All clear
All clear is the signal, generally given by a civil defence siren, which indicates that an air raid or other hazard has finished and that it is safe for civilians to leave their shelters.
During the First World War, the Metropolitan Police introduced their first air raid warning system in London in 1917, which consisted of police officers detonating maroons and the "all clear" was sounded by Boy Scout buglers.
Immediately before the Second 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.
