Beware the deadly force of a high-voltage summer

In Ancient Greece it was considered to be the weapon of Zeus, while the Vikings believed that it was tossed by Thor, the god of thunder, at his enemies. But it was a local postmaster in America called Benjamin Franklin who showed that lightning was in fact the dramatic discharge of static electricity from the sky.

In Ancient Greece it was considered to be the weapon of Zeus, while the Vikings believed that it was tossed by Thor, the god of thunder, at his enemies. But it was a local postmaster in America called Benjamin Franklin who showed that lightning was in fact the dramatic discharge of static electricity from the sky.

As events have shown this week, lightning is one of the most powerful and terrifying forces of nature. A single bolt can stretch for more than five miles, soar to a temperature of up to 50,000C and hold a charge equivalent to 100 million volts.

A lightning strike on Tuesday killed a 14-year-old boy in the back garden of his home in Bloxwich, West Midlands. Paramedics could not revive Joseph Wharton, who was pronounced dead at Walsall Manor Hospital. A separate bolt on the same day injured four girls in London's Hyde Park. Three suffered spinal injuries and the fourth teenager, who had momentarily stopped breathing, was left in a critical condition at St Mary's Hospital in Paddington.

Yesterday, lightning was also being blamed for a fire at a riding school in the French Alps in which seven teenagers and two adults are believed to have died. Storms with sheet and forked lightning had swept across the western Alps when they lashed the village of Lescheraines, north of Chambery, where the wooden dormitory of the riding school went up in flames.

Local police have not ruled out other possible causes of the fire, such as a discarded cigarette end, but an electrical fault caused by a lightning strike is the chief suspect. The French Sports minister, Jean-Françoise Lamour, said that the dormitory had not been registered with the authorities and did not meet required standards.

In Britain, between 30 and 60 people a year are struck by lightning. A handful of the victims die of their injuries, often as a result of a heart attack brought on by the shock of being hit by a huge bolt of electricity.

In America, where lightning can be more common and ferocious, the figures are worse. Over the past 30 years, about 67 Americans have been killed annually by lightning strikes - more deaths than from tornadoes. Another 300 are seriously injured each year.

Survivors suffer a spectrum of long-term health problems such as insomnia, severe headaches, deafness, memory loss, paralysis and other problems related to brain damage.

American health officials treat lightning far more seriously than in Britain. They estimate that the chances of an American being struck by lightning in a given year is one in 700,000, but these odds increase to one in 3,000 over a period of a lifetime lasting 80 years.

Such is the concern about lightning in America, the country has a lightning safety day. It is a little-known fact that as soon as you can hear thunder, you are theoretically in danger of being struck. During a thunderstorm, lightning can strike as far as 10 miles from the area where rain is falling. This is also about the same distance that the sound of thunder can travel.

In America, the public are advised to follow the 30-30 rule when visibility is good and there is nothing obstructing a person's view of an approaching thunderstorm: when you see lightning, you must count the time until you hear thunder. If that time is 30 seconds or less, the thunderstorm is within six miles of where you are and there is a potential risk of being struck. The second part of the rule takes into account another little-known fact: that lightning can still strike some time after a storm has passed over. Wait for 30 minutes after the last thunder clap before leaving shelter, the rule states, and do not be fooled by sunshine and blue skies.

We can thank Benjamin Franklin for understanding the nature of thunder and lightning. His experiment with a set of keys and a kite showed that lightning bolts were electrical discharges from the sky to the ground.

To generate the electrical charge necessary for lightning there needs to be a sharp difference in atmospheric temperatures. This difference in heat energy powers a thunderstorm, which is why about two-thirds of the 16 million storms that occur on the Earth each year take place in the tropics.

A thunderstorm forms in air that has three components - moisture, instability and something that causes the air to rise, such as an incoming weather front. Continual rising of the air builds storm clouds to a height of between 35,000 and 60,000 feet, where it is cold enough for ice crystals to form.

It is the ice crystals in these tall cumulonimbus clouds that appear to be the basis of the electrical charge leading to lightning strikes. One theory is that as the ice crystals rise and fall, they bump into one another, causing charged particles to separate, leading to a build-up of positive charge at the top of the cloud, and of negative charge at the bottom. The greater the temperature differences within the air, the greater the amount of convection movement inside clouds, causing the build up of a separated electrical charge.

An enormous electrical differential develops and, as the thunderstorm cloud moves over the land, a corresponding build-up of positive charge occurs on the ground. This positive charge tends to move up anything that pokes out above ground level, such as a golf umbrella or a fishing rod.

A person's hair can literally stand on end as the static electrical charge builds up underneath a thunderstorm cloud. As the electrical difference grows, it is only a question of time before the inherently unstable situation results in a lightning strike. The negatively charged area in the storm sends out a charge towards the ground called a stepped leader which is invisible to the human eye and moves downwards in steps of less than a second. When it nearly reaches the ground, it gets attracted by the build-up of positive charge and an electrical channel develops that opens a flow of charge, which appears as a bolt of lightning.

This is the typical "forked lightning" of a ground strike and it often flashes several times as the electrical channel opens and closes to complete a full discharge. Another form of lightning occurs as a discharge between or within clouds. This results in sheet lightning and produces dramatic flashes of light.

Thunder, meanwhile, is the sound of the rapid expansion of air caused when the lightning channel heats quickly to 50,000C. When the storm is some miles away we hear the thunder later than we see the lightning because sound travels far slower through air than light.

There are occasions when lightning can be caused by the discharge of positive charge from the top of a cloud. This "positive lightning" can be more dangerous because it can strike many miles from the area where it is raining. It can also occur over a longer duration and involve a higher current, meaning it is more likely to cause forest fires.

Monitoring the occurrence of lightning is an important part of weather forecasting because its presence is a good indication of how severe an approaching storm is likely to be.

Paul Taylor, who has been involved in lightning detection for 20 years at the Met Office, says that the system used by Britain can pick up lightning as far away as the east coast of America and the Gulf of Mexico, as well as the southern and northern extremities of Europe.

The detectors pick up the low frequency electromagnetic waves caused by lightning. By measuring the minuscule differences in the time it takes for the signal to arrive at different stations, the instruments can pinpoint the location and track of a thunderstorm.

The question also arises whether global warming is making lightning more common in Britain. Mr Taylor said that there is no indication that lightning in Britain is getting more common or more dangerous. This summer's storms are linked to a high-altitude wind, a jet stream, which is blowing bad weather further south than it would normally do at this time of the year, he said.

But Britain still has a long way to go before it can rival Houston, Texas. The lightning capital of the United States suffered some 1.6 million cloud-to-ground flashes in the 12-year period between 1989 and 2000.

For all its destructive power, scientists have shown that it was probably lightning that caused the creation of the vital building blocks of life which first resulted in a self-replicating molecule - the first lifeform - some four billion years ago. If they are right, then Mary Shelley was indeed prescient when she described lightning as the force that gave life to Dr Frankenstein's monster.