Why is lightning zigzagged instead of a straight line?

On a stormy day, look up at the sky. Lightning is never a straight bright line. It is a crooked, zigzagging path with branches. Why?

Here is the surprising part: lightning does not shoot down from the cloud. It feels its way.

Normal air does not carry electricity. Before charge can move, the air along the way must be torn apart into plasma, and that takes a very strong electric field. The charge cannot cross the whole sky in one jump, so it goes step by step. It breaks the air at the easiest spot and makes a short channel, only tens of metres long. Then it stops, builds up charge again, and jumps the next short step. Scientists call this a "stepped leader."

The problem is that air is never the same everywhere. Humidity, temperature, dust and electric charge all differ from place to place. So which way the next step goes is mostly luck. That is why the channel wanders and keeps forking. The thin branches you see in photos are paths that never reached anywhere.

When one branch finally touches the ground, the whole road is connected. Only then does the real flash begin. A current races back along the path already carved out, at about one third the speed of light. It heats the air to around 30,000 kelvin, five times hotter than the surface of the Sun. That bright streak is called the "return stroke." So the light of a cloud-to-ground flash actually travels upward.

In one line: lightning is crooked because it is feeling its way.

Try it yourself: pour a cup of water slowly down a slope. The water will not run straight down. It will curve and split into branches, because the ground is bumpy and water always takes the easiest path. Lightning finding its way through air works the same way.