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 (tm) 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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We are considering adding additional NEXRAD products in the future.

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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.

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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.

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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.

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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.

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Clear Air Mode

A NEXRAD Doppler radar will typically operate in the clear air mode when little or no precipitation is being detected within the effective range of the radar for reflectivity data (460 Km or 286 mi.). In clear air mode, the radar completes either seven or eight 360° sweeps at five unique elevation angles between .5° and 4.5° above the local horizon, comprising a "volume scan." NEXRAD data is updated every 10 minutes. Certain products do not update in Clear Air Mode, such as Echo Tops, VIL, and Surface Rainfall Accumulation, and the three layers of Layer Composite Reflectivity products. The radar is more sensitive when in clear air mode and is able to detect dry frontal boundaries, drizzle and snow (which typically shows up at reflectivities of 5 dBZ or lower) with greater precision and detail than when the radar is in precipitation mode.

Precipitation Mode

When the radar detects a significant amount of precipitation, it is then operated in the precipitation mode. In this mode, the volume scan of the radar is comprised of 11 full 360° sweeps at nine unique elevation angles (between .5° and 19.5° above the local horizon). NEXRAD data is updated every 6 minutes when the radar is in precipitation mode and all products are available.

Severe Weather Mode

During severe weather or suspected severe weather situations, the radar is operated in severe weather mode. In this mode, the NEXRAD Doppler radar's volume scan is comprised of 16 full 360° sweeps at 14 unique elevation angles between .5° and 19.5°. Each volume scan is completed in five minutes. Severe weather mode is a sub-mode of precipitation operating mode and is not regularly delineated from precipitation mode.

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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.