The sound of summer that spells SOS

It's As much a sound of summer as tennis balls fired into a net or a cricketer hitting a six - except in this case it's a matter of life and death. Early in the morning or late at night a BBC announcer will appeal for someone caravanning in Norfolk or sailing in the Irish Sea to phone home urgently because a mother or son is dangerously ill.

The BBC first broadcast an SOS message in 1923, one year after the Home Service of the British Broadcasting Corporation was founded. A six-year- old boy was missing, and his parents and the police were worried for his safety. The BBC allowed a 30-second broadcast, asking for news of sightings. As a result of reports from listeners the boy was found safe and well. This success, and the apparent willingness of listeners to help, encouraged the BBC to reserve a regular slot for such urgent appeals.

The Home Service's successor, Radio 4, broadcasts about 25 messages a year, with a peak in the summer months when holiday-makers cannot be contacted in any other way. According to a BBC spokesman: "Sometimes there are no SOS messages for weeks, sometimes four are broadcast consecutively. We get them out as fast as we can: it is possible that within an hour of a call coming in, the person who was out of touch will be turning the car around and heading home."

In recent years, the BBC has devised much stricter criteria for broadcasting the SOS messages. They no longer take calls for missing persons, only considering cases where someone is dangerously ill, and a close relative is out of contact. There must also a be a high probability that the person will be listening when the SOS goes out.

The SOS message, like the Shipping Forecast, is one of those Radio Four traditions which enhances its standing as a mouthpiece for the nation. No other national station has reserved airtime for such broadcasts.