If you've ever watched a spark of static electricity jump from an object like a doorknob to your finger, you perhaps noted that mini-bolt followed a fairly straight-line path from the object to your finger. Lightning bolts are essentially the granddaddy version of that static spark, so why do lightning bolts generally appear so jagged and forked?
The difference arises as a matter of scale, and the answer lies in the distance covered by the two sparks. The finger-to-doorknob path is but a few centimetres (inches) long, while the lightning bolt generally reaches five kilometres (three miles) or more in length. The reason for lightning's jaggedness arises in its formation process. To describe the process, I will use a basic cloud-to-ground lightning stroke as my example.
Photo Courtesy US National Weather Service
Air generally provides rather good electrical insulation against the movement of electricity through it. So when regions of static electrical charge build in thunderclouds, large voltage differentials, equalling millions of volts, will form between the cloud and ground below. During thunderstorms, the lower cloud generally contains a region that accumulates a large negative charge while the upper cloud becomes positively charged. A positive charge region develops on the surface of the normally neutrally-charged earth because the low cloud region of negative charge repels the negative charges beneath it while attracting the positive charges.
To relieve this voltage differential, a weak, negatively-charged streamer of ionized gas moves out from the cloud in search of the path of least resistance, or highest conductivity, to an area of positive charge. In a lightning flash this charge transfer occurs along a channel whose width is rather small, about the width of an adult finger, compared to its length. This streamer advances in relatively small, discrete steps about 50 metres (163 feet) long, creating an ionized channel called a stepped leader in the process. Each leader step takes about one microsecond to complete and about 50 microseconds elapse between succeeding steps.
Streamer Moves from Cloud to Ground in Discrete Stepped Leaders
The sinuous path of the stepped leader results because the conductivity of the air is not uniform. The leader, whose voltage potential at the leader tip can exceed 10 million volts, travels across the sky linking regions of lesser resistance in an irregular path — much like the non-straight path we would take while running through a forest to avoid the trees. Like trees, randomly strewn pockets of highly resistant air are also easier to go around than through. The individual steps are often far from vertical though usually head downward to some degree for a cloud-to-ground bolt. Should the stepped leader find two possible paths of better conductivity, it will fork exploring each split for the best route.
When a negatively-charged stepped leader finally connects with an area of positive charge on the ground (or in another cloud or the same cloud), a lightning bolt races back up the leader channel to relieve the electrical pressure, illuminating the jagged leader pathway in passing. Several flashes are usually required during a typical bolt to relieve all the potential charge stored in the cloud region. The complete lightning flash, composed of a series of individual strokes, typically lasts about 4/10 of a second. The duration of each individual lightning stroke varies, but typically averages about 30 microseconds.
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