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Frequently Asked Questions about Radar
NEXRAD (NEXt generation of weather RADar) Doppler radars have recently been installed throughout the country to replace the outdated network of conventional weather radars that were installed in the late 1950s. AccuWeather is offering access to the full network of nearly 150 NEXRAD Doppler radars, as each site is installed and test data made available from it by the government. NEXRAD's ability to detect wind patterns in storms and provide real time rainfall amounts are revolutionizing the way we keep up with and ahead of rapidly changing weather. Each NEXRAD radar generates dozens of data types, including higher resolution reflectivity data, storm total rainfall amounts, wind speeds and direction, wind gusts and much more, including early tornado detection capability.
AccuWeather has the most NEXRAD products available (20 basic products plus
value-added mosaics, StormTimer
What Are The Different Types Of Local NEXRAD Products AccuWeather Offers And What Do They Mean? AccuWeather provides access to the entire network of 150 NEXRAD Doppler Radars (operated by the U.S. Government) in the Continental US and San Juan, Puerto Rico. These radars, on AccuWeather.com, can have Roads, Cities, or Interstate Markers overlaid. More information on how to use the layers is available. In addition, the data from these radars is composited and offered as Metro, State, Regional, and National products. Following is a list of all radar products that are offered on AccuWeather.com
We are considering adding additional NEXRAD products in the future. How Are NEXRAD Images Displayed? AccuWeather makes NEXRAD data available in a scientific version which displays the full level and detail of NEXRAD data with a complete key, indicating the radar site and mode and the maximum intensity level detected. We have developed value-added NEXRAD displays in all resolutions by taking the unaltered NEXRAD data from the National Weather Service and adding visually pleasing base map features and keys. This makes this high-tech data easier for you to understand.
How Does Conventional Radar Work? Radar is an acronym that stands for RAdio Detection And Ranging. A weather radar consists of a parabolic dish (it looks like a satellite dish) mounted on a tower of up to five or so stories tall. The dish emits a pulsed beam of microwave radiation (analogous to a radio signal, only it is pulsed rather than continuous, and the signal has a shorter wavelength than radio signals). Most weather radars operate at wavelengths of either 5 cm or 10 cm. The signal is emitted in periodic pulses rather than continuously. The radar goes through a sequence in which it emits a burst of microwave radiation, then listens for any returned signal, then emits another burst, then listens once again. The radar very rapidly switches from sending out the signal to listening for any returned signal to sending out the signal again in quick succession. The burst of radiation travels out through the lower atmosphere and is scattered by particles in the atmosphere such as rain droplets and ice crystals. When the burst of microwave radiation encounters such particles, the signal's energy is scattered in all directions, and some of it is absorbed by the particle. A portion of the energy that is scattered is reflected back to the radar. This reflected signal is then received by the radar during its listening period, and is processed into the color digital display we are accustomed to seeing as radar data. Doppler radar uses the "Doppler effect" to determine the movement of the particles (rain drops, hail, ice crystals) that are reflecting the radar's signal back to the radar. Movement either toward or away from the radar along the radar beam, or radial, is all that can be detected, hence the term radial velocity. This movement is detectable because the frequency of the reflected signal is shifted, due to the movement of the rain or ice particles as they are blown around by the wind. This frequency shift is termed the Doppler effect after the Austrian physicist Christian Doppler, who discovered it. We experience this frequency shift in our lives every day. Picture yourself at a railroad crossing, waiting for the train to pass so you can drive on. The train is blowing its whistle as it approaches, and you can hear the pitch of the sound changing as the train nears. The pitch increases as the train approaches and then decreases as the train leads. You are standing still and the train, the source of the sound signal, is moving. The frequency of the train whistle's sound wave is being shifted due to the movement of the train, the source of that sound wave. The magnitude of the frequency shift is determined by the speed of the train. As long as there is wind blowing the rain drops around, the frequency of the reflected signal returned to the radar will be shifted. Doppler radar detects that frequency shift and measures its magnitude; conventional radar does not. How Does NEXRAD Doppler Radar Work? NEXRAD Doppler radar processes the radar's reflected signal to determine the location and intensity of precipitation (reflectivity products), the wind speeds toward and away from the radar site (radial velocity products) and many other products. This information goes directly to AccuWeather's computer systems, which create easy to read color displays of the NEXRAD information for each individual radar, and combine information from all radars to create mosaic displays. The Base Reflectivity, Tilt 1 product is the product most like that available from conventional radar; however, there are some significant differences. For one, NEXRAD Doppler radar data is gathered at a high resolution out to a farther distance from the radar site than conventional radar data, .6 by .6 of a mile resolution out to 143 miles for NEXRAD Base Reflectivity data, as opposed to similar resolution out to only 66 miles for conventional radar. And NEXRAD data depicts 16 data levels, as opposed to only 6 data levels that were available with conventional radar. Following is a table (Figure 1) outlining the significant differences between NEXRAD Doppler radar and conventional radar.
