Download The Science of Thunder and Lightning

 The Science of Thunder and Lightning Episode Two: Why lightning looks the way it does ­ crooked and forked? Written by: LEE Lap­shun In the first episode we explored why thunder roars. As thunder is always accompanied by lightning, let’s now take a look at
the spectacular display of lightning. The ionized air provides one or more lightning channels for the negative charges in the cloud to rush towards the ground. (The diagram was produced by Mr. L.S. Lee. Objects in the diagram are not to scale.) Lightning is a release of electric charges from a cloud to the air and then to the ground. Air itself is an electrical insulator but
the abundance of charges inside the cloud can induce a very high voltage across the cloud and the ground. This causes the
air to become ionized and conductive. A channel of ionized air emerges from the cloud at first and propagates towards the
ground in steps, each of typically tens of meters long. The propagation of the lightning channel follows a path of the least
resistance, which is not a straight but a zig­zag line because impurity or moisture in the atmosphere causes the air to be
ionized in different directions. Sometimes the lightning channel finds more than one conductive path and branches out, giving
lightning its forked appearance. Among all the branches of the lightning channel, the one which first reaches the ground establishes a conductive path all the
way from the cloud to the ground, as if there is a very long metal rod connecting them together. There will then be a sudden
and massive flow of electric current between the cloud and the ground and all the branches of the lightning channel will be lit
up, giving a bright flash across the sky. A forked and zig­zag lightning bolt (The picture was produced by Mr. L.S. Lee)