Thunderstorms

A thunderstorm is a powerful weather event involving towering storm clouds, heavy rain, lightning and thunder. They are some of the most spectacular natural events you can see from your back garden. At any moment around the world there are approximately 2,000 thunderstorms happening, producing roughly 100 lightning strikes every second. Most thunderstorms are short and harmless, but the strongest can produce devastating winds, large hail, flooding rains and tornadoes. They are nature's most dramatic display.

  • At any moment worldwideApproximately 2,000Thunderstorms in progress
  • Lightning strikes per secondApproximately 1008.6 million per day worldwide
  • Storm cloud typeCumulonimbusUp to 16 km tall
  • Lightning temperatureAround 27,000 degrees CHotter than the Sun's surface
  • Thunder causeAir rapidly heating then coolingFrom the lightning bolt
  • Most dangerous timeDuring the storm and right afterRisk continues for 30 minutes after last thunder

How thunderstorms form

A thunderstorm forms when three things come together:

  1. Warm wet air at ground level.
  2. An unstable atmosphere (cold air above warm air, so the warm air wants to rise).
  3. Something to lift the warm air (a weather front, a mountain slope, the Sun heating a patch of ground unevenly).

Once the warm wet air starts rising, it cools and water vapour condenses, releasing huge amounts of heat that pushes the air up even more. A cumulonimbus cloud can grow up to 16 km tall in less than an hour, full of swirling air, hail, ice and rain.

How lightning forms

Inside a thunderstorm cloud, strong updrafts and downdrafts carry water droplets and ice crystals up and down. As they bounce around, the lighter ice crystals tend to become positively charged and the heavier hail-like graupel tends to become negatively charged. The lighter crystals are carried to the top of the cloud, while the heavier graupel falls to the bottom. This creates a huge separation of electric charge: positive at the top of the cloud, negative at the bottom.

Eventually the charge difference becomes so large that it breaks down the insulation of the air, and a giant spark jumps. Most lightning happens inside or between clouds; only a small fraction strikes the ground. A single lightning bolt can be over 50 km long and contain up to a billion volts.

Why thunder follows lightning

The lightning bolt heats the air it passes through to approximately 27,000 degrees C in a few microseconds: about 5 times hotter than the surface of the Sun. The air expands explosively, then collapses back as it cools, producing a powerful sound wave: the thunder.

Light travels much faster than sound. Light travels at 300,000 km/s; sound travels at only 343 m/s in air. So you see the lightning flash almost immediately, but the sound takes a few seconds to reach you. By counting the seconds between flash and thunder, then dividing by 3, you can estimate the distance to the strike in kilometres. A 6-second gap means the lightning was about 2 km away.

How to stay safe in a thunderstorm

  • Get indoors if possible. A building or car is the safest place.
  • Avoid tall isolated objects: trees, hilltops, flagpoles, golf clubs, fishing rods.
  • Avoid water: do not swim or boat during storms. Water conducts electricity.
  • Crouch low on the balls of your feet if caught in the open, away from tall objects.
  • Wait out the storm: most thunderstorms pass in under an hour.
  • Stay inside for 30 minutes after the last clap of thunder. Lightning can still strike from a distance.
Fact Lightning kills approximately 2,000 people worldwide each year and injures many more. Around 90% of those struck by lightning survive, though many have lasting injuries. The chance of being struck in your lifetime is approximately 1 in 15,000, although it depends a lot on where you live and what you do. Roy Sullivan, a US park ranger, holds the record for surviving the most lightning strikes: 7 strikes between 1942 and 1977, with relatively minor injuries each time.

Severe thunderstorms

Most thunderstorms are short and relatively harmless. But the most powerful ones, called supercell thunderstorms, can produce dangerous weather including:

  • Large hail: tennis-ball or even grapefruit-sized hailstones that can damage cars and crops.
  • Tornadoes: most strong tornadoes are spawned by supercells.
  • Flash floods: heavy rain falling in a short time on top of already saturated ground.
  • Microbursts: sudden powerful downdrafts that can damage trees and aircraft.
  • Damaging winds: gusts over 100 km/h can rip off roofs and uproot trees.
Did you know? Lightning has been around since the very early Earth. Most scientists think lightning was important for the chemistry that produced the first building blocks of life approximately 4 billion years ago. The famous 1952 Miller-Urey experiment showed that passing electric sparks through a mixture of water vapour, ammonia and methane (similar to the early Earth atmosphere) produces amino acids: the building blocks of proteins. Lightning may literally have sparked life.
Deeper dive: the lightning capital of the world

If you wanted to find the place on Earth with the most lightning, you might guess somewhere tropical with regular thunderstorms. The answer is more specific: the Catatumbo river region of Venezuela, where the Catatumbo river meets Lake Maracaibo. This small area produces a unique phenomenon called Relampago del Catatumbo ("Catatumbo lightning"), which lights up the sky 140 to 160 nights per year, sometimes producing up to 250 lightning flashes per square kilometre per year.

The reason has to do with local geography. Warm moist air rising from Lake Maracaibo meets cool air rolling down from the surrounding Andes mountains. The mixing produces enormous thunderstorms that begin most evenings and last until the early morning. The lightning is so reliable and so spectacular that sailors used it as a natural lighthouse to navigate the lake for centuries.

NASA satellite measurements have confirmed Catatumbo as the world's most lightning-prone spot. The runners-up are the eastern foothills of the Congo basin in Africa, parts of the Himalayan foothills, and certain spots in the lakes of central Florida (the "lightning capital of the United States"). In each case, the same basic recipe applies: warm wet tropical air meeting cooler air at mountain or lake boundaries.

For more, see lightning, tornadoes and clouds.