How do clouds form electricity for lightning?

11 January 2009


How do clouds form electricity for lightning? How is the energy stored? Is it dynamically generated?


Chris - The answer is we don't 100% know.

Clouds are made of billions of tiny particles, ice crystals. They're called hydrometeors and these particles rub against each other in the cloud because the clouds are full of big currents of air. There are big ones and small ones.

In exactly the same way as if you take a balloon, or a comb, and run it through your hair it will transfer charge from one thing, your hair, to the balloon or comb, this enables you to have static electricity. The same thing happens with these particles in a cloud.

By a mechanism that no one understands that well, for some reason the big hydrometeors get a negative charge and the small ones get a positive charge. The small ones get pushed to the top of the cloud on upcurrents more than the bigger ones. That's how you get this distribution of charge within the cloud.

Some people speculate it might be something to do with the solar wind which is this charged stream of particles coming from the sun, past the Earth's magnetosphere. That aside, what you end up with is a big aggregation of static energy within the cloud which is separated, according to its charge. The bottom of the cloud is very negative.

The Earth, therefore, feels an electric field pushing towards it and this repels negative charge in the surface of the Earth, because the negatives can move away, leaving the surface of the Earth net positive.

This intensifies the electric field and the result is that eventually the potential difference that builds up between the cloud and the ground overcomes the natural insulation or the inherent resistance of the air and it begins to ionise. This is where you strip electrons away from gas molecules in the atmosphere and because electrons can move they can conduct.

You begin to carry a current and so you get a few "feelers" come down. When you've got a sufficient and contiguous connection between cloud and ground you'll get a full-on strike.

The lighting comes zipping down. The actual discharge is only about the size of a five pence piece and it lasts for a billionth of a second, microseconds at best.

The actual current that flows down it is something like 20,000 Amps and it's discharging between 1 and 10 billion joules of energy which is, in fact, enough to light a 100W bulb for 100 days.

It's not a huge amount but is enough to make a big bang. The reason that the lightning's nice and bright is that, as the electricity goes smashing though the air it causes the electrons in the atoms to get very excited. They then fall back to their original energy positions, giving us some light in the process. They also get very hot and because of the thermal expansion you get a shockwave. It's like a gun going off.


Cloud acquire a large amount of electric charge

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