How does lightning happen?

The principle

Lightning. It’s the same thing as the little zap you get from a doorknob after walking across a carpet, just millions of times bigger. Charge builds up in one place, and when it can’t wait any longer it jumps across the air as a spark.

Step 1: Ice bumps, charge splits. Inside a storm cloud, strong winds carry tiny ice crystals upwards while heavier soft hail (small, mushy hailstones) falls. Every time they bump, a little charge moves from one to the other. The light ice crystals end up plus and are carried to the top of the cloud; the soft hail ends up minus and stays lower down. Think of rubbing a balloon on your hair, done by billions of tiny pieces of ice at once.

Step 2: A leader creeps down. When the charge difference gets big enough, the air near the bottom of the cloud gives way. A faint channel, called a stepped leader, pushes towards the ground in jumps of about 50 m, pausing for a tiny moment between each jump. It branches and zigzags as it goes, because it only “feels” the air right around its tip. The whole trip takes about a twentieth of a second, too faint and fast for your eye to see.

Step 3: The ground reaches up. As the leader gets close, the ground below it builds up the opposite charge. Tall, pointy things such as trees, masts, towers and houses send a spark of their own upwards (an upward streamer). When one of them touches the leader, the path from cloud to ground is complete. That is why tall things are hit most often.

Step 4: The flash. Now a huge current races back up the finished path. This is the bright flash you see, called the return stroke. It heats the air in a channel only a few centimetres wide to about 30,000 °C, about 5 times hotter than the surface of the sun. Often it flashes several times along the same path, which is why lightning seems to flicker.

Step 5: Thunder. Air heated that fast has nowhere to go: it expands with a bang, sending out a wave of sound. Light reaches you almost instantly, but sound only travels about 1 km every 3 seconds. So count the seconds between the flash and the thunder and divide by 3 to get roughly how many kilometres away it was (9 seconds is about 3 km). If you hear thunder at all, you are close enough to be hit, so go inside.

Lightning in the cloud or down to the ground?

• In-cloud lightning: the most common kind, jumping between the plus and minus parts of the same cloud. Downside for spotters: it often just lights up the cloud from inside, so you see a glow rather than a bolt.
• Cloud-to-ground lightning: the kind shown in the video, with a clear bolt down to a tall point. Downside: it is the dangerous one, because it hits people, trees and buildings.

Try it yourself: shuffle across a carpet in socks on a dry day and touch a metal doorknob in a dark room. You may see and hear a tiny spark and snap: your own mini lightning and thunder.