All clear is the signal, generally given by an air raid siren, which indicates that an air raid or other hazard has finished and that it is safe for civilians to leave their

shelter A shelter is an architectural structure or natural formation (or a combination of the two) providing protection from the local environment. A shelter can serve as a home or be provided by a residential institution. It can be understood as both ...

s; it is commonly used in

radios Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to ...

as well.

United Kingdom

During the

First World War World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...

, the Metropolitan Police introduced their first air raid warning system in

London London is the Capital city, capital and List of urban areas in the United Kingdom, largest city of both England and the United Kingdom, with a population of in . London metropolitan area, Its wider metropolitan area is the largest in Wester ...

in 1917, which consisted of police officers detonating

maroon Maroon ( , ) is a brownish crimson color that takes its name from the French word , meaning chestnut. ''Marron'' is also one of the French translations for "brown". Terms describing interchangeable shades, with overlapping RGB ranges, inc ...

s (a type of loud

firework Fireworks are Explosive, low explosive Pyrotechnics, pyrotechnic devices used for aesthetic and entertainment purposes. They are most commonly used in fireworks displays (also called a fireworks show or pyrotechnics), combining a large numbe ...

) and the "all clear" was sounded by

Boy Scout A Scout, Boy Scout, Girl Scout or, in some countries, a Pathfinder is a participant in the Scout Movement, usually aged 10–18 years, who engage in learning scoutcraft and outdoor and other special interest activities. Some Scout organizatio ...

bugler The bugle is a simple signaling brass instrument with a wide conical bore. It normally has no valves or other pitch-altering devices, and is thus limited to its natural harmonic notes, and pitch is controlled entirely by varying the air and ...

s. Immediately before the

Second World War World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

, 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. The sirens remained in place during the

Cold War The Cold War was a period of global Geopolitics, geopolitical rivalry between the United States (US) and the Soviet Union (USSR) and their respective allies, the capitalist Western Bloc and communist Eastern Bloc, which lasted from 1947 unt ...

but instead of being the responsibility of local police forces, the alarms were activated by the

United Kingdom Warning and Monitoring Organisation The United Kingdom Warning and Monitoring Organisation (UKWMO) was a British civilian organisation operating to provide UK military and civilian authorities with data on nuclear explosions and forecasts of fallout across the country in the even ...

. Following an air raid, the "all clear" would only be sounded if no

nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear exp ...

s had been used, or when any

nuclear fallout Nuclear fallout is residual radioactive material that is created by the reactions producing a nuclear explosion. It is initially present in the mushroom cloud, radioactive cloud created by the explosion, and "falls out" of the cloud as it is ...

was deemed by the

Royal Observer Corps The Royal Observer Corps (ROC) was a civil defence organisation intended for the visual detection, identification, tracking and reporting of aircraft over Great Britain. It operated in the United Kingdom between 29 October 1925 and 31 Decembe ...

to have reached safe levels, which might have been up to two weeks later. The system was discontinued in 1992.

Other countries (using an All-Clear signalon their warning sirens)

The following is a list of countries that use All Clear on sirens: #

Denmark Denmark is a Nordic countries, Nordic country in Northern Europe. It is the metropole and most populous constituent of the Kingdom of Denmark,, . also known as the Danish Realm, a constitutionally unitary state that includes the Autonomous a ...

Sweden Sweden, formally the Kingdom of Sweden, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. It borders Norway to the west and north, and Finland to the east. At , Sweden is the largest Nordic count ...

Singapore Singapore, officially the Republic of Singapore, is an island country and city-state in Southeast Asia. The country's territory comprises one main island, 63 satellite islands and islets, and one outlying islet. It is about one degree ...

Kuwait Kuwait, officially the State of Kuwait, is a country in West Asia and the geopolitical region known as the Middle East. It is situated in the northern edge of the Arabian Peninsula at the head of the Persian Gulf, bordering Iraq to Iraq–Kuwait ...

U.S. The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 states and a federal capital district, Washington, D.C. The 48 contiguous ...

(some areas) #

Afghanistan Afghanistan, officially the Islamic Emirate of Afghanistan, is a landlocked country located at the crossroads of Central Asia and South Asia. It is bordered by Pakistan to the Durand Line, east and south, Iran to the Afghanistan–Iran borde ...

(for instance,

Kandahar Kandahar is a city in Afghanistan, located in the south of the country on Arghandab River, at an elevation of . It is Afghanistan's second largest city, after Kabul, with a population of about 614,118 in 2015. It is the capital of Kandahar Pro ...

) #

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

Israel Israel, officially the State of Israel, is a country in West Asia. It Borders of Israel, shares borders with Lebanon to the north, Syria to the north-east, Jordan to the east, Egypt to the south-west, and the Mediterranean Sea to the west. Isr ...

Other countries (not using an All-Clear signal on their warning sirens)

Netherlands , Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...

South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia. It constitutes the southern half of the Korea, Korean Peninsula and borders North Korea along the Korean Demilitarized Zone, with the Yellow Sea to the west and t ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

(some areas)

References

{{Mil-hist-stub Civil defense Emergency population warning systems