Radar Frequently Asked Questions
A behind-the-scenes look at how the doppler radar works
WHERE IS THE DOPPLER RADAR LOCATED?
The nearest doppler radar site is located atop a mountain in Floyd county and is maintained by the National Weather Service office in Blacksburg. All local stations receive this same data feed and display it in a variety of ways.
HOW DOES THE RADAR WORK?
NEXRAD (Next Generation Radar) obtains weather information (precipitation and wind) based upon returned energy. The radar emits a burst of energy. If the energy strikes an object (rain drop, bug, bird, etc), the energy is scattered in all directions. A small fraction 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 pulse, time it took to travel to the object and back, and phase shift of the pulse. This process of emitting a signal, listening for any returned signal, then emitting the next signal, takes place very fast, up to around 1300 times each second.
WHY IS THE RADAR NOT SHOWING RAIN WHEN IT'S RAINING HERE?
Weather radars can detect most precipitation within approximately 80 miles of the radar site, and intense rain or snow within approximately 150 miles.
However, very light rain or drizzle and even light snow from shallow cloud weather systems are not easily detected. There are a few other reasons why.
- The radar beam becomes further from the ground with distance (partly because of the Earth's curvature, and partly because the beam is angled upwards by a fraction of a degree), thereby sometimes missing the lower parts of the rain.
- The radar is also limited close in by its inability to scan directly overhead. Therefore, close to the radar, data are not available due to the radar's maximum tilt elevation of 19.5°. This area is commonly referred to as the radar's "Cone of Silence".
- The presence of mountains within the range of the radar can block part or whole of the radar beam, thus significantly reducing the echo intensity from rain on the other side of the mountains.
WHICH MOUNTAINS IMPACT THE RADAR BEAM?
While the doppler radar is located atop one of the higher mountains in the area, The one mountain that creates beam blockage for the local doppler radar is Poor Mountain. It is very noticeable if you look at any estimated precipitation total products. Here you can see a minimum along that radial beyond Poor Mountain.
This doesn't mean areas are totally in the dark. The radar still scans the sky at the higher levels and detects storms in and around the area. It's the lower clouds and precipitation that it often has trouble "seeing" due to the beam blockage.