Chris Smith

How do Thunderstorms Work?

The Science of Thunder and Lightning

At any one time, all around the world, there are 2,000 thunderstorms happening, producing over a 100 lightning strikes a second. That's over 8 million lightning bolts every day unleashing the power of 2 million tons of TNT. But how do clouds come by all this energy, and couldn't we put it to good use?

Figure 1: At any time there are over 2,000 thunderstorms occurring worldwide, each producing over a 100 lightning strikes a second. Thats over 8 million lightning bolts every day.

Scientists began to suspect that lightning must be a form of electricity as early as the 1700s because it looked similar to the sparks you could produce by rubbing certain materials together. Scottish scientist Robert Symmer had this off to a fine art and earned the dubious title of "the barefoot philosopher" because he was always removing his silk socks and rubbing them on things to produce sparks.

But it was a daring experiment by Benjamin Franklin in 1752, and one which he was lucky to survive, that proved it once and for all. Franklin flew a kite into a thundercloud and was rewarded with a stream of sparks flowing from the bottom of the kite string.

How is lightning generated?
Franklin's experiment worked because lightning is a multi-million volt electrical discharge between one cloud and another, or between a cloud and the Earth. It's produced when friction between tiny water and ice particles in clouds, called "hydrometeors", generates static electricity. For reasons that scientists don't fully understand, the smaller particles pick up a positive charge, and the larger particles pick up a negative charge.

As these hydrometeors jostle about, updrafts push the smaller positively-charged particles towards the top of the cloud, leaving the negative charges concentrated at the bottom. It's possible that the solar wind, a million mile an hour maelstrom of cosmic radiation streaming out of the sun, may help in this sorting process.

Before long the cloud accumulates a massive potential difference measured in millions of volts. This electrical potential creates a powerful electric field, a bit like the contour lines on a map, which stretches from the bottom of the cloud to the ground (Earth). As a result the ground becomes positively charged as electrons are repelled away by the negative charge in the clouds. Tall and sharp objects, like buildings, trees, lightning conductors, and even golfing umbrellas, deform the contour lines of the field and push them close together, concentrating the electric field around the top of the object and making it a target for a strike. This happens when the field becomes sufficiently strong to overcome the insulting properties of the air, and the cloud discharges to Earth, producing a lightning bolt.

Figure 2: Each lightning flash is about 3 miles long but only about a centimetre wide. It discharges about 1-10 billion joules of energy and produces a current of some 30,000 - 50,000 amps, which heats the surrounding air to over 20,000 degrees Celsius, three times hotter than the surface of the sun (6000 degrees Celsius).

So how much energy is loitering up there?
Each lightning flash is about 3 miles long but only about a centimetre wide. It discharges about 1-10 billion joules of energy and produces a current of some 30,000 - 50,000 amps, which heats the surrounding air to over 20,000 degrees Celsius, three times hotter than the surface of the sun (6000 degrees Celsius). In fact a single lightning bolt unleashes as much energy as blowing up a ton of TNT. And although it might look like a single flash, a strike is actually made up of between three and twelve individual lightning 'strokes', each lasting only a few thousandths of a second. This is what makes lightning appear to flicker.

And what about thunder ?
The intense heat of the lightning discharge superheats the surrounding air causing it to expand explosively. This creates a compression or 'shock' wave - the thunder - which spreads out through the air in all directions, travelling at about a fifth of a mile per second.

The flash and the thunder clap are produced simultaneously - as anyone unlucky enough to have ever got very close to a lightning strike can tell you - but the light from the flash travels much more rapidly (186,000 miles per second) than sound (0.2 miles per second approximately). The light therefore reaches you first, then a short while later (depending upon how far away the storm is), the thunder rolls in.

So with all that energy knocking around up there, surely we could collect enough lightning to power a town ?

Unfortunately not - simple maths shows that this is just not feasible :

100 joules of energy keeps a 100 watt lightbulb burning for 1 second. So 1 billion joules of energy (the amount in a modest lightning strike) would keep the same single light-bulb burning for just under 120 days.

Could you power a city on the electricity in a Lightning Bolt…?

The average household uses about 500-1000 kilowatt hours (kWh) per month. 1 kilowatt hour is 1000 Joules per second multiplied by 3600 seconds (the number of seconds in an hour); i.e. 3,600000 Joules.

So, the average household consumes about 500 x 3,600000 = 1.8 billion joules of energy per month. So if you could collect all of the energy contained in one lightning strike it would run just one home for a month. This sounds like good news, but not all of the energy in lightning is available as electricity - in fact probably less than 1% of the energy (10 million joules or so) could be harnessed as electricity because a large amount has already been wasted heating up the air.

Then you have to take into consideration the 'strike frequency' for any given area, the cost involved in building a tall tower to work as a lightning collector, and then tackle the problem of how to construct a sufficiently big capacitor to store all of the charge you collect. And who would want to live near a lightning collector? That would be one noisy neighbourhood!

And as to the claim that lightning never strikes twice, a few years back New York's Empire State building was hit 15 times in as many minutes; so you can draw your own conclusions about the validity of that statement.

- June 2007

About the Author

Chris Smith is a clinical lecturer in virology at Cambridge University and the founder and managing editor of the Naked Scientists