How Lightning Works, and How to Stay Safe

Lightning strikes the Earth about 44 times every second, roughly 1.4 billion flashes a year. Each flash heats the air around it to temperatures several times hotter than the surface of the Sun, and then it is gone in a fraction of a second.

For something so common, lightning is surprisingly complex. Here is how a storm builds up the charge, what happens in a strike, why thunder follows, and what the experts say to do when a storm approaches.

What lightning is

Lightning is a natural electrostatic discharge: a sudden flow of electricity between regions with different electric charges. Most flashes happen within a single cloud or between clouds; only about a quarter of all lightning worldwide travels between a cloud and the ground.

It is, in a sense, a giant version of the spark you feel when you touch a door handle after walking across a carpet. The difference is scale. A single flash can release somewhere between about 200 million and 7 billion joules of energy.

How a cloud becomes charged

The charge builds inside thunderstorm clouds. Strong updrafts carry small ice crystals upward, where they collide with larger, softer pellets of ice called graupel. In these collisions, the ice crystals tend to become positively charged and the graupel negatively charged.

Because the light crystals are carried higher, the upper part of the cloud tends to become positive, while the middle and lower parts become negative. When the difference grows large enough that the air can no longer insulate it, about 3 million volts per metre in damp air, the charge breaks through.

Anatomy of a strike

A cloud-to-ground flash begins with a faint channel called a stepped leader, which works its way down from the cloud in a series of jumps. As it nears the ground, a streamer rises to meet it from a tree, a building or the ground itself.

When they connect, a powerful current surges up the channel. This return stroke is the bright flash we see. It can travel at about a third of the speed of light, and an average flash carries a current of about 30,000 amperes. Most cloud-to-ground strikes carry negative charge; the rarer positive strikes, under 5 per cent, are more intense.

Hotter than the Sun

The return stroke heats the channel of air to around 30,000 °C, several times the temperature of the Sun's visible surface. Air heated that fast has nowhere to go but outward, and it expands explosively.

That explosion is thunder. The shock wave spreads out as a sound wave, a crack close to the strike and a long rumble further away, as sound arrives from different parts of a channel that may be kilometres long.

Counting the seconds

Light from a flash reaches you almost instantly, but sound travels at only about 340 metres per second. That is why thunder arrives after the flash, and why counting the seconds between them gives a rough distance: about three seconds for each kilometre.

It is a handy guide, but not a safety margin. If you can hear thunder at all, you are probably close enough to be struck, as weather services stress.

Where lightning strikes most

About 70 per cent of lightning happens over land in the tropics, where warm, moist air feeds powerful storms. The most lightning-struck place on Earth is Lake Maracaibo in Venezuela, where storms produce around 250 flashes a day on average, on about 297 days a year.

Flashes can be astonishingly long. The longest on record, identified in 2025, stretched 829 kilometres across Texas and Kansas during a storm in October 2017.

Staying safe

The US National Weather Service puts its advice in one sentence: when thunder roars, go indoors. Once a thunderstorm is nearby, no place outside is safe. Waiting until the rain starts, or until the storm looks close, is one of the main reasons people are struck.

In the United States, lightning strikes the ground about 25 million times a year and kills about 20 people, with hundreds more severely injured. Most lightning happens in summer, but people can be struck at any time of year.

The weather service also promotes the slogan Flash, Dash Inside, created by and for the Deaf and Hard of Hearing community, for whom the flash rather than the thunder is the warning. People who work outdoors face particular risk, and US workplace safety guidance covers how employers should plan for storms.

Myths and evidence

Lightning attracts plenty of myths, such as the belief that it never strikes the same place twice; tall structures are in fact struck again and again. Checking such claims against the evidence is a habit worth building, and our guide to how to evaluate a claim shows how.

Scientists are also studying how a warming climate may change thunderstorms and lightning. On the wider science of climate, see what scientists agree on about climate change.