Lightning

The world's atmosphere crackles with a steady flow of lightning -- at any moment, there are thunderstorms active somewhere on Earth, and these storms can produce hundreds or thousands of lightning strikes each hour. We are all familiar with lightning, that brilliant forking of electrical energy that splits the air and can turn night into day -- or illuminate the daylight more brightly than noon. There are many kinds of lightning, but all of them are powerful discharges of electricity that are dangerous to humans, can start forest fires, and produce ozone.

Lightning usually happens in association with a thunderstorm, but volcanic eruptions and the pyrocumulus clouds created by big forest fires can also spawn a few bolts of lightning.

How lightning forms, and its effects

Interestingly, although it is a very common and well-known weather phenomenon, scientists are still unsure of the exact cause of lightning in nature. One highly-regarded theory states that lightning may be caused by ice crystals in the storm gaining a positive electric charge, and the graupel a negative charge; the ice crystals are lifted to the top of the storm while the graupel sinks, and an electric field builds up between them which becomes strong enough to discharge out the bottom of the cloud -- this is the electrostatic induction hypothesis.

In any case, the ground beneath the thunderstorm builds up a charge of equal but opposite nature, and a stepped leader -- a channel of ionized air -- eventually drops from the cloud. When it reaches the ground, the electrical charge in the cloud discharges along it, and a bolt of lightning occurs.

Lightning is intensely hot -- about 54,000 degrees Fahrenheit, close to eight times the temperature of the sun's surface. It is also very fast and carries a lot of electricity, around 1 trillion watts. The most immediately noticeable effect of this is thunder -- which is caused either by violently rapid expansion of heated air, or an actual chemical explosion of the air itself. Lightning can easily destroy trees, damage buildings, kill people, and start fires in dry material when it strikes.

Lightning has several other effects. It can make fulgurites in the ground, or glass tubes made of melted sand, which can be nearly twenty feet long in some cases. Lightning also produces large amounts of nitrogen oxide and ozone, which are important parts of the world's air chemistry. How lightning affects such processes as global warming is not yet clear, since the chemical products of lightning would have both a warming and cooling effect, and the balance between the two has not been determined. Interestingly, though, lightning appears to be more frequent in industrially-polluted air.

Varieties of Lightning

There are a lot of different kinds of lightning, produced by different effects within a stormy environment. Some of these kinds of lightning are well-known and well-explained, while others are rare enough so that they are still mostly understood only through theory. Some of the more famous types of lightning include --

• Ball lightning is probably the most mysterious kind of lightning, since it is rare and has almost never been seen by a trained scientist. This kind of lightning is a small sphere of electricity that floats or bounces around near the ground before either fading or exploding. The size of ball lightning varies from tennis ball to beach ball size. It is seldom dangerous but sometimes melts its way through screens or glass and enters houses, or comes down a chimney -- an unsettling guest, to say the least. Thus far, only theories exist about what causes ball lightning, mostly revolving around plasma or chemical reactions.
• Heat lightning appears as noiseless flashes of light, usually on a hot summer night. This is caused either by direct observation of lightning too far away to be heard, or by the reflection of distant lightning flashes off the bottom of closer clouds.
• Cloud to cloud lightning occurs when lightning strikes between two areas of cloud with different electrical charges, without touching the ground. When the lightning is hidden in the cloud and is seen as a large splash or glow of light, rather than a bolt, it may be called sheet lightning..
• Blue jets are enormous, carrot-like lightning bolts that shoot upward from the tops of tropical thunderstorms to a height of fifty miles -- almost halfway to space. They are unlikely to be seen by most people but have become more famous because of unsubstantiated guesses that Air France Flight 447 was brought down in midair by a blue jet -- a guess which is likely to be wrong.