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Weather radar sends out radio pulses and maps the energy that returns. On U.S. National Weather Service (NWS) WSR-88D/NEXRAD displays, those returns can show precipitation echoes, radar-relative motion, and estimated accumulation—but they are samples of the atmosphere, not a direct picture of rain or wind at the ground.
How does weather radar work?
A WSR-88D antenna rotates while transmitting very short radio pulses and listening for echoes between transmissions. The radar records the antenna’s direction and the time between sending a pulse and receiving its return. Since the radio energy travels out and back, the elapsed time indicates how far away the target is; the antenna direction helps locate it on the map.
The strength of the returned energy depends on the targets in the sampled volume. More or larger targets can produce a stronger echo, but echo strength is not itself a rain-gauge reading. The radar cannot determine from reflectivity alone exactly how much precipitation is reaching a particular spot on the ground.
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Which radar product am I looking at?
Check the product name and legend before interpreting colors. Common NWS products show different properties of the radar sample; they are not interchangeable views of the same measurement.
| Product | What it represents | What to keep in mind |
|---|---|---|
| Base reflectivity | Strength of returned energy in the low, 0.5-degree elevation scan, commonly displayed in dBZ. | It is one low-elevation slice, not a direct surface rainfall measurement. |
| Composite reflectivity | The strongest reflectivity found at any scanned elevation in the radar volume. | A strong echo may be aloft even if the low-level reflectivity looks different. |
| Doppler velocity | Motion toward or away from the radar, along the radar beam. | It does not show the full wind vector or necessarily the total wind speed. |
| Storm-relative velocity | Radar-relative motion with storm movement subtracted. | It can make storm-scale circulation easier to inspect, but a signature is not proof of a tornado at the ground. |
| Precipitation estimates | Algorithmic estimates of precipitation over a stated time period. | Read the product’s time frame; an accumulation may cover a long period. |
NWS product explanations describe these views and their limitations in more detail: NWS JetStream: How to Read a Weather Radar.
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Where are the storms?
Use reflectivity to locate precipitation echoes
Reflectivity maps show returned energy, commonly in dBZ. Brighter or higher-reflectivity colors often accompany more intense precipitation or hail, but the color is not a guaranteed rainfall rate at the surface. Base reflectivity shows the low 0.5-degree scan; composite reflectivity shows the strongest return across scanned elevations. A composite image can therefore reveal a strong echo higher in the storm that is less evident in the low-level slice.
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Find the radar site on the map and locate yourself relative to it. Check the image timestamp and legend, and use a loop rather than a single frame to understand how echoes are changing. Radar coverage and the view of a storm vary by distance from the radar, so one map may not describe conditions uniformly across a large area.
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Are storms moving toward me?
Read Doppler velocity relative to the radar
Doppler radar detects a phase or frequency shift in returned pulses and estimates radial velocity: motion along the line between the target and radar. On NWS velocity displays, green or other cool colors commonly indicate motion toward the radar; red or warm colors commonly indicate motion away. Find the radar location first, since the colors describe motion relative to that site—not compass direction everywhere on the map.
Velocity does not capture motion perpendicular to the beam as inbound or outbound speed. A displayed radial speed can therefore be lower than the actual wind speed. Storm-relative velocity subtracts storm movement from the broader wind field to help reveal storm-scale circulation, but any rotation or tornado-related signature is a clue for forecasters to assess with other evidence, not confirmation of a tornado on the ground.
Use a loop cautiously to estimate arrival
A sequence of timestamped reflectivity images can help you judge whether a storm appears to be approaching and estimate its general movement. Storm motion can change, however, so extending a short loop forward does not guarantee an arrival time. For warnings and immediate hazard decisions, use current official NWS warnings and forecasts rather than relying on a radar image alone. The NWS event-ready guide recommends checking the timestamp, locating yourself, and looping reflectivity to assess approaching storms: NWS Event Ready Appendix C: Using Radar.
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Dual-polarization radar transmits and receives pulses oriented horizontally and vertically. Comparing their returns gives forecasters additional clues about target size, shape, and likely type. NWS products include hydrometeor classification, an algorithm’s most-likely category for targets in the radar volume; possible categories include rain, snow, hail, biological targets, clutter, and unknown.
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These data support precipitation estimates, precipitation-type assessment, and recognition of some non-weather echoes. A classification is not a direct visual identification of a particle, and no single map pixel makes the answer certain. NWS JetStream: Dual Polarization Radar
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It samples air aloft, not conditions at your feet
The radar beam spreads as it travels, and its height above ground increases with distance. The NWS Central Region event-ready guide notes that at far distances the beam may be 10,000 feet or more above ground. Precipitation seen aloft may weaken, evaporate, or otherwise differ before reaching the surface, so the echo may not match what a rain gauge or observer reports. The NWS guide cautions that the farther a location is from the radar, the greater the potential difference between the radar picture and surface conditions.
It does not measure everything in a storm
- Lightning: Radar does not provide lightning information; lightning is detected using a separate data source.
- Exact surface rainfall: Reflectivity measures returned energy from targets in the sampled volume, not precipitation collected at a specific ground location.
- The complete wind: Doppler velocity shows only the toward-or-away component relative to the radar.
- Certainty about hail or tornadoes: Algorithms that estimate hail size, rotation, or tornado potential can be wrong. Treat signatures as clues, not diagnoses.
NWS radar product descriptions and the event-ready guide explain these limits: NWS Raleigh: Radar Information and NWS Event Ready Appendix C: Using Radar.
How should I use a radar map?
- Confirm the product. Read its name and legend to distinguish reflectivity, velocity, storm-relative velocity, or a precipitation estimate.
- Check the timestamp. A useful-looking image may be stale; note when it was captured before interpreting it.
- Locate the radar and yourself. This is essential for reading inbound and outbound velocity and for judging how high the beam may be over your area.
- Loop multiple images. Use the time sequence to assess broad movement, while treating any projected arrival as an estimate.
- Check official alerts for decisions. Consult current NWS warnings and forecasts for hazards; do not use a radar image by itself as a warning or all-clear.
These products and conventions are those of the U.S. NWS WSR-88D/NEXRAD system. Other countries and radar networks may use different systems, products, legends, or display conventions.
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