Unlocked: Secret of why lightning strikes

The researchers analysed Met Office data on lightning strikes over the UK between 2000 and 2005. Photo / Thinkstock

It is said that lightning never strikes twice. But scientists have long been puzzled as to how lightning even strikes at all.

The laws of physics mean it should require a huge electrical current - much bigger than that in a storm cloud - for a lightning bolt to reach the earth.

Now British scientists believe they have the answer, and it may enable forecasters to predict when a lightning storm is due.

Scientists agree that lightning is caused by ice particles colliding within a storm cloud to create an electric charge. But they have been divided on how that charge creates a bolt big enough to reach the ground.

Now researchers at Reading University have suggested that a stream of high-energy particles pumped out by the sun, known as a solar wind, creates a 'pathway' needed for lightning to travel.

They say that the solar wind - in which the particles are propelled from the sun's atmosphere at around a million miles per hour - charges the air, meaning a lesser current is needed for a lightning bolt to strike.

Cosmic rays - another source of charged particles from exploding stars on the other side of the Universe - are thought to be another trigger for lightning.

The results could prove useful for weather forecasters, since solar winds synchronise with the rotation of the sun, sweeping into the Earth's atmosphere at regular intervals. As these streams are also tracked by Nasa, it offers the potential for predicting the severity of dangerous storms weeks in advance.

Dr Chris Scott, of Reading University's department of meteorology, said: "Getting a lightning bolt to jump through the air requires a much bigger charge than that in stormclouds. Laboratory tests show that it would require about 150 kilovolts per metre (kV/m) to generate lightning, but you would expect to generally see no more than 30 kV/m in a stormcloud.

"We think the high-speed solar wind streams provide just enough charge to trigger a lightning bolt in a cloud that is ready to release one."

He said that the sun's rotation means the streams of particles "wash past our planet with predictable regularity".

"Such information could prove useful when producing long-range weather forecasts," he added.

The particle stream from a solar wind varies in density, temperature and speed in a pattern that is dictated by the sun's rotation.

The researchers analysed Met Office data on lightning strikes over the UK between 2000 and 2005.

They compared this with information from Nasa's Advanced Composition Explorer spacecraft, which lies between the Sun and the Earth and measures solar winds.

There was an average of 422 lightning strikes across the UK in the 40 days after the arrival of each solar wind, compared with an average of 321 in the previous 40 days.

The rate of lightning strikes peaked between 12 and 18 days after the arrival of the solar wind.

Scientists hope to eventually use such data, along with weather forecasts, to pinpoint when lightning could strike.

Around 24,000 deaths around the world each year are caused by people being struck by lightning.

Professor Giles Harrison, co-author of the study published in the journal Environmental Research Letters, said: "By increasing our understanding of weather on earth we are learning more about its important links with space weather."

- Daily Mail