Does lightning go from cloud to the Earth or Earth to the cloud?

12 October 2008


Does lightning go from cloud to the Earth or Earth to the cloud?


Chris - Well, stats on lightning - lightning's fascinating and the amount of energy being unleashed all around the world all the time is absolutely huge. The stats suggest that there's something like two thousand thunderstorms raging all around the Earth at any given moment in time, unleashing about 100 lightning bolts per second. If you tot up the amount of energy that they're unleashing on us it's something like two megatonnes of TNT equivalent very day. It's huge amount of energy. Where does it all come from? It's pretty much down to static electricity. If you look at a big thundercloud there are lots of particles called hydro-meteors which are ice crystals, for want of a better expression. These are being jostled around, rubbed against each other and rubbed against the air by updraughts within the cloud. There're lots of different sizes, big ones and small ones. For some reason that scientists don't quite understand, the big ones stay at the bottom and get a negative charge. The smaller ones get a positive charge and they float towards the top of the cloud. You've now got a big charge difference inside the cloud. The concentration of negative charges at the bottom of the cloud creates an electric field which spreads out around from the cloud to involve anything that's near the cloud. That include the ground. The Earth's surface then feels this electric field pushing on it. This means that the negative charges in the surface of the Earth run away because they can move away. This makes the surface of the Earth net positive. Now there's an attraction between the two. The first thing that happens is when that charge in the cloud gets big enough to overcome the natural resistance in the air it starts to form a thing called a leader which is a very thin thread of electricity which runs down a bit of a disjointed, crackly path down to Earth. This starts ionising or stripping away from molecules of gas in the air. This creates a low-resistance pathway between the cloud and the Earth. Once that leader gets pretty close to the ground then you will get a lot of ionisation of positive from the ground coming up to meet it. Once you've made a connection then you've got a very low-resistance pathway. A lot of that charge from the cloud will come zipping down, the electrons rip down the lightning bolt creating a very big - what's called the return stroke - which then hits the ground and dissipates some of that energy. Interestingly the actual size of the lightning strike is only about the size of a one penny piece across, the actual pathway that the electricity comes down. It passes a current of something like 20,000 amps. A huge current which heats the air which it passes through to a scorching 30,000 degrees Celsius. This is something like 5 or 6 times the surface temperature of the sun. This creates this enormous shock wave because it heats up so fast. That's the thunder. The answer to the question is that it's, strictly speaking, dissipation of a lot of concentrated energy from the cloud. Electricity which is flowing away from the cloud to Earth or to another cloud or within the same cloud. Strictly speaking, the energy's flowing away from the cloud but it's a little bit of both. You've got this up-welling of positive. Charge near to the ground too.

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