Driving a thunderstorm’s powerful booms and thrilling light shows are amazingly high electric voltages. In fact, those voltages can be far higher than scientists had assumed. Scientists recently found this out by observing an invisible drizzle of subatomic particles.
Their new measurement found a cloud’s electric potential could reach 1.3 billion volts. (Electric potential is the amount of work necessary to move an electric charge from one part of the cloud to another.) That is 10 times the largest storm-cloud voltage previously found.
Sunil Gupta is a physicist at the Tata Institute of Fundamental Research in Mumbai, India. The team studied the inside of a storm in southern India in December 2014. To do this, they used subatomic particles called muons (MYOO-ahnz). They’re heavier relatives of electrons. And they constantly rain down upon Earth’s surface.
High voltages within clouds spark lightning. But even though thunderstorms often rage over our heads, “we really don’t have a good handle on what’s going on inside them,” says Joseph Dwyer. He’s a physicist at the University of New Hampshire in Durham who was not involved with the new research.
The previous highest voltage in a storm was measured using a balloon. But balloons and aircraft can monitor only part of a cloud at one time. That makes it tricky to get an accurate measurement of the whole storm. In contrast, muons zip right through, from top to bottom. Those that do become “a perfect probe for measuring the [cloud’s] electric potential,” explains Gupta.
Clouds slow the muon rain
Gupta’s team studied set up an experiment in Ooty, India. Called GRAPES-3, it measures muons. And in general, it recorded around 2.5 million muons every minute. During thunderstorms, however, that rate fell. Being electrically charged, the muons tend to get slowed by a thunderstorm’s electric fields. When those tiny particles finally reach the scientists’ detectors, fewer now have enough energy to register.
The researchers looked at the drop in muons during the 2014 storm. They used computer models to figure out how much electric potential the storm needed to show that effect on muons. The team also estimated the storm’s electric power. They found that it was about 2 billion watts! That’s similar to the output of a large nuclear reactor.
The result is “potentially very important,” Dwyer says. However, he adds, “with anything that’s new, you have to wait and see what happens with additional measurements.” And the researchers’ simulated thunderstorm — the one studied in the model — was simplified, Dwyer notes. It had just one area of positive charge, and another negatively charged area. Real thunderstorms are more complex than this.
If further research confirms that thunderstorms can have such high voltages, it could explain a puzzling observation. Some storms send bursts of high-energy light, called gamma rays, upward. But scientists don’t fully understand how this happens. If thunderstorms indeed reach a billion volts, that could account for the mysterious light.
Gupta and his colleagues describe their new findings in a study due to appear in Physical Review Letters.
Editor’s note: This story was updated March 29, 2019, to correct the definition of the cloud’s electric potential. Electric potential is the amount of work needed to move an electric charge, not an electron.
cloud (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds.
colleague Someone who works with another; a co-worker or team member.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
electric field A region around a charged particle or object within which a force would be exerted on other charged particles or objects.
electric potential Commonly known as voltage, electric potential is the driving force for an electrical current (or flow of electrons) in a circuit. In scientific terms, electric potential is a measure of the potential energy per unit charge (such as electron or proton) stored in an electric field.
electron A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.
field (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).
fundamental Something that is basic or serves as the foundation for another thing or idea.
gamma rays High-energy radiation often generated by processes in and around exploding stars. Gamma rays are the most energetic form of light.
lightning A flash of light triggered by the discharge of electricity that occurs between clouds or between a cloud and something on Earth’s surface. The electrical current can cause a flash heating of the air, which can create a sharp crack of thunder.
monitor To test, sample or watch something, especially on a regular or ongoing basis.
muon A type of unstable subatomic particle. Most on Earth formed when cosmic rays interact with atoms in the atmosphere. As leptons, muons belong to the same class of particles as the electron. Their mass, however, is roughly 200 times bigger. Muons tend to be short-lived. It tends to survive only 2.2 microseconds before it decaying (transforming) into an electron and two types of neutrinos.
particle A minute amount of something.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
physicist A scientist who studies the nature and properties of matter and energy.
subatomic Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).
voltage A force associated with an electric current that is measured in units known as volts. Power companies use high-voltage to move electric power over long distances.
watt A measure of the rate of energy use, flux (or flow) or production. It is equivalent to one joule per second. It describes the rate of energy converted from one form to another — or moved — per unit of time. For instance, a kilowatt is 1,000 watts, and household energy use is typically measured and quantified in terms of kilowatt-hours, or the number of kilowatts used per hour.
Journal: B. Hariharan et al. Measurement of the electrical properties of a thundercloud through muon imaging by the GRAPES-3 experiment. Physical Review Letters. Vol 122, March 15, 2019, p. 105101. doi: 10.1103/PhysRevLett.122.105101.