How Lightning Works
From colliding ice in a storm cloud to a channel of air five times hotter than the Sun's surface, here's what happens in the fraction of a second a bolt takes.
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Right now, somewhere on Earth, lightning is striking. Satellites count around 44 flashes every second, well over a billion a year. Each one is over in a fraction of a second, but a lot happens in that blink.
Step 1: a cloud full of colliding ice#
Lightning starts inside a thunderstorm cloud, called a cumulonimbus. Strong updraughts carry water high into the cloud, where it freezes. Up there you get two kinds of ice:
- Ice crystals: small and light, carried upwards by the wind.
- Graupel: soft, heavier pellets, a bit like tiny snowballs, that tend to sink.
As they bump past each other, the collisions knock electric charge from one to the other. The light crystals usually end up positive and get carried to the top of the cloud; the heavy graupel ends up negative and gathers lower down. The cloud becomes a giant battery, positive on top and negative at the bottom.
The negative base of the cloud also pushes away negative charge in the ground below, leaving the ground underneath it positively charged. That shadow of charge follows the storm as it moves.
Step 2: the stepped leader#
Air is normally an excellent insulator. It takes an enormous voltage, hundreds of millions of volts across a storm, to break it down.
When the charge is strong enough, a faint, branching channel of ionised air starts to creep down from the cloud. This is the stepped leader. It moves in jerky jumps of tens of metres at a time, branching and searching, and it's usually too dim to see with the naked eye.
Step 3: reaching up to meet it#
As the leader gets close to the ground, the electric field near the surface becomes intense. Tall, pointed objects such as trees, masts, buildings and sometimes people start sending up their own faint sparks, called upward streamers.
When one streamer touches the descending leader, the circuit is complete.
Step 4: the return stroke#
This is the part you actually see. A huge current, typically tens of thousands of amps, surges along the connected channel. Interestingly, the brightest pulse travels upwards, from the ground to the cloud, as the channel drains. It happens so fast that your eyes can't tell the direction.
The air in that channel, only a few centimetres wide, is heated to around 30,000 °C, about five times hotter than the surface of the Sun.
Often the same channel is used again. Several more strokes can follow within a fraction of a second, which is why lightning sometimes appears to flicker.
Most lightning never touches the ground at all. The majority of flashes happen inside a cloud or between clouds. Cloud-to-ground strikes are the minority, just the most dangerous and the easiest to photograph.
Where thunder comes from#
Heating air to 30,000 °C in a few millionths of a second makes it expand explosively. That sudden expansion sends out a shock wave, which slows down into the sound we call thunder. A sharp crack means the strike was close; a long rumble is the sound arriving from different parts of a long, winding channel at slightly different times.
Light reaches you almost instantly, but sound takes about 3 seconds per kilometre (or 5 seconds per mile). Count the seconds between the flash and the thunder, divide by three, and you have the distance in kilometres.
Lightning does strike twice#
The saying is wrong. Lightning favours tall, isolated, conductive points, and it will happily hit them again and again. The Empire State Building is struck around 20 to 25 times a year.
Some places are struck far more than others. The world's lightning capital is Lake Maracaibo in Venezuela, where warm, wet air and surrounding mountains produce storms on most nights of the year, a phenomenon known locally as Catatumbo lightning.
Staying safe#
- "When thunder roars, go indoors." If you can hear thunder, you are close enough to be struck.
- Wait 30 minutes after the last thunder before going back outside.
- A car with a metal roof is a safe shelter. That's not because of the rubber tyres; it's because the metal shell carries the current around the outside of the car and into the ground.
- Avoid open fields, hilltops, lone trees and water.
In short#
- Colliding ice in storm clouds separates charge: positive on top, negative below.
- A dim stepped leader creeps down from the cloud, and upward streamers reach up to meet it.
- When they connect, a massive return stroke heats the air to about 30,000 °C.
- That superheated air expands violently and produces thunder.
- Lightning can and does strike the same place twice.