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About NewtownWeather's Custom NEXRAD


What is NEXRAD?

NewtownWeather's custom NEXRAD (Next Generation Radar) radar uses Level 3 Data provided by AllisonHouse, and ultimately the National Weather Service. The regional radar product that you see on NewtownWeather, called Base Reflectivity, uses dBZ (also known as decibel), which measures echo intensity.

"Reflectivity" is the amount of transmitted power returned to the radar receiver after hitting precipitation, compared to a reference power density at a distance of 1 meter from the radar antenna. Base reflectivity images are available at several different elevation angles (tilts) of the antenna; the base reflectivity image currently available on this website is from the lowest "tilt" angle (0.5°). (Source: Weather Underground)


How does NEXRAD work?

NEXRAD is able to receive both wind and precipitation measurements, depending on energy returned back to the radar. First, the radar will release a short surge of energy. If that surge of energy comes into contact with an object (raindrop, snowflake, bird, bug, etc), the energy is scattered into many different directions. Some of the scattered energy will make its way back to the radar.

During the listening period of the radar, the reflected signal is received. Computers will then analyze the data by looking at the strength of the returned radar waves, time it took to travel to the object and back, and frequency shift of the pulse.

The National Weather Service's Doppler radars can detect most precipitation within approximately 90 mi of the radar, and intense rain or snow within approximately 155 mi. However, light rain, light snow, or drizzle from shallow cloud weather systems are not necessarily detected. (Source: Weather Underground)


What are the different modes of NEXRAD?

  • Clear Air Mode - Slow antenna roation; used when there is little or no activity in the radar coverage area. This is the most sensitive mode. During Clear Air Mode, the radar updates every 10 minutes.

  • Precipitation Mode - Faster antenna rotation; used when there is precipitation in the radar coverage area. During Precipitation Mode, the radar updates every 6 minutes.
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    VCP Scan Time (min) Elevation angles (°) Usage Special attributes
    11 5 0.5, 1.5, 2.4, 3.4, 4.3, 5.3, 6.2, 7.5, 8.7, 10, 12, 14, 16.7, 19.5 Convection, especially when close to the radar Has the best overall volume coverage.
    12 4 0.5, 0.9, 1.3, 1.8, 2.4, 3.1, 4.0, 5.1, 6.4, 8.0, 10.0, 12.5, 15.6, 19.5 Convection, especially activity at longer ranges Focuses on lower elevations to better sample the lower levels of storms.
    121 5.5 0.5, 1.5, 2.4, 3.4, 4.3, 6.0, 9.9, 14.6, 19.5 Large number of rotating storms, tropical systems, or when better velocity data is needed. Scans lower cuts multiple times with varying pulse repetitions to greatly enhance velocity data.
    21 6 0.50, 1.5, 2.4, 3.4, 4.3, 6.0, 9.9, 14.6, 19.5 Shallow precipitation Rarely used for convection due to sparse elevation data and long completion time.
    31 10 0.5, 1.5, 2.5, 3.5, 4.5 Detecting subtle boundaries or wintry precipitation Long-pulse
    32 10 0.5, 1.5, 2.5, 3.5, 4.5 Slow rotation speed allows for increased sensitivity. Default clear-air mode, reduces wear on antenna. Short-pulse

    (Source: Wikipedia)


    What radar products does NewtownWeather offer?
  • Base Reflectivity - Shows echo intensity, also known as reflectivity, which is measured in dBZ (decibel). This image is taken at a 0.5° tilt angle (lowest tilt).
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  • One Hour Rainfall - An estimate of of accumulated rainfall that has fallen in the past one hour.
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  • Storm Total Rainfall - An estimate of accumulated rainfall that has fallen over the duration of a storm event.
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  • Echo Tops - Shows the maximum height of precipitation echoes. This product is important when identifying areas of strong thunderstorm updrafts.
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  • Vertically Integrated Liquid - Amount of liquid water that the radar detects in a vertical column in the atmosphere for an area of precipitation. Normally, higher values of VIL would indicate larger hail or other types of severe weather.
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    (Source: Weather Underground & AMS Glossary)


    Why does the radar show precipitation when it's not raining?

    When the Base Reflectivity product shows precipitation (low dBZ - usually green in color) on a sunny day, for example, it is due to what is called ground clutter. The radar beam will send out a burst of energy (as explained before) and hit a false object, causing the ground clutter. Objects that may cause this "ground clutter" include buildings, hills, birds, insects, and aircrafts. This "ground clutter" is usually visible on radar within a 25-30 mile radius of the radar site (KDIX - Mt. Holly, NJ) in a circular pattern.

    (Source: Weather Underground)