Storm warnings as quick as a flash: It may not strike twice in the same place, but Declan Butler sees lightning being tracked

The hexagonal outline of France glows on the screen of a lone computer terminal in the heart of a chemical complex near Marseilles. It is 1.14am on 20 July and the system seems to be waiting in anticipation, like a coastal look-out scanning the horizon for enemy aircraft. But nothing stirs, just as nothing had stirred the day before, or the day before that.

Then, at 1.16am, a lone orange pixel blinks on to the screen, just off the Mediterranean coast. A few seconds later another. Within minutes, the outline of southern France is obscured by a frenzy of dancing orange dots, moving slowly eastwards. A tiny white ring, representing a 30km zone around the chemical plant, lies in the swarm's path. At 2.15am, a siren screeches from the computer as the first orange pixel breaches the plant's frontier. Instantly, relays and switches cough generators into life and isolate the complex from the national grid; a few seconds later, distant thunder echoes from up the valley.

This plant pays Fr100,000 ( pounds 10,400) a year for a real-time system which detects, measures, maps and records each individual impact of the 800,000 or so bolts of lightning that strike France every year, threatening costly delays to production through power cuts or damage to equipment by surges of electricity. Euro Disney, the military, space agencies, insurance companies and firefighters are among other customers.

Electronic systems for detecting lightning have been operating around the world for several years, but the French system is unique because of the sophisticated information processing system behind its network of 16 detectors. Details of every lightning flash are processed and disseminated to users in a few seconds; not only where and when it happened, but its amplitude and polarity, and the number of return strokes between the cloud and the ground.

This level of surveillance does not just give early warning of the atmosphere's electrical antics, but also opens up the flashes to serious scientific analysis; a far cry from the days when the British Met Office's thunderstorm statistics depended on a national network of human listeners.

The system is run by a private Paris company, Meteorage Franklin, set up in 1986. Not content with following French flashes, the company recently installed detectors in Switzerland, Austria, Belgium and Luxembourg, and is calling for a European lightning detection network.

Fortunately, the tracking of up to 57,000 bolts per day (more than 80,000 over two days last week) is more straightforward than it would appear. Each impact produces a powerful electromagnetic wave (1 to 500Khz) with a characteristic signal pattern that spreads over hundreds of kilometres, hugging the profile of the land like a Cruise missile. Detectors spaced at intervals of 150km-250km (90- 150 miles) around France pick up this radiation in much the same way as simple radio receivers do, but with greater discrimination; they recognise the characteristic radiation form of cloud-to-ground lightning and reject radiation from other sources, such as lightning between or within clouds, or human activity.

The detectors look like large toadstools 1.68m (5.5ft) high and with a elongated dome 34cm (13.4in) in diameter. They not only recognise the flash's fingerprint signal but also register the direction from which it came; a triangulation calculation using information from at least two detectors gives the location to within 4km (2.4 miles) at distances of up to 400km (248 miles). The network picks up more than 90 per cent of all cloud-to-ground lightning impacts on French territory.

Information from the detectors is relayed to four supercomputers at Meteorage Franklin's headquarters in Paris, via the French packet-switch communications network, Transpac. These computers perform the triangulation calculations, display the flash's location on a monitor and relay the information via Transpac to customers' screens.

France's electricity and telephone monopolies are the biggest customers: Electricite de France says that more than 75 per cent of all faults reported by customers are due to lightning. France Telecom reckons that surges of electricity due to lightning cost it about Fr170m ( pounds 18.2m) every year.

Lightning data comes in two forms, stored and fresh: the database holds the statistics of four million lightning impacts since 1987, and can be used to compile risk data; the real-time service is for those who need immediate information on weather conditions.

The database has been useful. Government guidelines specified protection against 100,000 amps as sufficient for sensitive installations, but a survey by Meteorage Franklin in 1990 showed that 12,562 impacts in France were greater than 200,000 amps.

The database has also been used to produce detailed maps of lightning activity, showing that most occurs in the Pyrenees, the Cote d'Azur and the Massif Central. Each square kilometre of the Cevennes region is struck more than five times a year, whereas one square kilometre of Brittany scores just 0.1 hit per year (but, perversely, Brittany and Normandy have the highest proportion of the more destructive positive bolts, rather than the much more common but milder negative bolts).

Electricite de France uses such information to help to decide which installations most need the protection of expensive equipment to detect and divert to earth incoming power surges.

France's insurance companies haven't missed their opportunity either. 'Lightning struck my cow' is a common insurance claim in France: too common, thought the companies. Half the claims were rejected as false after checking Meteorage Franklin's data in the system's first year, and even now lightning data saves the insurance companies more than a million francs per year, according to Meteorage Franklin.

But it is in real-time mode that the detector system shows its maximum power. Watching lightning storms develop on screen is almost better than the real thing; lightning fronts appear, sweep across the country and disappear before your eyes.

Electricite de France and France Te1ecom monitor the lightning to direct repair staff to areas where damage is likely and to warn staff already in risk zones. Electricite de France uses real-time lightning data to select the best paths to shunt supplies around the country to avoid the storms.

A zone of 30km (18.6 miles) around a factory can be programmed into the early-warning system. The first bolt of lightning to put its foot in this zone registers as a pixel within the zone drawn on screen; the pixel's appearance can be programmed to set off an alarm or switch the factory over to electricity supplied by generators, protecting equipment from damage by high voltages coming down the lines. Small companies can opt for a cheaper system whereby a telephone call is generated automatically from Paris when lightning is detected within 30km of the plant.

Euro Disney has subscribed so that it can cancel outdoor parades using electricity if a storm is coming. Firefighters in the Bordeaux area monitor the progression of lightning fronts to anticipate forest fires and send equipment in advance; 40 per cent of forest fires in this coniferous region are caused by lightning.

Most householders in France have the Minitel videotex service and many track nearby storms, for around 20p a minute, using the '3617 Meteorage' service, often simply out of curiosity. With such a precise tool in every household, is seems that the age-old children's game of measuring a storm's passage by counting the delay between flash and bang is now obsolete.

(Photograph omitted)