Some emergency situations can reach an entire population. This is the case of major climatic events (e.g. storms), industrial disaster (possible release of poisonous gas), nuclear disaster or warfare. In such cases, it is necessary to warn the general public so they can protect themselves.

When the event can be forecast and is coming within few days (e.g. flood, storm, heatwave or coldwave) it is possible to use usual mass media (radio, television, press). When the event is sudden, it is necessary to warn immediately the population. This can for example be performed by sirens such as air-raid sirens.

Prerequisites

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The prerequisites to have a population warning system is to have:

• a survey center that collects the warning signs and can start the alert;
• a network for the diffusion of the alert;
• an education program to teach the general public how to recognize an alert or what to do.

The warning signal normally occurs very rarely, and is likely to cause panic. Thus, in the general case, the education should be limited to a very simple attitude: home confinement and listening to the radio for additional instructions; there should be therefore only one type of warning signal. In specific areas with specific risks, the education can be more developed and there may be an additional signal for immediate evacuation (e.g. tsunami, risk of failure of a dam).

Population warning in the United States

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In the USA, the population warning is performed by radio, with a system called Emergency Alert System (EAS). This system replaced the Emergency Broadcast System (EBS) in 1997.

An Emergency Action Notification (EAN) is sent to all broadcast stations, cable systems, wireless cable systems, other regulated services of the FCC, participating industry entities, and to the general public that the EAS has been activated for a national emergency. An Emergency Action Termination (EAT) is sent when an EAN has terminated. These messages are originated by Primary: National, State or Local Pirmary (NP, SP, LP). State Relays (SR) relay the messages to local actors. Participating National sources (PN), i.e. broadcast stations, cable systems and wireless cable systems, relay the messages to the general public.

Population warning in France

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In France, the population warning is made by an air-raid siren. This network is called "National warning network" (Réseau national d'alerte, RNA). The system is inherited from the air-raid siren network developed before the Second world war (défense passive). It consists in about 4,500 sirens placed all over the territory. The siren can be an electronic siren with loudspeaker, or an electromechanical system (the sound is generated by a fan). In some cases, the warning signal may be played by a mobile system on a vehicle of the fire department.

The warning signal is described by the décret (by law) #90-394 of the 11 may 1990. It consists in a modulated sound going up and down during one minute, and repeated three times. The end of alert is a continuous signal lasting 30 seconds.

For an elecronic siren, the modulated signal consists in 4 seconds cycles:

• the tune rises from a frequency of 300 Hz (between D and D#) to 600 Hz (same tune, one octave higher) during 2 seconds;
• it goes back to 300 Hz in two seconds.

The signal has a square shape (hat function); it is played 15 times in a minute. For an electromechanic system:

• the motor is switched on during seven seconds, reaching 380 Hz (between F# and G);
• it is switched off during five seconds.

It is played 5 times in a minute. In both cases, the end of the alert is a 380 Hz sound.

The system is tested every month, the first wednesday at noon; in this case, the modulated signal is played only once.

When the warning signal sounds, people should confine home and listen to the radio: France Info (continuous information radio of the public service), local station or France Inter on longwave — 1,852 m, 162 kHz — (public service radio, the longwave broadcast can be received even when the local antena is not working). Instruction may also be spread by vehicles of the police department or the fire department.

Disaster preparedness

Alerte aux populations