Science & Environment

Thunderstorm numbers calculated

Thunderstorm in Iowa
Image caption The new research used a global network of monitoring stations

The Earth sees about 760 thunderstorms every hour, scientists have calculated.

The figure, unveiled at the European Geosciences Union meeting in Vienna, is substantially lower than numbers that have been used for nearly a century.

The new research uses a global network of monitoring stations that detect the electromagnetic pulses produced by major bolts of lightning.

It confirms that thunderstorms are mainly a tropical phenomenon - and the Congo basin is the global hotspot.

Thunderstorms also track the passage of sunlight across the world, with sunny conditions producing greater convection in the air.

"The monitoring stations might miss some bolts of lightning, but we think we're getting the big ones - and that's enough to tell you where the thunderstorms are," said Colin Price, head of the Geophysics and Planetary Sciences department at Tel Aviv University in Israel.

"And so with this global network we're able to improve on numbers that have been in standard use since the 1920s."

Global network

The first attempt to estimate thunderstorm numbers is thought to have been made by CEP Brooks in 1925.

At that time, it was customary for weather stations to note days when thunderstorms occurred nearby.

Collecting records where he could, the British climatologist calculated there were around 1,800 per hour on average across the world.

But his research suffered from incomplete data and mistaken assumptions - including that storms were equally distributed over land and sea, whereas the vast majority occur over land.

In the 1950s, OH Gish and GR Wait flew over the top of 21 thunderstorms in the US in aeroplanes carrying equipment capable of measuring voltages and currents in the air.

Extending their readings to the rest of the world, they came up with a global figure of 2,000-3,600 per year.

More recently, satellites have been deployed - but they do not see the whole world.

The new research uses a completely different technique, with more than 40 stations around the world geared up to detect electromagnetic pulses produced by strong lightning bolts.

Daytime peaks

Triangulating from groups of stations enables the World Wide Lightning Location Network (wwlln.net) to pinpoint flashes.

When they are clustered, a computer algorithm is deployed to assign flashes to their separate parent storms.

Analysing this data for September 2010 produced the average hourly figure of 760.

Each continent shows peaks during its daytime - and globally, the peak time is around noon GMT.

Thunderstorms cluster in the centre of continents in the tropics, with the Congo basin standing out.

"That's perhaps because it's drier there than in the Amazon, for example - thunderstorms seem to form more easily in drier conditions," Dr Price told BBC News.

The network is looking to add new observation points to improve results, and recently initiated a programme to detect explosive volcanic eruptions via the lightning flashes that occur in the ascending plumes of hot ash.

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