produces an abrupt change from one end of the color scale to the other. That is also what a tight couplet looks like, which is why aliasing can pass for rotation. The check is spatial continuity: an aliased patch is usually an island of strong opposite color inside a broad area of strong wind of one sign, surrounded by values near the Nyquist limit rather than by weaker ones.

For scale, a 2012 Radar Operations Center paper by Zittel and Jing gives WSR-88D Nyquist velocities of 21 to 35 m/s in the precipitation scan strategies and about 11.5 m/s in the long-pulse clear-air strategy. The radar’s product generator dealiases velocity automatically. The same paper describes the legacy algorithm failing under strong shear, in weak echoes, when the Nyquist velocity is far below the prevailing wind, and around moving clutter such as wind turbines, and reports that the ROC planned to start fielding a two-dimensional replacement in fall 2012. WDTD’s summary still applies: dealiasing works most of the time, and failures do occur.

Base velocity or storm-relative velocity

The NWS glossary describes the WSR-88D storm-relative velocity product as radial velocity with the average motion of all identified storms subtracted out, meant to show shear and rotation that storm motion might otherwise hide. The mesocyclone guide covers why that balances a couplet and why a wrong storm motion undermines it.

The first row comes from an NWS Nashville case study of a damaging 20 June 1998 storm, whose authors chose base velocity for the surface outflow because it “gives a more authentic estimate of the strength and motion of winds experienced near the surface.”

Couplets, convergence and divergence

When inbound and outbound maxima sit close together, their positions relative to the radar tell you what the air is doing. WDTD’s storm-scale rules:

Any other orientation is a mix. WDTD’s example of a supercell that became tornadic shows cyclonic convergence at 0.5°, nearly pure cyclonic rotation at 1.8° and 3.1°, and storm-top divergence at 10.0°.

Rotational velocity and shear

Rotational velocity is half the sum of the strongest inbound and outbound speeds, as the mesocyclone guide explains. In an NWS Central Region applied research paper from WFO Hastings (Herald and Drozd), a 15 May 1999 tornado in Rooks County, Kansas showed 26 m/s inbound beside 7 m/s outbound, reported as 17 m/s of rotation. The same paper defines azimuthal shear as the velocity difference along an azimuth divided by the azimuthal length. That is why a couplet’s diameter matters as much as its speeds: on the next scan the storm still showed 16 m/s of rotation, but the distance between maxima had grown from 2 to 4 km, which halves the shear (our arithmetic).

Numbers from single events do not transfer as thresholds. An NWS Nashville study of the 16 April 1998 tornadoes linked gate-to-gate rotational velocity near 50 knots to F3 damage close to the radar, and its authors describe it as one event.

WeatherStory Radar decodes velocity and storm-relative velocity from raw Level II, and lets you click any gate for its exact value and beam height.