The NEXRAD Doppler radar is also much more sensitive than conventional radar, allowing users to see meteorological phenomena never before visible in radar data, such as blowing dust and dry frontal boundaries that have no associated precipitation. What Other NEXRAD Products Are Available Through AccuWeather? The reflected signal of the NEXRAD Doppler radar is processed by computer programs to yield a wealth of information that has never before been available. For example, the strength of the reflected signal is an indication of the intensity of precipitation. NEXRAD Doppler radar uses this information to interpret how much rain has fallen and where. Rainfall totals for the past 1 hour, the past 3 hours and the entire duration of a particular storm are available. These precipitation accumulation products are potentially valuable to emergency managers, hydrologists and others concerned with flooding and flash flooding. Real-time reports of rainfall have never before been available with such detail and coverage. NEXRAD Doppler radar also uses the strength of the reflected signal to interpret the amount of liquid water contained within the clouds (the Vertically Integrated Liquid Water product) which can be very useful in determining the relative strength and hail-producing potential of storms. The radial velocity data is one of the most valuable products available from each NEXRAD Doppler radar. It is the radial velocity product that allows meteorologists to detect circulations inside thunderstorms that often precede the development of a tornado. The exact location of cold fronts and other meteorological boundaries, such as sea breeze fronts, can also be determined with the use of this data. Directions for interpreting radial velocity data are included in this FAQ in the Base Velocity section. Vertical wind profiles are also available from NEXRAD. These profiles allow meteorologists, aviators and others to monitor changes in wind speed and direction with height directly above the radar site. Such information can alert meteorologists about atmospheric changes that could result in severe weather. This information is also extremely valuable to aviators in determining the magnitude of vertical wind sheer. Access to all products for all NEXRAD sites are available for download via AccuData. What Are The Different Modes Used By NEXRAD Doppler Radar? While conventional radar displays reflectivity data gathered at only one elevation angle from one 360° sweep, NEXRAD Doppler radar, also known as WSR-88D (Weather Surveillance Radar - 1988 Doppler), is operated in one of three possible volume scan strategies.
Why Is There Data On My Local NEXRAD Radar Image When There Is No Precipitation Outside? If you see data on your AccuWeather.com Local NEXRAD Doppler Radar but that location is not receiving precipitation, this could be for a number of reasons. AccuWeather.com does not attempt to alter the Local NEXRAD Doppler Radar data because scientists may be interested in this data, even if it is not "real" precipitation. 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 (explained above). This non-precipitation data may also affect the MAX dBZ reading so be sure to use the color scale to confirm the MAX dbZ reading if ground clutter is present. To see examples of, and learn more about Ground Clutter, click here. 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. As mentioned in the Base Relectivity section, 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. |
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and numerous other
value-added products) from the most NEXRAD sites, all in real time.
AccuWeather's NEXRAD products will provide you with superior resolution, more
detail, higher accuracy and a wide range of information than ever before
available from existing radar technology. 
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