Lightning is a giant static discharge
Lightning is electricity built up inside a cloud suddenly rushing through the air. In a towering thundercloud, strong updrafts carry ice crystals and small soft hail called graupel up and down, and as they collide, electric charge gets separated. Generally the light ice crystals take on a positive charge and gather near the top, while the heavier graupel turns negative and collects lower down. When the difference becomes more than the air can hold back, current breaks through the air, and that's lightning. Much of it stays inside a cloud or jumps between clouds; strikes that reach the ground are only part of the total.
Thunder is the sound of air exploding
The air along a lightning channel is heated to tens of thousands of degrees in a tiny fraction of a second. That suddenly superheated air expands explosively and shoves the surrounding air outward, and the shock spreads as sound: thunder. A nearby strike makes a sharp, tearing crack, while distant thunder is a low rumble that rolls on and on. A lightning channel can be several kilometers long, so sound from different parts of it reaches you at different moments, and echoes off hills and buildings pile on top of that.
Light arrives at once, sound takes its time
Lightning and thunder happen at the same instant, but they reach you at very different times. Light travels at about 300,000 km per second, so the flash from a strike a few kilometers away reaches your eyes essentially immediately. Sound, on the other hand, moves through air at about 340 m per second. That figure shifts with temperature, a bit faster when it's warm and slower when it's cold, but around 340 is close enough for everyday conditions. Sound takes about 3 seconds to cover a kilometer, so the longer the wait after the flash, the farther away the strike was.
Counting seconds to estimate distance
Timing the gap between flash and thunder lets you estimate how far away a strike was. A 9-second gap means roughly 3 km; 3 seconds means about 1 km. In countries that use miles, people divide by 5 instead. Your reaction time adds some delay at both ends, so treat the result as a rough figure only.
- Start counting the moment you see the flash
- Stop when you first hear the thunder
- Divide the seconds by 3 for an approximate distance in kilometers
- Repeat a few times; if the count keeps shrinking, the storm is coming closer
Why far away isn't the same as safe
Being able to measure the distance doesn't mean a distant storm is harmless. Lightning can strike more than 10 km from the center of a storm, sometimes while the sky overhead still looks clear. That's why weather services and safety agencies say that, whatever your count tells you, if you can hear thunder you are already close enough to be struck and should move to shelter right away. Lightning can also come from the trailing edge of a storm after the rain stops and the sky brightens, so it's best to wait about 30 minutes after the last thunder before heading back out.
Where to go and where to avoid
The safest place is inside a solid building with electrical wiring and plumbing. Open-sided shelters like gazebos and bus stop canopies, and tents, keep off the rain but not lightning. A car with a hard metal roof is also relatively safe, not because of the rubber tires but because the metal body carries the current around the outside. Close the windows and keep your hands off metal parts.
- The middle of open fields, sports grounds and golf courses
- Mountain summits, ridges and high rocks
- Under a lone tall tree
- In or near water such as the sea, rivers and pools
- Near barbed wire, metal fences and steel towers
Indoor precautions
A building is safe because lightning current runs through its wiring and pipes down into the ground. The flip side is that if you're touching that wiring or plumbing, the current can pass through you. While thunder is rumbling, don't use a corded phone or touch electronics that are plugged in, and put off showering or washing dishes for a while. Stay away from windows and doors, and don't lean against concrete walls or lie on a concrete floor. If you want to protect devices like computers, unplug them before the storm arrives, not while thunder is already crashing.
Caught outside, and first aid
There is no truly safe position or spot outdoors. If you can't reach a building or car, get off high ground, move away from lone trees and water, and if you're in a group, spread out so a single strike can't injure several people. A person who has been struck carries no leftover electricity, so it's safe to go to them right away. Call emergency services immediately, and if they aren't breathing or responding, start CPR. Cardiac arrest is the biggest danger in a lightning strike, so fast action can decide whether someone survives. Before hiking or camping, check the official forecast for thunderstorms and change your plans if warnings are out.
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