A thunderstorm is one of the most dramatic things the atmosphere does. A bright flash, a delay, then a rolling boom: the sequence is familiar, yet the details of how lightning forms are still being researched. The broad picture, however, is well understood.
Key takeaways
- Lightning is a huge electrical discharge that happens when charge separates inside a storm cloud.
- Thunder is the sound made when lightning rapidly heats the air around its path.
- Light travels far faster than sound, so you see the flash before you hear the thunder.
- Counting the seconds between flash and bang gives a rough idea of distance.
- Lightning can be dangerous, and simple precautions reduce risk.
How a storm cloud becomes charged
Tall thunderclouds contain strong rising air currents, ice crystals, soft hail pellets and water droplets. When these particles collide, they exchange electric charge. Lighter ice crystals tend to be carried upward and gather positive charge near the top, while heavier pellets sink and collect negative charge in the middle and lower parts of the cloud. The ground below often develops an opposite, positive charge. Scientists continue to study the exact details of how this charge separation happens.
The lightning flash
Air normally stops electricity from flowing, but when the voltage difference becomes large enough, the air breaks down and forms a conducting channel. A faint path called a stepped leader works its way downward in short jumps. When it connects with an upward streamer from the ground or a tall object, a bright return stroke surges back up the channel. What we see as a single flash is often several strokes in quick succession along the same path. Much lightning stays inside clouds or jumps between clouds and never reaches the ground.
What makes thunder
The lightning channel can reach temperatures several times hotter than the surface of the Sun for a very short moment. The air around it expands explosively and then contracts, sending out a pressure wave that we hear as thunder. Because the channel is long and sound arrives from different distances along it, thunder often rumbles and echoes instead of arriving as a single bang.
Judging how far away a storm is
Light reaches you almost instantly, while sound travels roughly one kilometre in three seconds, or about one mile in five seconds. Count the seconds between the flash and the thunder and divide by three for kilometres or by five for miles. This is only a rough guide, and storms can move quickly.
Staying safer
Safety agencies give consistent advice: when thunder is audible, move indoors to a substantial building or into a hard-topped vehicle with the windows closed. Avoid open fields, hilltops, isolated trees and water. Indoors, avoid contact with plumbing and corded electronics during a storm. Follow official local guidance, as recommendations can vary by country.
Kinds of lightning
Most lightning never touches the ground. Intracloud lightning flashes inside a single storm cloud, and cloud-to-cloud lightning jumps between clouds, lighting up the sky. Cloud-to-ground lightning is the type that causes damage and injuries. Rarer phenomena include ball lightning, glowing spheres reported by witnesses for centuries and still not well explained, and transient luminous events high above storms, nicknamed sprites, jets and elves, which were first photographed in the late twentieth century. Heat lightning is simply distant lightning too far away for its thunder to be heard.
Lightning rods and protection
In the eighteenth century, Benjamin Franklin proposed that a pointed metal rod connected to the ground could protect buildings, and his lightning rods were soon installed on churches and houses. The rod does not attract lightning so much as provide a safe, low-resistance path to the earth if a strike happens. Modern buildings use grounding systems that carry current away from the structure. Inside, surge protectors help shield electronics from voltage spikes that travel along power lines, although they cannot stop a direct strike.
Safety myths and facts
Some common beliefs are wrong. Rubber tyres do not protect a car; its metal body does. Using a mobile phone is safe, though a corded landline during a storm is not. Lightning can strike far from the storm centre, sometimes ten kilometres or more ahead of the rain, so the saying that when thunder roars, you should go indoors is good advice. A common rule is to wait for about thirty minutes after the last thunder before going back outside. Avoid showers, baths and plumbing during a storm. People struck by lightning do not carry an electrical charge and can be helped safely, and anyone who is unresponsive needs emergency help and CPR.
Lightning and the planet
Earth experiences tens of lightning flashes every second on average, concentrated over land in the tropics. Lightning helps fix nitrogen from the air into forms plants can use, and it can ignite wildfires. In Venezuela, a spot where the Catatumbo River meets Lake Maracaibo has an extraordinary number of storms for much of the year, producing near-constant lightning. Satellites and ground networks now track lightning and help forecast severe weather.
Thunder and distance
The sound of thunder is affected by the distance and the surroundings. Close strikes sound like sharp cracks, while distant ones rumble because the sound waves take different routes and bounce off terrain and layers of air. Thunder is generally heard from distances of up to about fifteen to twenty-five kilometres, depending on conditions. Recording thunder with microphones lets scientists estimate the structure and path of a lightning channel.
Frequently asked questions
Can lightning strike the same place twice?
Yes. Tall structures and exposed points are struck repeatedly, sometimes several times in a single storm.
Is heat lightning a different kind of lightning?
The term usually describes distant lightning whose thunder is too far away to hear.
Does rubber tyre insulation protect a car?
A car's metal body usually carries the current around the occupants, which is why a closed hard-topped vehicle is considered safer than being outside.
How far away can you hear thunder?
Usually up to around fifteen to twenty-five kilometres, depending on weather, terrain and background noise. If you can hear thunder, you are close enough to be struck, so take shelter.
What is ball lightning?
Ball lightning refers to rare reports of glowing spheres seen during or after thunderstorms, lasting seconds. Because it is so rare and hard to observe, its cause is still debated, and only a few scientific measurements exist.
Is it safe to use a phone during a storm?
Mobile and cordless phones are safe indoors, because they are not connected to the wiring. Corded phones and devices plugged into mains power can carry a surge, so avoid them. Do not stand outside to use a phone during a storm.
Sources and further reading
Written by
FactNest Editorial Team
FactNest is operated by Muonix. The FactNest Editorial Team publishes general-interest explainers with AI-assisted drafting and illustrations. Contact contact@muonix.co for questions or corrections; we respond within 2 business days.
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