Radar colour measures the strength of an echo. To read a weather radar map well, you also need to know what the beam sampled, how the echoes are moving and which frames are forecasts. Start with the legend, then play the loop.
Why the same colour can mean different rain
A radar sends out microwave pulses and measures the returning echo. For small raindrops, the contribution to reflectivity grows with the sixth power of the drop’s diameter. Double the width and the contribution grows 64 times; triple it and it grows 729 times.
NOAA’s worked example compares 729 one-millimetre drops with a single three-millimetre drop in the same sampling volume. Both produce 29 dBZ, even though the small drops together contain 27 times as much water. Reflectivity alone cannot tell those two mixtures apart.
That is why a colour is not an exact rainfall rate. Dual-polarization radar adds information about particle shape, but the familiar reflectivity image is still a map of echo strength.
Read dBZ and the map’s own legend
Reflectivity is displayed in dBZ, a logarithmic unit. An increase of ten dBZ means ten times the reflectivity. Standard rain-only relationships translate that into roughly four to five times the rainfall rate, but hail makes that conversion unreliable.
In Forecast Bar’s palette, green is around 20 dBZ, yellow 30, orange 40, red 50 and magenta 65. Broadly, the lower colours indicate lighter precipitation; the highest can mean very heavy precipitation or hail. Always read the legend for the map in front of you.
The beam may be well above your head
The lowest typical NEXRAD scan points half a degree above horizontal. As the beam travels, Earth curves away beneath it. At a range of a hundred miles, the beam centre is nearly two miles above the radar.
Shallow rain or snow can sit below that beam. Conversely, precipitation seen aloft can evaporate before it reaches the surface, a process called virga. A radar echo and rain at your feet are different observations.
Play the loop, then separate past from future
Suppose a line is seventy miles west at 2 PM and moving east at 35 mph. If that motion holds, it reaches town around 4 PM. This illustrative estimate comes from the loop, not the colour of a single frame.
A bowing segment can indicate damaging wind; a hook is a classic indication of rotation. These clues do not replace official warnings. Future radar frames are model forecasts, whose uncertainty grows with lead time, rather than observations of rain that has already happened.
What to take away
Use these checks next time:
- Read the legend before assigning meaning to a colour.
- Remember that distant radar beams sample higher in the atmosphere.
- Play the loop and check the time label: observations and forecasts serve different purposes.
