The Next Generation Weather Radar system (NEXRAD) comprises approximately 160 Weather Surveillance Radar-1988 Doppler (WSR-88D) sites throughout the United States and selected overseas locations.

Doppler radar is different than conventional radar in that it not only measures the distance and density of clouds, but the speed and direction (towards/away from the radar) of them. This gives much more useful information to those watching for severe weather activity.

More information about NEXRAD is available here.