A bright white flash splits the dark sky, and a few seconds later a deep rumble rolls across the town. Every child asks the same question: what causes lightning and thunder? The answer is a mix of ice, air and electricity inside a storm cloud. Lightning is a huge electric spark. Thunder is the sound that spark makes as it blasts the air around it. This guide explains how a storm cloud builds up electric charge, why you see the flash before you hear the sound, and how to stay safe.

What Is Lightning?

Lightning is a sudden, powerful discharge of static electricity. It is the same kind of spark you sometimes feel when you touch a door handle after walking on a carpet, only enormously larger. A single bolt can be several kilometres long and carry a current of tens of thousands of amperes. Earth is hit by lightning roughly 40 to 50 times every second, and most of these flashes happen over land in the tropics.

What Is Thunder?

Thunder is the sound wave produced by lightning. The bolt heats a thin channel of air to about 30,000°C, which is several times hotter than the surface of the sun. The air expands explosively and then cools and contracts, sending a shock wave outward. Close up, thunder is a sharp crack or bang. From far away it becomes a long, low rumble, because the sound from different parts of the bolt reaches you at different times and bounces off hills and clouds.

How Does Lightning Form? 5 Steps

1. A tall storm cloud grows. Warm, moist air rises quickly and builds a towering cumulonimbus cloud. Water droplets are carried high enough to freeze.
2. Ice particles collide. Tiny ice crystals rise with the updraft, while heavier soft hail pellets (called graupel) fall. When they collide, electric charge is transferred between them.
3. The cloud separates its charge. The light ice crystals, which carry positive charge, are lifted to the top. The heavier graupel, which carries negative charge, collects in the lower part of the cloud. The ground beneath the storm becomes positively charged.
4. A path opens. When the electric field becomes strong enough, a faint channel of charge called a stepping leader moves down from the cloud in short jumps. A matching streamer reaches up from the ground, often from a tall object such as a tree, tower or building.
5. The bright flash. When the two meet, a huge current surges up the channel. This return stroke is the brilliant flash you see. A single flash often repeats several times in a few tenths of a second, which is why lightning can appear to flicker.

How Does Thunder Form?

The same flash that lights up the sky also heats the air around it almost instantly. The air has no time to move out of the way, so its pressure rises sharply and creates a shock wave. That wave travels outward as sound. Because sound moves far slower than light, you always see lightning first and hear thunder later.

Why Do We See Lightning Before We Hear Thunder?

Light travels at about 300,000 kilometres per second, so it reaches your eyes almost at the same moment the flash happens. Sound travels at about 343 metres per second in air, which is roughly one kilometre every three seconds. The gap between the flash and the bang tells you how far away the strike was.

To estimate the distance, count the seconds between the flash and the first rumble, then divide by 3. The result is the distance in kilometres.

Types of Lightning

• Cloud-to-ground: the bolt that strikes the earth. It is the most dangerous type for people and buildings.
• Intra-cloud: a flash inside one cloud. It is the most common type, and it lights up the whole cloud, which is sometimes called sheet lightning.
• Cloud-to-cloud: a flash that jumps between two separate clouds.
• Cloud-to-air: a bolt that leaves the cloud and fades into the clear air nearby.
• Ground-to-cloud: a bolt that starts from a tall structure, such as a mountain-top tower, and rises into the cloud.

Lightning Myths and Facts

Does lightning never strike the same place twice?

It does. Tall, pointed or isolated objects are struck again and again. Some tall towers are hit many times in a single year.

What is “heat lightning”?

It is simply lightning from a faraway storm. The flash can be seen at night, but the thunder is too far away to be heard. It is not a different kind of lightning.

Can lightning happen without rain?

Yes. A bolt can strike several kilometres from the storm’s rain area, which is why people sometimes call it a “bolt from the blue”. If you can hear thunder, you are close enough to be struck.

Are rubber tyres or rubber shoes protection?

No. The protection of a car comes from its metal roof and body, which direct the current around the passengers and into the ground. Shoes offer no real protection.

Where and When Does Lightning Strike Most?

