Weather radar sends out short radio pulses and maps the energy that bounces back. On U.S. National Weather Service (NWS) WSR-88D/NEXRAD displays, that return can show where precipitation is aloft, how echoes are moving toward or away from the radar, and clues about what the targets may be. It is not a camera view of the ground: radar samples a volume of atmosphere, and its beam rises higher above the surface with distance.
How does weather radar work?
A WSR-88D antenna rotates while transmitting very short radio pulses and listening for echoes between transmissions. The antenna direction indicates where a return came from; the elapsed time between transmission and return indicates its distance because the radio wave travels out and back. Stronger returned energy can come from more or larger targets, but it does not by itself give a precise rainfall rate at the surface. The NWS Birmingham explanation of radar says precipitation-mode volume scans take about 4–6 minutes, depending on the volume coverage pattern (VCP). That describes the radar’s scan context, not a guaranteed refresh interval for every app or viewer.
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The radar makes measurements at multiple elevation angles. Taken together, those slices form a three-dimensional sample around the radar, which is why different radar products can show different aspects of the same storm.
Where are the storms?
Reflectivity shows echo strength
Reflectivity, commonly displayed in dBZ, represents the strength of energy returned by targets in the sampled volume. Higher or brighter reflectivity often accompanies heavier precipitation or hail, but it is not a direct measurement of how much rain is reaching a particular spot on the ground.
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Two common reflectivity products differ in the part of the scan they show:
| Product | What it represents | How to read it |
|---|---|---|
| Base reflectivity | The low-elevation, 0.5-degree scan. | A lower slice of the radar sample; it may differ from stronger echoes higher up. |
| Composite reflectivity | The strongest reflectivity found at any scanned elevation. | Useful for seeing the strongest echo in the scanned volume, but that maximum may be aloft rather than near the ground. |
These product descriptions follow the NWS radar product guide and NWS radar products overview. A bright spot on a composite image therefore does not establish that the same intensity is occurring at the surface.
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Estimated precipitation is not a rain-gauge reading
NWS one-hour and storm-total precipitation products are algorithmic estimates. The NWS Birmingham explanation describes storm-total accumulation as updating continuously since the last one-hour break in precipitation. In a prolonged event, that interval can span multiple days, so read the product’s stated accumulation period before interpreting its total.
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Are the storms moving toward me?
Doppler velocity shows motion relative to the radar
Doppler processing uses a phase or frequency shift in returned pulses to estimate radial velocity: movement toward or away from the radar. On NWS velocity displays, green or cool colors commonly indicate motion toward the radar, while red or warm colors commonly indicate motion away. Find the radar location on the map before interpreting the colors.
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Velocity is not a complete wind vector. It captures motion along the radar beam, not motion perpendicular to it, so displayed radial speed can be lower than the full wind speed. Storm-relative motion subtracts storm movement from the overall wind field to make storm-scale circulation easier to inspect. A circulation signature is a clue for forecasters to assess alongside other evidence—not proof that a tornado is on the ground.
What can dual-polarization radar tell me?
Dual-polarization radar transmits and receives pulses in horizontal and vertical orientations. Comparing their returns adds clues about the size, shape and likely type of targets. NWS hydrometeor-classification products label the algorithm’s most likely category in the sampled volume; categories can include rain, snow, hail, biological targets, clutter and unknown.
This information can support precipitation estimates, precipitation-type assessment and the identification of some non-weather echoes. The classification is an inference from radar returns, not a direct visual identification of an object in a particular pixel. See the NWS dual-polarization overview and NWS product descriptions.
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A radar loop can help you see the direction and approximate movement of storms. Check the image timestamp and loop interval, locate yourself, and compare several frames rather than projecting a single position forward. Storm motion can change, so a visual extrapolation is an estimate, not a guaranteed arrival time. The NWS Event Ready Appendix C recommends checking the timestamp, locating yourself and looping reflectivity imagery to assess whether storms are approaching.
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What can’t radar tell me?
- Exactly what is happening at the surface: Radar samples targets aloft. The beam gets higher above ground farther from the radar; the NWS guide notes that at far distances it may be 10,000 feet or more above ground. Conditions below the beam can differ from the echo.
- A precise surface rainfall amount from reflectivity alone: Echo strength is not a direct rain-gauge measurement, and precipitation aloft may not match what reaches the ground.
- The full wind speed or direction from one velocity value: Doppler velocity is only the toward-or-away component relative to the radar.
- Lightning: Radar does not detect lightning; lightning data comes from a separate source. The NWS Event Ready Appendix C states that lightning is detected using a different source of information.
- A definitive hail-size, rotation or tornado diagnosis from an algorithm: Such estimates and signatures can be wrong and should be treated as clues, not confirmation.
The NWS Event Ready guide cautions that what appears on radar does not always match what is happening at the surface, and that the potential difference grows with distance from the radar.
How to use a radar map responsibly
- Check the timestamp. Confirm when the displayed scan was collected; a map can be stale even if it is still visible.
- Locate the radar and yourself. Distance matters because the beam samples higher above ground farther away.
- Read the product name and legend. Distinguish reflectivity from velocity, storm-relative motion or estimated accumulation before drawing conclusions.
- Use a loop for movement. Note its time span and interval; treat any arrival estimate as approximate.
- Use official NWS forecasts and warnings for hazard decisions. A radar image alone is not a warning or a substitute for current official alerts.
This article describes U.S. NWS WSR-88D/NEXRAD products; other countries and radar networks may use different systems and display conventions. For current conditions, consult the NWS radar viewer at radar.weather.gov alongside current local NWS warnings and forecasts.
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