All clear is the signal, generally given by anair raid siren, which indicates that anair raid or other hazard has finished and that it is safe for civilians to leave theirshelters; it is commonly used inradios as well.
During theFirst World War, theMetropolitan Police introduced their firstair raid warning system inLondon in 1917, which consisted of police officers detonatingmaroons (a type of loudfirework) and the "all clear" was sounded byBoy Scoutbuglers.[1]
Immediately before theSecond World War, a network of sirens had been established in towns across the United Kingdom; the "alert" was given by a rising and falling tone and the "all clear" by a continuous tone.[2] The sirens remained in place during theCold War but instead of being the responsibility of local police forces, the alarms were activated by theUnited Kingdom Warning and Monitoring Organisation. Following an air raid, the "all clear" would only be sounded if nonuclear weapons had been used, or when anynuclear fallout was deemed by theRoyal Observer Corps to have reached safe levels, which might have been up to two weeks later. The system was discontinued in 1992.[3]
The following is a list of countries that use All Clear on sirens:
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