Why does lightning zigzag?

A lightning bolt begins with a stepped leader, an invisible channel of ionised air that extends downward from a cloud in roughly 50-metre steps, each lasting a few millionths of a second. At each step, the leader branches in the direction where it is easiest to ionise the next pocket of air, which depends on tiny, random differences in air density, moisture and dust. This creates the characteristic zigzag shape. When the leader nears the ground, a return stroke shoots upward along the channel at about a third the speed of light, producing the bright flash we see. The channel is only a few centimetres wide but can reach temperatures of about 30,000 °C.