How do you read rainfall radar colors?
Colors on a precipitation radar map represent the strength of energy returned to the radar from targets in the atmosphere. Without the product name and its legend, a green, yellow, or red patch cannot be translated into an exact amount of rain at the surface; the same color may represent another variable on a different product.
Reflectivity, sampling height, and the legend
1. What physical measurement produces the colors? Weather radar sends short electromagnetic pulses and receives energy scattered back by raindrops, snowflakes, or hailstones. On a base reflectivity product, the numbers are displayed on a logarithmic scale called dBZ: a larger value means a stronger echo in the volume sampled by the beam. The palette turns ranges of those numbers into visible colors. Green is not universally light rain and red is not universally hail. Read the color key for the corresponding dBZ range together with the observation timestamp; colors alone do not identify precipitation rate or type. 2. Is one pixel the amount of rain reaching the ground? A radar beam samples an atmospheric volume at a certain height and width. As it travels farther it becomes wider and higher, so it may overshoot shallow rain at the surface or detect ice aloft that evaporates before reaching the ground. Base reflectivity generally uses a low elevation scan, whereas composite reflectivity takes the strongest echo somewhere in a vertical column. A composite image can therefore emphasize intense echoes above the ground; it is not interchangeable with a ground rain gauge or the lowest elevation scan. Distance and terrain blockage matter. 3. Do the colors prove whether it is rain, snow, or hail? Both a large hailstone and many raindrops can produce a strong echo, while snowflake shape and the melting layer also change the return. Dual-polarization radar products provide additional clues about particle shape and type, but a surface temperature profile, observations, and an official forecast remain important. A red or green region on a Doppler velocity product may instead mean motion away from or toward the radar, rather than heavy or light precipitation. Check the title and units before interpreting the color.
Reading order, false echoes, and safety
4. In what order should a radar map be checked? First identify whether the display is base reflectivity, composite reflectivity, radial velocity, or an estimated accumulation. Then read its units, legend, scan time, distance from the radar, and movement in an animation. Several successive frames help reveal the direction of a line of showers, but an observation is not a forecast for later in the day. Compare the image with official warnings and nearby instruments. A ground gauge and the radar can disagree at the same nominal place because one measures a point at the surface and the other samples a beam volume higher up. 5. Can a colored patch appear where no rain falls? Buildings and terrain near the radar, birds and insects, and unusual refraction during temperature inversions can produce non-weather echoes. If a spot is nearly stationary or circular, compare neighboring radars, satellite imagery, and surface reports before calling it rain. A surface station also covers only one point, and a localized shower may miss it. Conversely, a blank patch is not proof that the sky is dry: equipment may be offline or a beam may be blocked by mountains. Quality-control algorithms reduce false returns but do not remove all uncertainty. 6. What is the limit for real-world decisions? Radar can help locate an approaching precipitation area and track its recent trend, but it cannot guarantee that a particular street will be free of flash flooding. Short heavy rain interacts with antecedent soil moisture, stream channels, slope, pavement, and drainage. Follow official weather and emergency alerts; never drive into a flooded road. A fading color does not itself cancel an alert, because updates can be delayed and radar can have blind spots. Use the map as one observation within a wider warning system, not as a substitute for local emergency instructions.