How Doppler Radar Works:
NEXRAD (Next Generation Radar) can measure both precipitation and wind. The radar emits a short pulse of energy, and if the pulse strike an object (raindrop, bug, bird, etc), the radar waves are scattered in all directions. A small portion of that scattered energy is directed back toward the radar.
This reflected signal is then received by the radar during its listening period. Computers analyze the strength of the returned radar waves, time it took to travel to the object and back, and frequency shift of the pulse. The ability to detect the "shift in the frequency" of the pulse of energy makes NEXRAD a Doppler radar. The frequency of the returning signal typically changes based upon the motion of the raindrops (or bugs, dust, etc.).
Why Is There Data On My NEXRAD Radar Image When There Is No Precipitation Outside?
If you see data on NEXRAD Doppler Radar but that location is not receiving precipitation, this could be for a number of reasons.
The most common explanation for the seemingly anomalous data on the image is "ground clutter." Ground clutter usually appears near the center of the radar image when the radar beam intersects trees, buildings, mountains, insects, pollution, etc. near the site. Every NEXRAD radar has ground clutter, but it is much more evident when the radar is operating in Clear Air Mode.
Anomalous data, often high in intensity, which is not limited to a circular pattern near the radar site may be "anomalous propagation," or "AP." AP occurs when atmospheric conditions cause the radar beam to be reflected back to the earth, where it bounces off the ground and reports the terrain back as radar echoes. It is often hard to distinguish from actual precipitation data.
Precipitation not reaching the ground (virga) may be occurring. Also, the NEXRAD Doppler radar's sensitivity also allows users to view cold fronts, sea breeze fronts and thunderstorm outflow boundaries that have no precipitation associated with them. Boundaries such as these appear as thin continuous lines on the Base Reflectivity product at reflectivities of 10 dBZ or lower. Other non-weather related phenomena are also detected periodically in NEXRAD reflectivity data due to the high sensitivity and high resolution of the radar, such as smoke plumes from grass and forest fires and movements of large flocks of migrating birds.