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Weather Radar Reflectivity: Colors, Intensity and Common Limits

Reflectivity is the strength of energy returned to weather radar, expressed on a logarithmic dBZ scale.

Timeline

  1. Radar pulse: The radar sends microwave energy and measures the power returned by targets in each sampled volume.
  2. Volume scan: Multiple elevation angles build base, composite, velocity and dual-polarization products with different strengths.
  3. Decision time: Forecasters combine radar trends with observations, warnings and other sensors rather than using one color alone.

Weather-radar reflectivity measures how strongly targets return transmitted energy to the receiver. It is reported as dBZ, a logarithmic expression of reflectivity factor Z, because returns span a very large range. On many displays, warmer colors represent larger dBZ values, but the legend is the authority because palettes and operating modes can differ. A colored pixel describes a sampled volume in the atmosphere; it is not a direct observation of rain reaching one address. [1][2][3]

Higher reflectivity generally corresponds to more or larger hydrometeors, so it often indicates heavier precipitation. NOAA’s broad guidance ranges from very light echoes below about 20 dBZ to heavy precipitation or some hail above roughly 50 dBZ. Those are interpretation aids, not fixed rain-rate conversions. Large drops or hail return disproportionately strong energy, while numerous small tropical drops can produce intense rainfall with less dramatic reflectivity than a hail-bearing thunderstorm. [1][2][4]

Base and composite reflectivity answer different questions. Base reflectivity commonly uses the lowest elevation scan and can preserve low-level boundaries, hook-like shapes and other detail. Composite reflectivity chooses the maximum dBZ found through the radar’s vertical volume above each location, making elevated storm cores easier to see. Composite colors can therefore depict strong precipitation held aloft that has not reached the ground, and the maximum can mask low-level structure. [1][3][5]

Distance changes what the radar sees. The beam becomes wider and generally higher above the ground as it travels, partly because of its elevation angle and Earth’s curvature. NOAA shows that small low-level features visible near one radar can be unresolved or overshot by a distant radar. Mountains and structures can block energy and leave shadows. A weak or empty distant pixel does not prove calm surface conditions below the beam. [4][6][7]

Not every echo is precipitation. Buildings, terrain, wind turbines, birds, insects and airborne debris can return energy. Temperature and moisture layers can bend the beam toward the ground, producing anomalous propagation that resembles rain. Ground clutter often looks stationary and speckled near a radar, but appearance alone is imperfect. Forecasters compare neighboring radars, satellite, observations, motion and dual-polarization variables to separate weather from contamination. [3][6][8]

Reflectivity also does not by itself establish a tornado, damaging wind or exact hail size. Doppler velocity measures motion toward or away from the radar, while dual-polarization products describe characteristics of returned horizontal and vertical energy and help distinguish rain, hail, snow and non-weather targets. Even recognizable storm signatures are evidence to evaluate, not a substitute for an official warning, spotter report or the full three-dimensional storm analysis. [2][4][9]

To read a radar image, check its timestamp, dBZ legend, radar or mosaic source, product name and animation. Ask whether it is base or composite reflectivity, how far the feature is from a radar and whether echoes move consistently with weather. Then use the current NWS forecast and warnings for decisions. Radar can update quickly but still samples intervals, elevations and volumes; delays, missing data and processing filters can change what an app displays. [3][5][6]

Sources

  1. NOAA JetStream — Radar Images: Reflectivity
  2. NOAA NCEI — Next Generation Weather Radar Product Descriptions
  3. National Weather Service Northern Indiana — About WSR-88D Reflectivity
  4. NOAA JetStream — How Radar Works
  5. National Weather Service — RIDGE2 Radar FAQ
  6. NOAA JetStream — Radar Beams
  7. NEXRAD Radar Operations Center — Wind Farm Impacts and Beam Geometry
  8. NOAA JetStream — Anomalous Propagation
  9. NOAA JetStream — Dual Polarization

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