Lightning

Lightning is a giant spark of electricity that can pass between clouds, through the air, or between cloud and ground. Although thunderstorms are its familiar setting, lightning has also been observed in volcanic eruptions, intense forest fires, surface nuclear detonations, heavy snowstorms and hurricanes.

Satellite research indicates about 40 flashes per second worldwide, a figure that revised an earlier estimate of 100 per second made in 1925. At any moment about 2,000 thunderstorms are active around the globe, amounting to more than 14.5 million storms a year.

The electrification of a thunderstorm begins with its vertical motion. Updrafts carry small water droplets to heights between 35,000 and 70,000 feet, miles above the freezing level, while downdrafts bring hail and ice down from the frozen upper storm. Within this churning mixture, collisions shear electrons from ascending particles onto descending ones, giving a storm cloud a negative base and a positive top. The process is thought to begin with graupel pellets about a quarter millimetre to a few millimetres in diameter.

The separation of charge follows the sorting of ice by weight. Lighter ice crystals acquire positive charge and rise while heavier hail carries negative charge downward, leaving the cloud top positive and its middle and base negative. Collisions and air movements reinforce this arrangement, so that the top of a thunderstorm is positively charged and its middle and lower parts negatively charged.

The visible flash begins with an invisible precursor: a stepped leader that descends toward the ground in short segments, reaching the surface in about a hundredth of a second. As it nears the ground an upward connecting discharge of opposite polarity rises to meet it about 30 metres above the surface. The junction completes the conductive path between cloud and earth.

Along that path the return stroke then travels back to the cloud at about 60,000 miles per second, roughly one-third the speed of light, and produces more than 99 percent of the flash's luminosity. A typical flash contains three or four leader-return sequences, and a single negative flash may hold up to 20 return strokes. Peak temperatures in the return-stroke channel reach about 30,000 degrees Celsius.

Subsequent strokes are carried by dart leaders, which follow the decaying channel at intervals of about a tenth of a second and are relatively straight rather than stepped.

The electrical scale of a lightning discharge is set by the potential difference between cloud and ground, which reaches 10 to 100 million volts. Peak return-stroke currents approach 30,000 amperes. Before air can break down and conduct, tens or hundreds of millions of volts must accumulate.

Within the discharge channel the temperature climbs to about 30,000 °C (54,000 °F), roughly five times hotter than the surface of the Sun. Measured more broadly, channel temperatures span 18,000 to 60,000 degrees Fahrenheit.

Air heated within a fraction of a second expands explosively, compressing the air ahead of it into a shock wave much like a sonic boom. That shock wave decays within about ten yards and becomes the sound wave heard as thunder, which can be audible 25 miles away.

A nearby channel yields a sharp crack, while a discharge several miles away gives a prolonged rumble. Sound needs about five seconds to travel a mile, so dividing the flash-to-thunder delay by five gives the distance in miles.

Cloud-to-ground flashes divide into two polarities with distinct behaviour. Positive cloud-to-ground flashes normally have a single return stroke but are more likely to carry a sustained current. Because of that sustained current they are more likely to strike 5 to 10 miles from the rain core, transfer more charge and ignite fires more easily.

A bolt from the blue leaves the side of a storm and reaches the ground under clear sky several miles from the cloud. Almost all lightning occurs within 10 miles of the parent storm, yet detection equipment has confirmed bolts striking almost 50 miles away.

Transient luminous events occur high above storms. Sprites are mostly red, last no more than a few seconds and take shapes resembling jellyfish, carrots or columns. Elves are disk-shaped glows up to 300 miles across that last less than a thousandth of a second and appear above active cloud-to-ground lightning.

Lightning is often observed during explosive volcanic eruptions and is commonly associated with ash plumes. Within an eruption column the discharges are not confined to a single level: they occur in the gas-thrust region above the vent, in the rising convection column and in the umbrella region.

