There are 159 NEXRAD radars systems deployed in the United States and around the world. Each of these radars implements the WSR-88D technology. The "Doppler" capability of these radars uses shifts in the phase of the reflected energy to determine the velocity of the particles towards or away from the radar. The effective detection range is between 80 and 140 nautical miles, depending on the intensity of the precipitation. In clear air mode, these radars transmit data every 10 minutes. In precip mode, they transmit every 4 to 6 minutes.
All of the images on the ADDS radar page are generated by the NWS. ADDS simply provides a custom interface to the information already available on the NWS website.
The display is an image representation of the Nexrad radar reflectivity overlayed with cloud tops and centroid movement. The Nexrad Radar Coded Message (RCM) product originates as an automated text message generated at each NWS radar site. The RCM reflectivity data has spatial resolution of about 12 km and is updated every 30 minutes. The RCM is a legacy product which will be replaced sometime in the near future. The raw RCM has considerable false echoes, so an automated process that checks the RCM for meteorological validity when compared to current satellite images, synoptic conditions, neighboring radar sites, and lightning.
The displayed image is the edited reflectivity. The RCM includes the max top for each radar's area of coverage. The other tops shown on the display are derived from the satellite images at the centers of convective activity. Movements shown are the radar centroid movements as generated by the Nexrad processor.
NOTE: The echo tops shown can in some cases be erroneous. The algorithm that computes this value uses any echo within the range of the radar. This can lead to spurious values when precipitation is far from the site. Values greater than 50,000 feet (500 on the plot) can be disregarded especially if their locations don't correspond to any precipitation.
The images are updated twice hourly and are posted at HH:15 and HH:45. The main radar page will show the latest image or loop the latest 4 hours of images.
There are roughly 140 radar sites across the U.S. Each site has static images create for base reflectivity, composite reflectivity and total precipitation. Clicking on a site on the map will bring up the latest image for that site and allow the user to loop the latest 2 hours of images.
The national radar is a composite of the 140 or so radar sites. This provides a single view of the entire country or 12 regional views. The page will show the latest image or loop through the latest 2 hours of images.