• Question: What causes lightning/how is lightning generated?

    Asked by Beverly to Chris, Josh, Rebecca, Rob, Susan on 22 Jun 2015.
    • Photo: Rebecca Dewey

      Rebecca Dewey answered on 22 Jun 2015:

      Electric charge can build up during a storm because big volumes of air molecules are moving against each other, and as they rub against each other through friction, electrostatic charge is produced. Think about this like when you rub a balloon against your hair or your jumper and a “static electricity” builds up that you can feel as a little cloud around the balloon, or that you can use to lift someone’s hair up or stick the balloon to the ceiling. It’s the same sort of charge but in weather fronts, it is much MUCH bigger!

      Then when this charge reaches a certain amount, a certain volume of air might need to *discharge* its electric charge. It does this by earthing or equalising itself with the ground. A very highly charged part of the cloud will produce a stream of ionised gas (i.e. gas where the atoms/molecules have one or more extra electron(s) on them or have had one or more of their own electrons removed). This stream will work its way to the ground, through the shortest, fastest, easiest route. The easiest route is through a conductive material. Air is not very conductive, but metal is very conductive, so the path of least resistance can sometimes be down a lightening rod (say, on a church spire) or down a pole (say a radio mast or a mobile phone mast). Therefore most tall buildings will have lightening conductors on them designed to equalise the charge without doing any damage to the building. If the lightening is happening over a wide open space, and there are no obvious conductors, it might find a different route, say through a tree or even a person.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 22 Jun 2015:

      It’s quite a complicated process, though the general principle is simple: static charge builds up in a cloud and then discharges, either to earth or to another cloud.

      The initial condition is that the cloud develops some static charge. How it does this isn’t all that well understood, though some sort of friction such as Rebecca describes is likely. Usually, the base of the cloud winds up with net negative charge, and the top with net positive charge.

      When the charge build-up is big enough, a “leader” is produced. This is a channel of partially ionised air that gradually propagates outward from the cloud. This is another thing that is not wholly understood: although a sufficiently high concentration of charge can spontaneously ionise a gas such as air, the leaders seem to form at concentrations that aren’t high enough to expect this. Maybe there is some local extra concentration, e.g. around a nice pointy ice crystal (electric fields are magnified around pointy things). The leader may travel out towards another cloud, or down towards the ground; it often branches into a tree-like shape as it does.

      When the leader gets close enough to the ground or the other cloud, the positive charge that has been attracted by the negative charge of the leader becomes concentrated enough to start another track of partially ionised air, the upward streamer, this time with net positive charge.

      When the negative leader and an upward streamer meet, a conductive path has been established between the originating cloud and the ground or the other cloud. Electric current can now flow between the two opposite charges. This is the actual lightning strike. It involves extremely large currents – about 30 kA on average.

      The conductive path can be used by several discharges (re-strikes): apparently most lightning strikes actually contain 3 or 4 distinct discharges, and some can be many more.

      The most common type of lightning is from one cloud to another, and the second most common type is from one part of a cloud to another part of the same cloud. Cloud to ground lightning is only the third most common type.

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