A "Black Warning", also for manual sirens, was either a Morse code 'D' (— · ·) or three quick tones, indicating imminent danger of fallout.
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The organizations activate public warning systems such as sirens and the Emergency Alert System, and forward the reports to the NWS, which does directly disseminate information and warnings through its NOAA Weather Radio All Hazards network.
This considerably smaller device was produced by the Gamewell Corporation, of Newton, Massachusetts, for use as a municipal alarm, especially at fire stations, to alert firemen and the public during emergencies.