How to read Doppler

National Weather Service Doppler radar is perhaps the most sophisticated weather observing tool on this planet. Every few minutes, an entire suite of products is generated which gives forecasters information about precipitation rates, wind velocity, turbulence, and accumulated rainfall. There are 123 National Weather Service Dopplers at work in the United States. Nothing can compare to this in any other country.

Dopplers save lives and property by allowing forecasters to see into storms with significantly more detail than ever before. Many times, warnings for severe weather can be issued BEFORE the severe weather even occurs. Of course, some storms still escape detection, but not nearly as many as before.

Dopplers use the Doppler effect... the phase shift caused by a beam of energy bouncing off an object that is moving toward or away from the Doppler tower. The resulting Radial Velocity can indicate rotation in storms, a frequent sign of severe weather or tornadoes. Further detailed analysis of the storms's structure can indicate whether or not a tornado is likely developing.

The Doppler imagery shown here is known as Base Reflectivity. This shows the brightness of precipitation echoes in units known as dbz. The scale on the right side of the screen indicates units of dbz. The higher the number, the heavier the rain.

There are two modes of operation: Precipitation mode and Clear Air mode. During "Precipitation" mode, most of what you see is precipitation. Some of the lightest echoes are likely evaporating before reaching the ground. Occasionally, temperature differences over small distances can bend the radar beam into the ground, producing false echoes. The presence of rain and ice together can create unusually high-intensity reflections called bright banding, which can contaminate parts of the image. Thin lines usually indicate fronts or thunderstorm outflow boundaries.

During "Clear Air" mode, the radar is still very useful. By making use of more detailed scans, subtle features of the atmosphere can be seen. Mid-level cloud formations, which can be a sign of developing precipitation, can be detected. Light snow, which does not reflect well, can be seen. During summer, weak air mass boundaries, which often breed thunderstorms, can be detected.

— WDAY Chief Meteorologist John Wheeler