Historical Warning Tones
During World War II for a "Red Warning" of approaching danger, the siren produced a single continuous tone that rose and fell regularly between one high and one low (All Clear). Another alternating tone signified a "take cover" warning for immediate danger.
After World War II, two further warnings were introduced for nuclear attack – a "Grey Warning" indicated approaching nuclear fallout with a 2½ minute warning of short steady tones divided by equal periods of silence, the silence being created with a manual shutter or electric solenoid.
A "Black Warning," also for manual sirens, was either a Morse code 'D' (— · ·) or three quick tones, indicating imminent danger of fallout.
These signals are described in the video Civil Defence Bulletin - No. 5.
Read more about this topic: Civil Defense Siren
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