National Geographic

As clouds gathered over the city of Philadelphia in 1752, Benjamin Franklin stood outside with a simple kite-borne experiment and proved the electric nature of lightning. Now, more than 250 years later, scientists have uncovered a shocking secret about the awesome power of thunderstorms.

With the help of charged particles originating from space, a team in India has for the first time accurately measured the electric properties of a giant thundercloud, determining that the behemoth contained 10 times more energy than any previously investigated storm. Along with discovering a novel connection between cosmic and terrestrial events, the findings might help solve a 25-year-old mystery in high-energy physics.

THUNDERSTORMS 101At any moment, about 2,000 thunderstorms are occurring worldwide. Learn how thunderstorms form, what causes lightning and thunder, and how these violent phenomena help balance the planet’s energy and electricity.

Since 2001, physicists in Udagamandalam, India, have been using the Gamma Ray Astronomy PeV EnergieS phase-3, or GRAPES-3, telescope to monitor subatomic particles called muons. Cascades of these naturally occurring particles rain down on Earth when cosmic rays from the distant universe hit our upper atmosphere. (Here’s how scientists recently used muons to discover a previously unknown void inside the Great Pyramid of Giza.)

Intriguingly, the highly sensitive GRAPES-3 instrument often detected slight decreases in the muon shower’s intensity between April and June, and again between September and November—just when the subtropical region receives its highest rainfall.

The GRAPES-3 muon telescope.

PHOTOGRAPH BY GRAPES-3 COLLABORATION

“This was more of an amusing episode for us than anything serious,” says study coauthor Sunil Gupta, a high-energy physicist at the Tata Institute of Fundamental Research in Mumbai, India, whose team described their work last month in Physical Review Letters. “We were studying high-energy cosmic rays and interplanetary space, and not so much the thunderstorms.”

Packing a punch

Muons carry negative charge, meaning their paths are distorted by electric fields. Gupta wondered if that property could be used to calculate how much energy the thunderclouds contained.

Back in 1929, Nobel prize-winning physicist Charles Thomson Rees Wilson measured the electric field inside a thundercloud and found it to be a surprisingly large 12,700 volts per inch. This implied that the storms, which can stretch for miles, should have enormous total electric potentials of around a gigavolt, or the equivalent of nearly a billion AA batteries.

But measuring voltage across an object usually requires placing two wires at either end, and nobody had figured out how to do that for a large and amorphous thing like a cloud. Airplane and balloon experiments, which have flown through thunderstorms taking readings at various locations, found electric potentials of tens of millions of volts, with the largest previously recorded event having 130 million volts.

Study coauthor Balakrishnan Hariharan devised a model that determined how powerful an electric field would need to be to alter the number of muons detected in GRAPES-3. Working backward, the team could then use their muon observations to estimate the electric field inside the clouds above the experiment.

In the GRAPES-3 data, the researchers saw the electrical effects of 184 thunderstorms over the course of three years. The muons indicated that one particular leviathan, which appeared on December 1, 2014, briefly contained an electric potential of nearly 1.8 gigavolts. That’s enough energy to run all of New York City for half an hour, Gupta says.

“To achieve such high voltages on the ground is almost impossible,” he adds. “But nature seems to know how to do it almost effortlessly.”..Read More at

National Geographic