Two networks map lightning in real time over the United States: Vaisala's National Lightning Detection Network and the Earth Networks Total Lightning Network. Ground-based systems of this kind locate a stroke by comparing the time of arrival of radio signals at three or more stations. Vaisala reports detecting and locating more than two billion lightning events every year, within seconds of each flash.

Lightning Mapping Arrays take a different approach, locating very high frequency radiation near 60 MHz from six or seven stations and resolving sources in three dimensions.

The World Meteorological Organization has certified a single flash that spanned 768 kilometres across the southern United States on 29 April 2020 as the longest distance ever recorded for lightning ^{14WMO certifies two megaflash lightning records · World Meteorological Organization ↗}. That mark displaced the previous distance record of 709 kilometres across southern Brazil on 31 October 2018 which was sixty kilometres shorter than the new figure ^{14WMO certifies two megaflash lightning records · World Meteorological Organization ↗}. For duration the longest-duration flash lasted 17.102 seconds over Uruguay and northern Argentina during June 2020 according to the World Meteorological Organization ^{14WMO certifies two megaflash lightning records · World Meteorological Organization ↗}.

Where the Catatumbo River meets Lake Maracaibo, lightning recurs at the same time and place on about 300 days a year. Shortly after dusk the lake is struck about twenty-eight times a minute for up to nine hours. People around Lake Maracaibo are struck by lightning three to four times more often than people in the United States.

A typical thunderstorm flash produces about 250 moles of nitrogen oxides, equivalent to 3.5 kilograms of nitrogen. The best estimate of the global nitrogen mass released as lightning-produced nitrogen oxides is 5 teragrams a year, with an uncertainty of 3 teragrams.

A strike into sandy soil can fuse dirt and clay into a glassy, convoluted tube known as a fulgurite.

In a direct strike part of the current flashes over the skin while the rest passes through the body, often along the cardiovascular or nervous systems. Most indoor lightning casualties come from conduction along wiring, plumbing or metal surfaces connected to the outside.

Over the thirty years from 1989 to 2018 the United States averaged 43 reported lightning fatalities each year.

The 30-30 rule gives a simple test for when a storm has come close enough to threaten. A flash-to-thunder delay of 30 seconds or less means the storm is close enough to be dangerous, so shelter should be sought without delay. The rule also governs when it is safe to resume activity: at least 30 minutes should pass after the last thunder before leaving shelter.

Shelter is not equal in value. A fully enclosed, substantially constructed building with wiring and plumbing offers the best protection from lightning. A car with a metal roof and sides gives second-best protection because the metal shell, not the rubber tires, shields the occupants. The lightning crouch is no longer recommended.

The study of lightning as an electrical phenomenon rests on a small set of documented experiments and their consequences. On a June afternoon in 1752 Franklin flew a silk kite in a Philadelphia field during a thunderstorm to test the electrical nature of lightning. A month before Franklin's experiment Thomas-Francois Dalibard demonstrated the electrical nature of lightning in northern France. The protective device that followed from this line of inquiry was tested in practice: the large pointed rod on the Maryland State House, the biggest Franklin rod of his lifetime, had only one recorded instance of lightning damage.

• 1746
  Franklin encounters electrical experiments
  Franklin meets electrical experiments in Boston.
• July 1747
  Positive and negative
  Franklin is using the terms positive and negative.
• 1750
  The iron rod proposed
  Franklin proposes an iron rod about 8 or 10 feet long.
• 1752
  Dalibard in northern France
  A month before Franklin's experiment, Dalibard demonstrates the electrical nature of lightning.
• June 1752
  The silk kite
  Franklin flies a silk kite in a Philadelphia field during a thunderstorm.
• a year after
  Richmann killed
  Richmann is killed by ball lightning.
• reign of George III
  The blunt rod at the palace
  King George III fits his palace with a blunt rod.
• Franklin's lifetime
  The Maryland State House rod
  The largest Franklin rod of his lifetime records only one instance of lightning damage.