Lightning

Lightning is a giant electric spark that jumps through the sky during a thunderstorm. It is one of the most powerful natural phenomena on Earth, with each bolt carrying up to 1 billion volts and reaching temperatures of approximately 27,000 degrees C: about five times hotter than the surface of the Sun. Lightning has been part of the Earth's weather for billions of years, possibly even helping create the first chemicals of life. It is also extremely dangerous: lightning kills approximately 2,000 people worldwide every year, although careful safety habits can dramatically reduce the risk.

  • VoltageUp to 1 billion voltsPer typical bolt
  • CurrentUp to 200,000 ampsFor a few millionths of a second
  • TemperatureApproximately 27,000 degrees C5x hotter than the Sun's surface
  • Strikes per second worldwideApproximately 1008.6 million per day
  • Length of a boltUp to 100+ kmMost are 5 to 10 km
  • Speed of boltAbout a third the speed of lightAbout 100,000 km/s

How lightning forms

Lightning forms inside towering cumulonimbus thunderstorm clouds. Strong updrafts and downdrafts inside the cloud carry water droplets, ice crystals and hail up and down. As these particles collide and bounce around, they swap electric charge: smaller ice crystals tend to gain positive charge, while heavier graupel (small soft hail) tends to gain negative charge.

The updrafts carry the positively charged crystals to the top of the cloud, while the heavier negative graupel sinks to the bottom. This creates a huge separation of charge: positive at the top, negative at the bottom. The negative bottom of the cloud also induces a positive charge on the ground directly below.

Eventually the charge difference becomes so big that the insulation of the air breaks down, and a powerful electric current jumps to neutralise it. The result is a lightning bolt: the air glows white-hot along the path of the discharge.

The types of lightning

  • Cloud to ground: the most familiar (and most dangerous) type. Strikes the surface.
  • Cloud to cloud: jumps between two different clouds.
  • Intracloud: stays within a single cloud. Most lightning is this type, often seen as flickering light inside a cloud.
  • Ground to cloud: rare; starts from a tall object on the ground and travels upwards into the cloud.
  • Sheet lightning: a bright flash that lights up a whole cloud without a visible bolt.
  • Heat lightning: lightning far away whose thunder is too distant to hear.
  • Ball lightning: rare glowing balls of light that float through the air. Still not fully understood by science.

How to estimate distance using lightning

You see the lightning flash almost instantly because light travels at 300,000 km/s. But sound travels much slower: only 343 m/s in air. So thunder reaches you a few seconds after the flash, depending on how far away the strike was.

To estimate the distance:

  1. Watch for the lightning flash.
  2. Count the seconds until you hear the thunder.
  3. Divide by 3 to get the distance in kilometres, or divide by 5 to get the distance in miles.

A 6-second gap means the strike was approximately 2 km away. A 3-second gap means 1 km. A gap of less than 30 seconds means the storm is close enough to be dangerous, and you should seek shelter immediately.

How dangerous is lightning?

Lightning kills approximately 2,000 people worldwide each year and injures many more. Most strikes happen when people are caught outdoors during a storm. The most dangerous places include:

  • Under or near tall trees.
  • On hilltops or open fields.
  • In or on water.
  • Holding metal objects (umbrellas, golf clubs, fishing rods).

The safest places are inside solid buildings or hard-topped vehicles. Cars are surprisingly safe because the metal body of the car acts as a Faraday cage, channelling the electric current around the outside and away from the passengers inside.

Fact Lightning hits the same place again far more often than people think. The Empire State Building in New York is struck by lightning approximately 25 times a year. The Eiffel Tower is hit approximately 20 times a year. The old saying that "lightning never strikes twice in the same place" is completely wrong: tall, isolated, conductive objects are struck again and again, year after year.

Lightning and life

Lightning may have been important in producing the chemistry of life on early Earth. In 1952, American chemists Stanley Miller and Harold Urey ran a famous experiment in which they passed electric sparks through a mixture of water vapour, ammonia and methane (similar to the early Earth's atmosphere). After just a week, they found that the mixture contained amino acids: the building blocks of proteins.

This suggested that lightning storms on the young Earth could have produced the basic chemistry needed for life to begin. Modern experiments using updated guesses about the early atmosphere have produced even more interesting molecules. Lightning may have literally helped spark life on Earth.

Did you know? Lightning also produces fulgurites: long tube-shaped glassy structures that form when a lightning bolt strikes sand and instantly melts it into glass. Fulgurites can be over a metre long and are sometimes called "fossil lightning". They preserve the exact path the lightning took through the sand for thousands of years.
Deeper dive: Benjamin Franklin and the invention of the lightning rod

For most of human history, lightning was a complete mystery. People believed it was thrown by angry gods (Zeus, Thor, Jupiter, Indra and many others). Lightning was thought to be unstoppable: it killed people, burnt down churches, sank ships and was considered a punishment from heaven.

This started to change in 1752 when the American scientist and politician Benjamin Franklin performed his famous (and incredibly dangerous) kite experiment. Franklin flew a kite with a metal key attached during a thunderstorm. When the storm clouds passed overhead, electric charge flowed down the wet kite string and produced sparks from the key. The experiment proved that lightning was the same kind of electricity that could be made in a laboratory.

This discovery led directly to Franklin's invention of the lightning rod: a pointed metal rod attached to the top of a tall building, with a wire running down to the ground. The rod attracts lightning strikes that would otherwise hit the building, and the wire safely carries the charge into the earth without harming the building.

The lightning rod was one of the first major scientific inventions to save lives in a measurable way. Within a few decades of its introduction, churches and tall buildings that had previously been struck regularly were almost never damaged. Today every tall building in the world uses some form of Franklin's basic idea. He never patented the lightning rod, refusing to profit from a device that he felt belonged to humanity.

For more, see thunderstorms and clouds.