At any given moment, some 2000 thunderstorms rumble around the planet. Their most obvious feature is the appearance in the sky of intense streaks of light – flashes of lightning. One day, will it be possible to envisage harnessing these powerful discharges?
Sizing up the phenomenon
The power and energy of the discharges that sear across a stormy sky are absolutely colossal: a current of 30,000 amps at a potential of 100 million volts.
Lightning flashes travel over distances of several kilometres and can attain speeds of 40,000 km/sec, a bit more than a tenth of the speed of light. The air along their path is heated to 30,000 °C – five times the temperature to be found on the surface of the sun. During a thunderstorm, the quantity of energy present in a cumulonimbus cloud is equivalent to that consumed by the whole of France during 5 minutes!
Charges lead to a discharge
Thanks to Benjamin Franklin, the electrical nature of lightning flashes has been known for three centuries. But the mechanisms behind the potential that builds up in storm clouds are still badly-understood. One thing scientists are sure of is that cumulonimbus clouds constitute enormous reservoirs of electrical charges. On the other hand, there is no agreement about the reasons for the appearance of these charges. According to some researchers, they result from friction of the dust particles, water droplets and crystals making up the clouds - friction that is caused by air currents in the heart of the cloud. For others, it is the changes in the state of the water in the cloud (gas, liquid and solid) that are responsible for the creation of the electrical charges.
One thing is certain, negative charges accumulate in the lower part of cumulonimbus clouds (1), whereas positive charges gather at the top. The presence of negatively-charged particles at the base of a cloud causes an accumulation of positive charges on the ground (opposite charges attract). A potential is therefore created between the two giant electrodes formed by the earth’s surface and the lower part of the cloud. The layer of air separating the two is far from being a perfect insulator. As a result, when the force of attraction between the charges becomes too strong, some negative charges make their way downwards towards earth, constituting what is known as a “stepped leader”. On the ground, positive charges gather to meet them on high points (such as the top of a tree). When the leader is sufficiently close, a “streamer”, an upward stream of charges, is created. At this stage, nothing is visible to the naked eye. But when the leader and streamer meet, they establish a conducting bridge between the earth and the sky. An intense electric current propagates along the channel so formed, producing a violent illumination – lighting strikes.
More often than not, the lightning flashes produced in a storm do not touch the ground. They are formed within a single cloud, or between two different clouds. Whatever the case, they are always caused by the attraction of two groups of charges with opposite signs.
The 50,000 thunderstorms which break out every day all around the globe fulfil a very useful role – that of permanently ensuring the equilibrium of the Earth’s electric field.
However, whilst lightning is useful, it is also very dangerous. An excellent means of protection would be to manage to control its trajectory. But how to harness such a highly energy-intense phenomenon? Researchers are currently on the track of an interesting solution. They have managed to control the direction of lightning flashes generated under laboratory conditions. The tool of these “lightning tamers” – a laser …
(1)
A cumulonimbus is a cloud characteristic of thunderstorms. In temperate latitudes, its base is at an altitude of between 300 and 3000 metres and its height from top to bottom can attain 12000 m. Its top often spreads out in the form of an anvil or an unkempt plume or tail and its base is very dark and frequently accompanied by ragged clouds.
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