Lightning needs thunderstorms, so it is most common where warm, moist air rises strongly. This means tropical land areas, mountain slopes and places with strong afternoon heating. Storms are most frequent in the late afternoon and evening, when the ground has been heated all day, and in the warm months of the year.

Lightning and Thunder in Pakistan

In Pakistan, lightning is most common in the pre-monsoon and monsoon months, from about May to September, when the plains are hot and moist air arrives from the sea. Thunderstorms can form over the plains of Punjab and Sindh, the hills of Khyber Pakhtunkhwa and Kashmir, and the desert area of Tharparkar. People who work in open fields, herd animals, sit under isolated trees or shelter in small huts are especially at risk. Lightning also often comes with strong winds, hail and sudden heavy rain, so it is often followed by flash floods in the hills. Always follow warnings from the Pakistan Meteorological Department when a thunderstorm is expected.

Lightning Safety Tips

The best rule is simple: “When thunder roars, go indoors.” If you can hear thunder, you are within range of a strike.

• Go into a strong building with a roof and walls, or into a hard-topped metal vehicle with the windows closed. Open shelters, sheds, tents and bus stops do not protect you.
• Follow the 30-30 rule: if the gap between flash and thunder is 30 seconds or less, go inside. Wait 30 minutes after the last thunder before going out again.
• Stay away from windows, doors, and plumbing. Avoid showers, baths and washing dishes during a storm, because the current can travel through pipes.
• Unplug sensitive electronics, or use a surge protector, and avoid using corded phones.
• If you are caught outdoors, move away from open fields, hilltops, water, and isolated trees. Do not shelter under a lone tree, and do not hold a metal pole or umbrella.
• Stay out of lakes, rivers, the sea and swimming pools. Water conducts electricity.
• If a group is outside, spread apart so a single strike does not harm everyone.

If someone is struck by lightning

A person who has been struck does not carry any electric charge, so it is safe to touch them. Call for emergency help at once. If the person is not breathing, begin CPR and continue until help arrives. Many lightning victims survive when they receive help quickly.

Keep watching the sky and the forecast with the live weather, hourly forecast and weather maps on WeatherWorld.

Can Lightning Be Predicted?

Forecasters can tell when the atmosphere is likely to produce thunderstorms, and radar and lightning detection networks track storms as they move. They cannot say exactly where the next bolt will strike. That is why you should act on the first sound of thunder instead of waiting for the rain to start.

Learn More

For official safety advice, visit the National Weather Service lightning safety page. To see how the same storms can spin into something more dangerous, read what a tornado is and how it forms. To understand the pressure changes that come with storms, read about atmospheric pressure, and to see how big storms form over the sea, read what a tropical cyclone is.

Frequently Asked Questions

What causes lightning and thunder in simple words?

Ice particles in a storm cloud rub together and build up electric charge. When the charge becomes too strong, it jumps as a spark, which is lightning. The spark heats the air so fast that it booms, which is thunder.

Why do we see lightning before we hear thunder?

Light travels much faster than sound. The flash reaches you almost instantly, while the sound takes about three seconds to travel each kilometre.

How hot is lightning?

A lightning channel can reach about 30,000°C for a very short moment, which is several times hotter than the surface of the sun.

How far away can you hear thunder?

Usually about 10 to 15 kilometres. Beyond that, the sound fades or bends upward and is lost before it reaches you.

Can you be struck by lightning indoors?

It is rare, but possible through wiring, plumbing and corded devices. Staying away from water pipes and electronics during a storm lowers the risk.

Is it safe to use a mobile phone during a thunderstorm?

Using a mobile phone indoors is safe. Avoid corded phones and avoid using any device that is plugged in and connected to the wiring.

How long should I wait after thunder before going outside?

Wait at least 30 minutes after the last thunder you hear. Storms can still strike after the rain has stopped.

Final Thoughts

Lightning and thunder are two parts of the same event. Ice and air inside a storm cloud build up an enormous electric charge, the charge releases as a spark of lightning, and the heated air makes the sound of thunder. The science is fascinating, but the safety message is simple. If you can hear thunder, go indoors, stay there until 30 minutes after the last rumble, and keep away from water, open ground and tall isolated objects. A little respect for the storm can protect you and your family.